IGCSE Physics Topic 6: Space Physics

6.1.1 The Earth ▼

Before we look deep into space, let us start right here at home. Our planet Earth spins on its axis, orbits the Sun, and has a companion -- the Moon -- circling around it. These three motions explain day and night, the seasons, and the phases of the Moon. Imagine you are standing at the Royal Observatory in Greenwich, looking up at the sky, and let us work out exactly what is happening.

Earth's Rotation and Day/Night

Why Do We Have Day and Night?

The Earth rotates (spins) on its axis once every approximately 24 hours. The axis is an imaginary line running from the North Pole to the South Pole. As the Earth spins, different parts of the surface face towards and away from the Sun.

When your part of the Earth faces the Sun, you experience daytime.
When your part faces away from the Sun, you experience night-time.

This rotation also explains the apparent daily motion of the Sun across the sky. If you stand at Greenwich and watch the Sun, it appears to rise in the east, travel across the southern sky, and set in the west. But the Sun is not actually moving around us -- it is the Earth rotating that creates this apparent motion. It is like sitting on a spinning roundabout: the trees seem to move past you, but really you are the one spinning.

Exam Tip

The key word is "apparent" motion. The Sun does not actually move across the sky -- the Earth rotates. If an exam question asks you to "explain the apparent daily motion of the Sun," always mention that the Earth rotates on its axis once every 24 hours, and this makes the Sun appear to move from east to west.

Earth's Orbit and the Seasons

Why Do We Have Seasons?

The Earth orbits the Sun once every approximately 365 days (one year). The Earth's axis is tilted at about 23.5 degrees to the perpendicular of its orbital plane. This tilt is the key to understanding seasons.

As the Earth orbits the Sun, the tilt means that different hemispheres receive different amounts of sunlight at different times of the year:

Summer in the Northern Hemisphere (June-August): The North Pole is tilted towards the Sun. Sunlight hits the Northern Hemisphere more directly and for longer hours each day. That is why London has long, warm summer days.
Winter in the Northern Hemisphere (December-February): The North Pole is tilted away from the Sun. Sunlight arrives at a lower angle and days are shorter. That is why December days in Edinburgh are short and cold.
When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere, and vice versa.

The periodic (repeating) nature of seasons happens because the Earth makes a complete orbit every 365 days, so the same pattern of seasons repeats each year in the same order: spring, summer, autumn, winter.

Exam Tip

Seasons are NOT caused by the Earth being closer or further from the Sun. The Earth's orbit is nearly circular, and the distance change is tiny. Seasons are caused by the tilt of the Earth's axis. This is a very common exam mistake -- do not fall for it!

Memory Trick

TILT causes seasons, not distance. Think: "The Tilt Is Letting in The light at different angles." The hemisphere tilted towards the Sun gets more concentrated, direct sunlight -- that is summer.

The Moon's Orbit and Phases

How Does the Moon Create Its Phases?

The Moon orbits the Earth approximately once every month (about 27.3 days for one complete orbit, though the cycle of phases takes about 29.5 days because the Earth is also moving around the Sun).

The Moon does not produce its own light -- it reflects sunlight. As the Moon orbits the Earth, we see different amounts of its sunlit side, creating the cycle of phases:

New Moon: The Moon is between the Earth and Sun. The sunlit side faces away from us, so the Moon appears dark.
Waxing Crescent: A thin sliver of the sunlit side becomes visible.
First Quarter: We see half the sunlit side (right half from the Northern Hemisphere).
Waxing Gibbous: More than half is visible, still growing.
Full Moon: The Earth is between the Moon and Sun. We see the entire sunlit face.
Waning Gibbous: The visible lit portion starts shrinking.
Last Quarter: We see the other half lit (left half from the Northern Hemisphere).
Waning Crescent: A thin sliver remains, then back to New Moon.

Memory Trick

"Waxing" = growing (getting bigger), "Waning" = shrinking (getting smaller). Think of wax dripping onto a candle, building up -- waxing means getting bigger. The full cycle takes about one month, which is where the word "month" originally comes from (month = Moon-th).

Supplement

Average Orbital Speed

For objects moving in circular (or approximately circular) orbits, we can calculate the average orbital speed using:

v = 2πr / T

v = average orbital speed (m/s) r = average radius of orbit (m) T = orbital period (s)

This equation works because the circumference of a circle is 2πr, and speed = distance / time. So the orbital speed is the circumference divided by the time for one complete orbit.

Worked Example The Earth orbits the Sun at an average radius of 1.50 x 10¹¹ m. Calculate the average orbital speed of the Earth. (Take T = 365 days.)

Step 1: Convert T to seconds

T = 365 days x 24 hours x 3600 seconds = 365 x 86 400 = 3.154 x 10⁷ s

Step 2: Write the equation

v = 2πr / T

Step 3: Substitute values

v = (2 x π x 1.50 x 10¹¹) / (3.154 x 10⁷)
v = (9.425 x 10¹¹) / (3.154 x 10⁷)

Step 4: Calculate

v = 29 900 m/s (approximately 30 km/s)

v ≈ 30 000 m/s (about 30 km/s or 108 000 km/h)

Worked Example The Moon orbits the Earth with an average orbital radius of 3.84 x 10⁸ m and a period of 27.3 days. Calculate the Moon's average orbital speed.

Step 1: Convert T to seconds

T = 27.3 x 24 x 3600 = 27.3 x 86 400 = 2.359 x 10⁶ s

Step 2: Substitute into v = 2πr / T

v = (2 x π x 3.84 x 10⁸) / (2.359 x 10⁶)
v = (2.413 x 10⁹) / (2.359 x 10⁶)

Step 3: Calculate

v = 1023 m/s

v ≈ 1020 m/s (about 1.0 km/s)

Worked Example The International Space Station (ISS) orbits at a height of 408 km above the Earth's surface. The Earth's radius is 6370 km. The ISS completes one orbit in 92 minutes. Calculate the orbital speed of the ISS.

Step 1: Find orbital radius

r = Earth's radius + height above surface = 6370 + 408 = 6778 km = 6.778 x 10⁶ m

Step 2: Convert T to seconds

T = 92 x 60 = 5520 s

Step 3: Substitute

v = (2 x π x 6.778 x 10⁶) / 5520 = (4.259 x 10⁷) / 5520

Step 4: Calculate

v = 7714 m/s

v ≈ 7700 m/s (about 7.7 km/s or 27 700 km/h)

Exam Tip

Always convert T into seconds before substituting into v = 2πr / T. The most common mistake is forgetting to convert days or minutes into seconds. Also check that r is in metres.

🌎 Apply It: Real-World Physics

Can you connect classroom physics to the real world? Tap each scenario to reveal the answer.

At the Royal Observatory in Greenwich, London, visitors can stand on the Prime Meridian line -- the line of zero degrees longitude. As the Sun crosses this meridian, it marks local noon. A visitor notices that the Sun rises in the east over the Thames and sets in the west over central London.

Why does the Sun appear to move from east to west across the sky each day? Is the Sun actually moving around the Earth?

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Identify the Physics

This is about the apparent daily motion of the Sun caused by the Earth's rotation.

Work It Out

The Sun does not move around the Earth. The Earth rotates on its axis from west to east once every 24 hours. Because the Earth spins eastward, the Sun appears to move westward across the sky. This is exactly like looking out of a train window -- the trees appear to move backwards, but really you are the one moving forwards. The Sun appears to rise in the east, reach its highest point around noon at the meridian, and set in the west.

Connect to the Syllabus

This directly addresses syllabus point 6.1.1 Core (1): Earth rotates on its axis once in approximately 24 hours, which explains the apparent daily motion of the Sun and the day/night cycle.

Aha! Moment

The Royal Observatory at Greenwich was founded in 1675 by King Charles II specifically to solve the problem of determining longitude at sea. Astronomers would carefully record when stars crossed the meridian, using the Earth's rotation as a giant clock. The Greenwich Mean Time (GMT) standard, used worldwide, came from this observatory -- all because the Earth spins reliably once every 24 hours.

In June, Edinburgh in Scotland enjoys nearly 18 hours of daylight, while in December daylight drops to about 7 hours. Meanwhile, in the Falkland Islands (Southern Hemisphere), the pattern is reversed.

Explain why Edinburgh has long days in June and short days in December. Why is the Falkland Islands' pattern reversed?

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Identify the Physics

This is about seasons caused by the tilt of the Earth's axis combined with the Earth's orbit around the Sun.

Work It Out

The Earth's axis is tilted at about 23.5 degrees. In June, the Northern Hemisphere (where Edinburgh is) is tilted towards the Sun. This means: (a) the Sun climbs higher in the sky, so its rays hit Edinburgh more directly, and (b) the Sun is above the horizon for more hours each day -- nearly 18 hours. In December, the Northern Hemisphere tilts away from the Sun, so the Sun stays low in the sky and is above the horizon for only about 7 hours. The Falkland Islands are in the Southern Hemisphere, so when the Northern Hemisphere tilts towards the Sun, the Southern Hemisphere tilts away. Their winter is our summer, and their summer is our winter.

Connect to the Syllabus

This covers syllabus point 6.1.1 Core (2): the Earth orbits the Sun once in 365 days, and the tilt of its axis explains the periodic nature of seasons.

Aha! Moment

Edinburgh is at about 56 degrees north latitude. In midsummer, the Sun barely sets -- the sky stays light all night in a phenomenon called "simmer dim" in Shetland. Meanwhile, at the Tropic of Cancer (23.5 degrees N), the Sun is directly overhead at noon on the summer solstice. The further from the equator you are, the more extreme the difference between summer and winter daylight hours.

Tim Peake, the British ESA astronaut, spent 186 days aboard the International Space Station (ISS). The ISS orbits 408 km above the Earth at about 7.7 km/s, completing one orbit every 92 minutes. Tim reported seeing 16 sunrises and sunsets every day.

Why did Tim Peake see 16 sunrises each day? Use the orbital speed equation to verify the ISS orbital period.

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Identify the Physics

This combines the Earth's rotation (24-hour day) with the orbital speed equation v = 2πr / T.

Work It Out

The ISS orbits the Earth every 92 minutes. In a 24-hour day, the number of orbits = 24 x 60 / 92 = 1440 / 92 = 15.7, which is about 16 orbits. Each orbit, the ISS passes from the sunlit side of Earth to the dark side and back. So Tim saw approximately 16 sunrises and 16 sunsets every day.

Verification: r = 6370 + 408 = 6778 km = 6.778 x 10⁶ m. Rearranging v = 2πr / T gives T = 2πr / v = (2 x π x 6.778 x 10⁶) / 7700 = 4.259 x 10⁷ / 7700 = 5531 s = 92.2 minutes. This matches the stated 92-minute period.

Connect to the Syllabus

This uses v = 2πr / T (Supplement) and connects it to the Earth's rotation period (Core). The ISS orbits much faster than the Earth rotates, so multiple sunrises occur in one Earth-day.

Aha! Moment

Tim Peake became the first official British ESA astronaut to visit the ISS in December 2015. During his Principia mission, he ran the London Marathon on a treadmill aboard the ISS, completed a spacewalk, and inspired millions of school students across the UK. He travelled about 125 million km during his 186-day mission -- further than the distance from Earth to the Sun!

The BBC Sky at Night programme, originally presented by Sir Patrick Moore for over 50 years, regularly features the Moon. A viewer in Bristol notices that over several weeks, the Moon changes from a thin crescent to a full disc and back again.

Explain why the Moon goes through phases. Why does a Full Moon always rise at roughly sunset?

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Identify the Physics

This is about the Moon's orbital motion around the Earth and the resulting phases.

Work It Out

The Moon orbits the Earth approximately once a month. It does not produce its own light -- it reflects sunlight. At any time, exactly half the Moon is lit by the Sun and half is in darkness. As the Moon moves around the Earth, we see different proportions of the lit side.

At New Moon, the Moon is roughly between Earth and Sun, so the lit side faces away from us. At Full Moon, the Earth is roughly between the Moon and the Sun, so we see the entire lit face. The phases in between show gradually more (waxing) or less (waning) of the lit side.

A Full Moon rises at about sunset because the Moon is on the opposite side of the Earth from the Sun. As the Sun goes down in the west, the Full Moon comes up in the east -- they are on opposite sides of the sky.

Connect to the Syllabus

This covers syllabus point 6.1.1 Core (3): the Moon orbits the Earth in approximately one month, which explains the Moon's cycle of phases.

Aha! Moment

Sir Patrick Moore presented The Sky at Night on the BBC from 1957 until his death in 2012 -- over 700 episodes, making it the longest-running programme with the same presenter in television history. He made astronomy accessible to millions. The Moon was one of his great passions, and his lunar maps were so detailed that NASA consulted them when planning the Apollo missions.

ESA's Rosetta spacecraft travelled to Comet 67P/Churyumov-Gerasimenko, which orbits the Sun at an average distance of about 5.0 x 10¹¹ m with a period of about 6.5 years. Scientists needed to calculate the comet's orbital speed to plan the rendezvous.

Calculate the average orbital speed of Comet 67P. Why is this speed much slower than the Earth's orbital speed?

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Identify the Physics

This uses the orbital speed equation v = 2πr / T, and relates speed to orbital distance.

Work It Out

r = 5.0 x 10¹¹ m, T = 6.5 years = 6.5 x 365 x 24 x 3600 = 6.5 x 3.154 x 10⁷ = 2.05 x 10⁸ s.

v = 2πr / T = (2 x π x 5.0 x 10¹¹) / (2.05 x 10⁸) = (3.14 x 10¹²) / (2.05 x 10⁸) = 15 300 m/s ≈ 15 km/s.

This is about half the Earth's orbital speed (30 km/s). Objects further from the Sun orbit more slowly because the Sun's gravitational pull is weaker at greater distances. Less gravitational force means less centripetal force, so the object needs a lower speed to maintain its orbit.

Connect to the Syllabus

This uses v = 2πr / T (Supplement) and connects to the idea that orbital speeds decrease with distance from the Sun (6.1.2 Supplement).

Aha! Moment

ESA's Rosetta mission launched in 2004 and took 10 years to reach Comet 67P. It was the first spacecraft to orbit a comet and deploy a lander (Philae) onto the surface. The mission was controlled from ESA's European Space Operations Centre in Darmstadt, Germany, with significant contributions from UK scientists. The comet turned out to be shaped like a rubber duck!

Practice Questions: Section 6.1.1

Test your understanding of Earth's rotation, orbit, and the Moon's phases.

Score 0 / 20

QUESTION 1

The Earth rotates once on its axis in approximately:

A. 1 hour

B. 24 hours

C. 365 days

D. 1 month

The Earth rotates on its axis once every approximately 24 hours. This rotation causes day and night. 365 days is the time for one orbit around the Sun.

QUESTION 2

The apparent motion of the Sun across the sky from east to west is caused by:

A. The Sun orbiting the Earth

B. The Earth rotating on its axis

C. The Earth orbiting the Sun

D. The Moon orbiting the Earth

The Sun appears to move across the sky because the Earth rotates (spins) on its axis. The Sun is effectively stationary relative to the Earth's rotation. The Earth rotates from west to east, making the Sun appear to move from east to west.

QUESTION 3

Seasons on Earth are caused by:

A. The Earth being closer to the Sun in summer

B. The Sun producing more heat in summer

C. The tilt of the Earth's axis as it orbits the Sun

D. The Moon blocking sunlight in winter

Seasons are caused by the tilt of the Earth's axis (about 23.5 degrees). As the Earth orbits the Sun, the hemisphere tilted towards the Sun receives more direct sunlight and has longer days -- this is summer. The distance to the Sun barely changes and does NOT cause seasons.

QUESTION 4

A Full Moon occurs when:

A. The Moon is between the Earth and the Sun

B. The Earth is roughly between the Moon and the Sun

C. The Moon produces its own light

D. The Moon is at its closest point to the Sun

At Full Moon, the Earth is roughly between the Moon and the Sun. We can see the entire sunlit face of the Moon. When the Moon is between Earth and Sun, that is a New Moon -- the lit side faces away from us.

QUESTION 5

The Moon orbits the Earth approximately once every:

A. 24 hours

B. 7 days

C. 1 month

D. 1 year

The Moon orbits the Earth in approximately one month (about 27.3 days for the orbital period, 29.5 days for the full phase cycle). The word "month" itself comes from "Moon."

QUESTION 6

Why do we always see the same side of the Moon from Earth?

A. The Moon does not rotate at all

B. The Moon's rotation period equals its orbital period

C. The far side of the Moon is always in darkness

D. The Moon is too far away to see both sides

The Moon rotates on its own axis in exactly the same time it takes to orbit the Earth (about 27.3 days). This is called synchronous rotation or tidal locking. Both sides of the Moon receive sunlight -- the "far side" is not permanently dark.

QUESTION 7

It is winter in the UK (Northern Hemisphere). At the same time, in Australia (Southern Hemisphere) it is:

A. Summer

B. Winter

C. Spring

D. Autumn

When the Northern Hemisphere is tilted away from the Sun (winter), the Southern Hemisphere is tilted towards the Sun (summer). The seasons are reversed between the hemispheres because of the Earth's axial tilt.

QUESTION 8

The Earth takes one complete orbit around the Sun in approximately:

A. 24 hours

B. 30 days

C. 365 days

D. 12 months exactly

The Earth orbits the Sun in approximately 365.25 days (which is why we have a leap year every 4 years). This orbital period defines one year.

QUESTION 9

"Waxing" Moon means the visible lit area is:

A. Getting larger

B. Getting smaller

C. Staying the same

D. Completely dark

Waxing means growing or increasing. A waxing Moon shows more of its sunlit face each night. Waning means shrinking -- the lit part decreases.

QUESTION 10

The average orbital speed equation is v = 2πr / T. If the orbital radius doubles and the period stays the same, the orbital speed will:

A. Halve

B. Double

C. Stay the same

D. Quadruple

v = 2πr / T. If r doubles and T stays the same, v = 2π(2r) / T = 2 x (2πr / T), so v doubles. Speed is directly proportional to the radius when the period is constant.

QUESTION 11

A satellite orbits the Earth at a radius of 7000 km with a period of 98 minutes. What is its approximate orbital speed?

A. 1200 m/s

B. 3500 m/s

C. 7500 m/s

D. 15 000 m/s

T = 98 x 60 = 5880 s. r = 7000 km = 7.0 x 10⁶ m. v = 2πr / T = (2 x 3.14 x 7.0 x 10⁶) / 5880 = 4.4 x 10⁷ / 5880 ≈ 7500 m/s.

QUESTION 12

At the equator, the Sun rises at about 6:00 AM and sets at about 6:00 PM every day of the year. At London (52 degrees N), summer days are much longer than winter days. Why?

A. London is further from the Sun

B. The Sun moves faster across the sky in London

C. The axial tilt changes how long the Sun is above the horizon at different latitudes

D. London rotates faster than the equator

The Earth's axial tilt means that at higher latitudes like London, the tilt has a bigger effect on how long the Sun stays above the horizon. In summer the Northern Hemisphere tilts towards the Sun, giving London very long days. At the equator, the tilt has minimal effect on day length.

QUESTION 13

A New Moon occurs when:

A. The Moon is roughly between the Earth and the Sun

B. The Earth is between the Moon and the Sun

C. The Moon is behind the Earth's shadow

D. The Moon has stopped reflecting light

At New Moon, the Moon is roughly between the Earth and the Sun. The sunlit side faces away from Earth, so we cannot see the Moon. It is NOT in the Earth's shadow -- that would be a lunar eclipse, which is a different (and rare) event.

QUESTION 14

Mars orbits the Sun at a radius of 2.28 x 10¹¹ m with a period of 687 days. What is Mars' approximate orbital speed?

A. 12 000 m/s

B. 24 000 m/s

C. 30 000 m/s

D. 48 000 m/s

T = 687 x 86 400 = 5.935 x 10⁷ s. v = 2πr / T = (2 x 3.14 x 2.28 x 10¹¹) / (5.935 x 10⁷) = 1.433 x 10¹² / 5.935 x 10⁷ ≈ 24 100 m/s ≈ 24 km/s.

QUESTION 15

Which statement correctly explains day and night?

A. The Sun orbits the Earth, passing behind it at night

B. The Sun switches off at night

C. The Earth rotates, so each side alternately faces towards and away from the Sun

D. The Moon blocks the sunlight at night

The Earth rotates on its axis. The side facing the Sun has daytime; the side facing away has night-time. This cycle repeats every 24 hours as the Earth continues to spin.

QUESTION 16

The Moon appears to shine because:

A. It produces its own light like a star

B. It reflects sunlight

C. It is very hot

D. It reflects starlight

The Moon does not produce its own light. It shines by reflecting sunlight off its rocky surface. Only about 12% of the sunlight that hits the Moon is reflected back.

QUESTION 17

The Earth's axis is tilted at approximately:

A. 0 degrees

B. 10 degrees

C. 23.5 degrees

D. 45 degrees

The Earth's axis is tilted at approximately 23.5 degrees to the perpendicular of its orbital plane. This tilt is crucial for causing the seasons. If the tilt were 0 degrees, there would be no seasons at all.

QUESTION 18

What is the correct order of Moon phases starting from New Moon?

A. New Moon, Waxing Crescent, First Quarter, Full Moon

B. New Moon, Full Moon, First Quarter, Last Quarter

C. Full Moon, Waxing Crescent, New Moon, Last Quarter

D. New Moon, Waning Crescent, Full Moon, First Quarter

The correct sequence from New Moon is: New Moon → Waxing Crescent → First Quarter (half lit) → Waxing Gibbous → Full Moon → Waning Gibbous → Last Quarter → Waning Crescent → back to New Moon.

QUESTION 19

A planet has an orbital radius of 6.0 x 10¹¹ m and an orbital period of 3.0 x 10⁷ s. Its average orbital speed is approximately:

A. 63 000 m/s

B. 20 000 m/s

C. 126 000 m/s

D. 12 600 m/s

v = 2πr / T = (2 x 3.14 x 6.0 x 10¹¹) / (3.0 x 10⁷) = 3.77 x 10¹² / 3.0 x 10⁷ = 1.26 x 10⁵ = 126 000 m/s.

QUESTION 20

The "periodic nature of seasons" means:

A. Seasons happen randomly

B. Seasons only occur in certain countries

C. The same pattern of seasons repeats at regular intervals

D. Each season lasts for one month

"Periodic" means repeating at regular intervals. Because the Earth orbits the Sun once every 365 days with a fixed axial tilt, the same pattern of spring, summer, autumn, and winter repeats every year in the same order.

6.1.2 The Solar System ▼

Our Solar System is a remarkable neighbourhood in space. It contains one star, eight planets, dwarf planets, asteroids, comets, and moons -- all held together by gravity. Let us explore what makes up this system, why the inner and outer planets are so different, and how gravity shapes everything from orbits to surface conditions.

What Is in the Solar System?

The Solar System consists of:

One star -- the Sun, at the centre
Eight planets (in order from the Sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
Minor planets (dwarf planets) such as Pluto, Ceres, Eris
Asteroids -- rocky bodies, mostly found in the asteroid belt between Mars and Jupiter
Comets -- icy bodies with elongated orbits that develop tails when near the Sun
Moons (natural satellites) -- bodies that orbit planets

Memory Trick

My Very Educated Mother Just Served Us Nachos -- Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. This classic mnemonic gives you the planets in order from the Sun.

Inner Planets vs Outer Planets

The four inner planets (Mercury, Venus, Earth, Mars) are:

Rocky (terrestrial) -- made of rock and metal
Small -- relatively small diameter and mass
Close to the Sun

The four outer planets (Jupiter, Saturn, Uranus, Neptune) are:

Gaseous (gas giants or ice giants) -- mostly hydrogen, helium, and other gases
Large -- much larger diameter and mass
Far from the Sun

The Accretion Model: How Did the Planets Form?

The Solar System formed about 4.6 billion years ago from a huge cloud of gas and dust called a solar nebula. The accretion model explains how:

The cloud contained many different elements -- hydrogen, helium, iron, silicon, carbon, water ice, and more.
The cloud began to collapse under its own gravity.
As it collapsed, it began to rotate, and the material flattened into a spinning accretion disc (like pizza dough spinning and flattening).
The centre became very hot and dense, forming the Sun.
In the disc, small particles collided and stuck together (accreted), gradually forming larger and larger bodies -- planetesimals, then protoplanets, then planets.

Why are inner planets rocky and outer planets gaseous?

Close to the Sun, it was too hot for gases like hydrogen and helium to condense. Only materials with high melting points (rock and metal) could survive, so the inner planets formed from these dense, rocky materials.
Far from the Sun, it was cold enough for gases and ices to condense and accumulate. The outer planets grew large enough for their gravity to capture and hold onto huge atmospheres of hydrogen and helium, becoming gas giants.

Exam Tip

When explaining the accretion model, make sure to mention: (1) gravity causing collapse, (2) many elements in the interstellar cloud, (3) rotation of material, and (4) formation of an accretion disc. These are the four key points the syllabus requires.

Planet	Type	Avg Distance from Sun (AU)	Orbital Period	g (N/kg)
Mercury	Rocky	0.39	88 days	3.7
Venus	Rocky	0.72	225 days	8.9
Earth	Rocky	1.00	365 days	9.8
Mars	Rocky	1.52	687 days	3.7
Jupiter	Gas giant	5.20	11.9 years	23.1
Saturn	Gas giant	9.58	29.5 years	9.0
Uranus	Ice giant	19.2	84.0 years	8.7
Neptune	Ice giant	30.1	165 years	11.0

Gravitational Field Strength

What Determines Surface Gravity?

The gravitational field strength at a planet's surface depends on the mass of the planet. A more massive planet has a stronger gravitational pull at its surface. However, the planet's size also matters -- if a very massive planet is also very large, the surface may be far from the centre, which reduces the field strength.

Key facts:

Gravitational field strength at the surface depends on the planet's mass (and radius).
Gravitational field strength decreases with distance from the planet. The further you are from the planet's centre, the weaker the gravitational pull.

Speed of Light and Distance Calculations

Calculating Light Travel Time

Light travels at a speed of 3.0 x 10⁸ m/s in a vacuum. We can calculate how long light takes to travel between objects in the Solar System using:

time = distance / speed

Worked Example The Sun is 1.50 x 10¹¹ m from the Earth. How long does light take to travel from the Sun to the Earth?

Step 1: Write what you know

distance = 1.50 x 10¹¹ m, speed of light = 3.0 x 10⁸ m/s

Step 2: Use time = distance / speed

time = (1.50 x 10¹¹) / (3.0 x 10⁸) = 500 s

Step 3: Convert to minutes

500 / 60 = 8.3 minutes

Light takes about 500 seconds (8.3 minutes) to travel from the Sun to the Earth

Worked Example Mars is approximately 2.28 x 10¹¹ m from the Sun. How long does it take for sunlight to reach Mars?

Step 1: Write what you know

distance = 2.28 x 10¹¹ m, speed of light = 3.0 x 10⁸ m/s

Step 2: Calculate

time = (2.28 x 10¹¹) / (3.0 x 10⁸) = 760 s = 12.7 minutes

Light takes about 760 seconds (12.7 minutes) to travel from the Sun to Mars

Worked Example Neptune is about 4.50 x 10¹² m from the Sun. How long does light take to reach Neptune?

Step 1: Calculate

time = (4.50 x 10¹²) / (3.0 x 10⁸) = 15 000 s

Step 2: Convert

15 000 / 60 = 250 minutes = 4.2 hours

Light takes about 15 000 seconds (4.2 hours) to travel from the Sun to Neptune

Why Do Planets Orbit the Sun?

The Sun contains about 99.86% of the total mass of the Solar System. Because it is so massive, its gravitational attraction is enormous. This gravitational pull provides the centripetal force needed to keep all the planets, minor planets, and comets in orbit around it.

In other words: the force that keeps a planet in orbit is the gravitational attraction between that planet and the Sun. Without this force, the planet would fly off in a straight line (Newton's first law).

Supplement

Elliptical Orbits

The orbits of planets, minor planets, and comets are elliptical (oval-shaped), not perfectly circular. The Sun is at one focus of the ellipse, not at the centre. For most planets, the orbits are nearly circular, so the Sun is approximately at the centre. But for comets and some minor planets, the orbits are highly elliptical.

Orbital Speed and Distance from the Sun

As you move further from the Sun:

The Sun's gravitational field decreases
Orbital speeds decrease -- planets further from the Sun move more slowly

Mercury (closest) has the highest orbital speed at about 48 km/s, while Neptune (furthest) moves at only about 5.4 km/s.

Elliptical Orbits and Conservation of Energy

When an object in an elliptical orbit moves closer to the Sun, it speeds up. When it moves further from the Sun, it slows down. This can be explained using conservation of energy:

As a comet falls closer to the Sun, it loses gravitational potential energy.
This lost potential energy is converted into kinetic energy, so the comet speeds up.
As it moves away from the Sun, kinetic energy converts back to gravitational potential energy, and the comet slows down.
Total energy (kinetic + gravitational potential) stays constant.

Interpreting Planetary Data

In the exam you may be asked to analyse data about the planets. Here are some patterns to look for:

Orbital distance increases → orbital period increases (further planets take longer to orbit)
Orbital distance increases → orbital speed decreases
Orbital distance increases → surface temperature generally decreases (further from the Sun)
Inner planets have higher density (rocky), outer planets have lower density (gaseous)
More massive planets generally have higher gravitational field strength at the surface

Planet	Distance (AU)	Period (years)	Orbital Speed (km/s)	Density (kg/m³)	Surface Temp (°C)
Mercury	0.39	0.24	47.4	5427	167 (mean)
Venus	0.72	0.62	35.0	5243	464
Earth	1.00	1.00	29.8	5514	15
Mars	1.52	1.88	24.1	3933	-65
Jupiter	5.20	11.9	13.1	1326	-110
Saturn	9.58	29.5	9.7	687	-140
Uranus	19.2	84.0	6.8	1271	-195
Neptune	30.1	165	5.4	1638	-200

Exam Tip

Venus is hotter than Mercury despite being further from the Sun. This is because Venus has a very thick atmosphere of carbon dioxide that traps heat (runaway greenhouse effect). If the exam gives you a data table, do not assume temperature always decreases with distance -- look at the actual data!

🌎 Apply It: Real-World Physics

Connect your knowledge of the Solar System to real missions and observations.

The Lovell Telescope at Jodrell Bank Observatory in Cheshire has been tracking spacecraft since the 1950s. When NASA's Curiosity rover landed on Mars in 2012, Jodrell Bank helped track its signals. A radio signal from Mars took about 14 minutes to reach Earth at the time.

Using the speed of light, estimate the distance between Mars and Earth at that time. Why does this communication delay make rover driving on Mars so challenging?

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Identify the Physics

This is about calculating distance using the speed of light and the practical effects of light travel time.

Work It Out

Radio signals travel at the speed of light: 3.0 x 10⁸ m/s.
Time = 14 minutes = 14 x 60 = 840 s.
Distance = speed x time = 3.0 x 10⁸ x 840 = 2.52 x 10¹¹ m ≈ 2.5 x 10¹¹ m.

This delay means if the rover sends a message saying "I see a rock ahead," controllers on Earth would not receive it until 14 minutes later. If they then send back "turn left," that command takes another 14 minutes to arrive. A round trip is 28 minutes. The rover could drive off a cliff in that time! This is why Mars rovers use autonomous navigation software -- they must make some driving decisions themselves.

Connect to the Syllabus

This covers syllabus point 6.1.2 Core (4): calculate the time for light to travel significant distances in the Solar System. It also shows a practical application of these calculations.

Aha! Moment

Jodrell Bank Observatory, home to the 76-metre Lovell Telescope, has been at the forefront of space tracking since 1957, when it was the only telescope in the West able to track the rocket that launched Sputnik. It became a UNESCO World Heritage Site in 2019. The communication delay to Mars varies from about 3 minutes (when Mars is closest to Earth) to over 22 minutes (when Mars is on the opposite side of the Sun).

ESA's Huygens probe was released from the Cassini spacecraft and landed on Titan, Saturn's largest moon, in January 2005. Titan has a surface gravitational field strength of only 1.35 N/kg, compared to Earth's 9.8 N/kg. Titan's atmosphere is thicker than Earth's, despite its weak gravity.

Why is Titan's gravitational field strength so much lower than Earth's? Why is it surprising that Titan has a thick atmosphere despite its weak gravity?

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Identify the Physics

This is about gravitational field strength depending on mass and the relationship between gravity and atmosphere retention.

Work It Out

Titan's gravitational field strength (1.35 N/kg) is much lower than Earth's (9.8 N/kg) because Titan has much less mass than Earth. The surface gravitational field strength depends on the mass of the body -- less massive bodies have weaker surface gravity.

Normally, a body with weak gravity struggles to hold onto an atmosphere because gas molecules can reach escape velocity more easily. However, Titan is extremely cold (about -180 degrees C). At such low temperatures, gas molecules move much more slowly. Even with weak gravity, the molecules are too slow to escape. This is why Titan retains a thick nitrogen atmosphere despite its small size.

Connect to the Syllabus

This covers syllabus point 6.1.2 Core (3): gravitational field strength at the surface depends on planet mass, and it decreases with distance from the planet.

Aha! Moment

The Huygens probe was built in Europe and was a key ESA contribution to the Cassini-Huygens mission. It took 2 hours 27 minutes to descend through Titan's thick atmosphere and sent back images of a landscape with methane rivers and lakes. Titan is the only body in the Solar System other than Earth known to have stable surface liquids -- but instead of water, the lakes are filled with liquid methane and ethane!

Halley's Comet has a highly elliptical orbit around the Sun. At its closest approach (perihelion), it is about 8.8 x 10¹⁰ m from the Sun. At its furthest point (aphelion), it is about 5.2 x 10¹² m away. Its orbital period is about 76 years. The last time it was visible from Britain was in 1986.

Explain why Halley's Comet moves much faster at perihelion than at aphelion. Use conservation of energy in your answer.

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Identify the Physics

This is about elliptical orbits and conservation of energy explaining the change in speed.

Work It Out

As Halley's Comet falls towards the Sun (moving from aphelion to perihelion), it gets closer to the Sun. Its gravitational potential energy decreases (becomes more negative). By conservation of energy, the total energy stays constant, so the lost potential energy is converted into kinetic energy. More kinetic energy means higher speed. So the comet moves fastest at perihelion (closest to the Sun).

As it moves away from the Sun (perihelion to aphelion), kinetic energy converts back to gravitational potential energy, and the comet slows down. It moves slowest at aphelion (furthest from the Sun).

This is analogous to a ball rolling down and up hills -- it speeds up going downhill (losing PE, gaining KE) and slows down going uphill (losing KE, gaining PE).

Connect to the Syllabus

This covers Supplement points: elliptical orbits with the Sun at one focus, and the explanation using conservation of energy for why objects travel faster when closer to the Sun.

Aha! Moment

Halley's Comet is named after Edmond Halley, the British astronomer who predicted its return in 1758 using Newton's laws of gravitation. Halley was also the Astronomer Royal. The comet was visible from Britain in 1986, and the next appearance will be in 2061. ESA's Giotto spacecraft flew past the comet in 1986 and took the first close-up images of a comet's nucleus.

The UK Space Agency helped fund ESA's JUICE (Jupiter Icy Moons Explorer) mission, which launched in 2023. JUICE will study Jupiter's moons Ganymede, Europa, and Callisto. Jupiter is 5.2 AU from the Sun (1 AU = 1.50 x 10¹¹ m). Its orbital speed is about 13 km/s, much slower than Earth's 30 km/s.

Explain why Jupiter's orbital speed is lower than Earth's, even though Jupiter is much more massive. Calculate the time for light to travel from the Sun to Jupiter.

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Identify the Physics

This is about orbital speed decreasing with distance and light travel time.

Work It Out

Why is Jupiter's orbital speed lower? The Sun's gravitational field decreases with distance. At Jupiter's distance (5.2 AU), the gravitational pull from the Sun is much weaker than at Earth's distance (1 AU). A weaker gravitational pull means less centripetal force, and a lower speed is needed to maintain a stable orbit. Jupiter's own mass does not determine its orbital speed -- it is the Sun's gravity at that distance that matters.

Light travel time: Distance = 5.2 AU = 5.2 x 1.50 x 10¹¹ = 7.8 x 10¹¹ m.
Time = distance / speed of light = (7.8 x 10¹¹) / (3.0 x 10⁸) = 2600 s = 43.3 minutes.

Connect to the Syllabus

This covers Supplement: the Sun's gravitational field decreases and orbital speeds decrease as distance from the Sun increases. It also covers Core: calculating light travel time.

Aha! Moment

JUICE will take 8 years to reach Jupiter, arriving in 2031. British scientists and engineers contributed key instruments, including the JANUS camera and the J-MAG magnetometer built at Imperial College London. At Jupiter's distance, sunlight is 25 times weaker than at Earth, so JUICE needs the largest solar panels ever built for an interplanetary mission -- each one is the size of a double-decker bus!

Students at a school in Cambridge are studying the Solar System. They have data showing that Saturn's density (687 kg/m³) is less than that of water (1000 kg/m³), while Earth's density is 5514 kg/m³. Saturn is a gas giant with a mass 95 times greater than Earth's.

Why does Saturn have such a low density despite being 95 times more massive than Earth? Relate this to the accretion model.

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Identify the Physics

This is about planetary composition, density, and the accretion model.

Work It Out

Density = mass / volume. Saturn is 95 times more massive than Earth, but it is also enormously larger -- about 764 times the volume of Earth. Because its volume is proportionally much greater than its mass, its density is very low.

Saturn is mostly made of hydrogen and helium -- very light gases. According to the accretion model, Saturn formed far from the Sun where temperatures were low enough for these gases to condense and be captured by gravity. The inner planets formed closer to the Sun where only dense rocky and metallic materials could survive the high temperatures. So inner planets are dense (rock and iron) while outer planets are low-density (gas and ice).

Saturn's density being less than water is a famous fact -- it would float if you could find a bathtub big enough!

Connect to the Syllabus

This covers Core: inner planets are rocky and small, outer planets are gaseous and large. It also covers the accretion model and interpreting planetary data (Supplement).

Aha! Moment

The University of Cambridge has a rich astronomical history. The Cavendish Laboratory, where the neutron was discovered, continues cutting-edge astrophysics research. Cambridge's Institute of Astronomy studies how planets form in other solar systems, and they have found that the same rocky-inner / gaseous-outer pattern seems to apply to many other planetary systems too -- the accretion model explains it every time.

Practice Questions: Section 6.1.2

Test your understanding of the Solar System, gravity, orbits, and planetary data.

Score 0 / 20

QUESTION 1

The correct order of planets from the Sun is:

A. Mercury, Venus, Mars, Earth, Jupiter, Saturn, Uranus, Neptune

B. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

C. Venus, Mercury, Earth, Mars, Jupiter, Saturn, Neptune, Uranus

D. Mercury, Venus, Earth, Mars, Saturn, Jupiter, Uranus, Neptune

The correct order from the Sun is: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Remember: My Very Educated Mother Just Served Us Nachos.

QUESTION 2

The inner planets are rocky and small because:

A. They are too close to the Sun to have any atmosphere

B. Close to the Sun, it was too hot for gases to condense, leaving only rocky materials

C. The Sun pushed all the gas away

D. Rocky planets always form first

In the accretion model, close to the Sun temperatures were too high for hydrogen, helium, and ices to condense. Only materials with high melting points (rock and metal) could form solid particles and accrete into planets.

QUESTION 3

The Sun contains approximately what percentage of the Solar System's total mass?

A. 50%

B. 75%

C. 90%

D. Over 99%

The Sun contains about 99.86% of all the mass in the Solar System. This enormous mass is why all other objects orbit the Sun -- its gravitational attraction dominates everything.

QUESTION 4

The force that keeps the Earth in orbit around the Sun is:

A. Magnetic force

B. Gravitational attraction between the Earth and the Sun

C. Nuclear force

D. Electrostatic force

The gravitational attraction between the Earth and the Sun provides the centripetal force that keeps the Earth in orbit. This force acts towards the Sun, constantly pulling the Earth towards it and preventing it from flying off in a straight line.

QUESTION 5

Light from the Sun takes approximately 8 minutes to reach Earth. How long would it take to reach a planet 3 times further from the Sun than Earth?

A. 8 minutes

B. 16 minutes

C. 24 minutes

D. 64 minutes

Time = distance / speed. If the distance is 3 times greater and the speed of light is constant, the time is also 3 times greater: 3 x 8 = 24 minutes.

QUESTION 6

Pluto is now classified as:

A. The ninth planet

B. A dwarf planet (minor planet)

C. An asteroid

D. A comet

In 2006, the International Astronomical Union reclassified Pluto as a dwarf planet (minor planet). It does not meet the criteria for a full planet because it has not "cleared the neighbourhood" around its orbit.

QUESTION 7

As you move further from the surface of a planet, the gravitational field strength:

A. Increases

B. Decreases

C. Stays the same

D. First increases then decreases

Gravitational field strength decreases with distance from the planet's centre. The further you are from the planet, the weaker the gravitational pull. This is why astronauts in orbit experience microgravity.

QUESTION 8

In the accretion model, the formation of planets involved:

A. The Sun ejecting material that cooled into planets

B. Small particles in a rotating disc colliding and sticking together to form larger bodies

C. Planets being captured from other solar systems

D. Asteroids combining in a single explosion

In the accretion model, an interstellar cloud collapsed under gravity and formed a rotating disc. Within this disc, small particles collided and stuck together (accreted), gradually forming planetesimals, then protoplanets, then full planets.

QUESTION 9

A comet in a highly elliptical orbit moves fastest when it is:

A. Closest to the Sun (perihelion)

B. Furthest from the Sun (aphelion)

C. Halfway between perihelion and aphelion

D. Its speed is constant throughout the orbit

By conservation of energy, as a comet falls towards the Sun it converts gravitational potential energy into kinetic energy, speeding up. It moves fastest at perihelion (closest to the Sun) where it has maximum kinetic energy.

QUESTION 10

Which planet has the highest orbital speed?

A. Mercury

B. Jupiter

C. Neptune

D. Earth

Mercury is closest to the Sun, so it experiences the strongest gravitational pull and has the highest orbital speed (about 48 km/s). Orbital speed decreases with distance from the Sun.

QUESTION 11

The asteroid belt is located between the orbits of:

A. Earth and Venus

B. Mars and Jupiter

C. Jupiter and Saturn

D. Saturn and Uranus

The main asteroid belt lies between the orbits of Mars and Jupiter. Jupiter's strong gravity prevented the material in this region from accreting into a full planet.

QUESTION 12

The orbits of planets around the Sun are:

A. Perfectly circular

B. Elliptical (oval-shaped)

C. Square

D. Random curves

All planetary orbits are ellipses with the Sun at one focus. For most planets, the ellipse is very close to a circle, but it is still technically elliptical. Comets have much more elongated (eccentric) elliptical orbits.

QUESTION 13

A planet is 3.0 x 10¹² m from the Sun. How long does light take to reach it?

A. 100 s

B. 1000 s

C. 10 000 s

D. 100 000 s

Time = distance / speed = (3.0 x 10¹²) / (3.0 x 10⁸) = 10 000 s (about 2.8 hours).

QUESTION 14

Which of the following is NOT a component of the Solar System?

A. Asteroids

B. Comets

C. Other stars

D. Dwarf planets

The Solar System contains only one star -- the Sun. Other stars are outside our Solar System. The nearest star to the Sun is Proxima Centauri, about 4.2 light-years away.

QUESTION 15

Jupiter has a much higher surface gravitational field strength (23.1 N/kg) than Earth (9.8 N/kg) mainly because:

A. Jupiter has much more mass than Earth

B. Jupiter is closer to the Sun

C. Jupiter rotates faster

D. Jupiter has more moons

Gravitational field strength at a planet's surface depends primarily on the planet's mass. Jupiter is about 318 times more massive than Earth, giving it a much stronger gravitational pull at its surface.

QUESTION 16

A comet at aphelion (furthest from the Sun) has:

A. Maximum kinetic energy and minimum potential energy

B. Minimum kinetic energy and maximum gravitational potential energy

C. Maximum kinetic energy and maximum potential energy

D. Zero kinetic energy and zero potential energy

At aphelion (furthest from the Sun), the comet is at its highest point, so it has maximum gravitational potential energy. By conservation of energy, it has minimum kinetic energy (lowest speed). At perihelion, the opposite is true.

QUESTION 17

In an elliptical orbit, the Sun is located at:

A. The exact centre of the ellipse

B. One focus of the ellipse

C. The edge of the ellipse

D. Outside the ellipse

In an elliptical orbit, the Sun sits at one focus of the ellipse. An ellipse has two foci, and the Sun is at one of them. When the orbit is nearly circular, the Sun is approximately at the centre.

QUESTION 18

Which of the following correctly describes comets?

A. Rocky bodies that orbit between Mars and Jupiter

B. Small stars that orbit the Sun

C. Icy bodies with elongated orbits that develop tails near the Sun

D. Moons that have escaped from their planet

Comets are icy bodies (sometimes called "dirty snowballs") with highly elliptical orbits. When they approach the Sun, the ice vaporises and forms a glowing tail pointing away from the Sun.

QUESTION 19

From the planetary data table, which pattern is correct?

A. Density always increases with distance from the Sun

B. Temperature always decreases with distance from the Sun

C. Orbital speed decreases with distance from the Sun

D. Orbital period decreases with distance from the Sun

Orbital speed consistently decreases with distance from the Sun: Mercury (48 km/s) to Neptune (5.4 km/s). Temperature does NOT always decrease -- Venus is hotter than Mercury due to its greenhouse effect. Orbital period increases with distance.

QUESTION 20

The accretion disc formed during the Solar System's formation was the result of:

A. An explosion from the Sun

B. Planets colliding with each other

C. A collapsing, rotating cloud of gas and dust flattening under gravity

D. The Moon breaking apart

The Solar System formed from an interstellar cloud that collapsed under gravity. As it collapsed, conservation of angular momentum caused it to rotate faster and flatten into a disc shape -- the accretion disc from which the planets formed.

6.2.1 The Sun as a Star ▼

The Sun is the star at the centre of our Solar System, but what exactly is a star? In this section, we will look at the Sun's composition, the radiation it emits, and the nuclear reactions that power it. The Sun is not special among stars -- it is a medium-size, middle-aged star. But to us, it is everything.

The Sun: Our Star

What Kind of Star Is the Sun?

The Sun is a medium-size star (also called a main sequence star or yellow dwarf). It is not particularly large or small compared to other stars. It is:

Made up of mostly hydrogen (about 73%) and helium (about 25%), with small amounts of heavier elements.
It radiates energy across a range of the electromagnetic spectrum, but most of its energy is in the infrared, visible light, and ultraviolet regions.
Its surface temperature is about 5500 degrees C, and its core temperature is about 15 million degrees C.

Exam Tip

The syllabus specifically says the Sun radiates most energy in infrared, visible light, and ultraviolet. Do not forget infrared -- many students only mention visible light. The Sun is NOT just a visible-light source.

Supplement

Nuclear Fusion: What Powers the Sun?

Stars are powered by nuclear reactions -- specifically, the fusion of hydrogen into helium. In the Sun's core:

The temperature is about 15 million degrees C and the pressure is enormous.
Under these extreme conditions, hydrogen nuclei (protons) are forced close enough together to fuse (join together).
Four hydrogen nuclei fuse to form one helium nucleus.
The helium nucleus has slightly less mass than the four hydrogen nuclei combined.
This "missing" mass is converted into energy according to Einstein's equation E = mc².
This energy is what makes the Sun shine.

In a stable star (like the Sun in its current phase), the inward pull of gravity is balanced by the outward pressure from the high-temperature core. This balance keeps the star at a constant size.

Memory Trick

FUSION = FUSING small nuclei together. Think of "fusing" as welding or joining. Hydrogen nuclei are fused into helium. This is the opposite of fission (splitting), which you studied in Topic 5. Remember: Fusion = joining (like "fuse together"), Fission = splitting (like "fissure" = a crack).

🌎 Apply It: Real-World Physics

See how our understanding of the Sun connects to real science and exploration.

The Herschel Museum of Astronomy in Bath, Somerset, is located in the house where William Herschel discovered infrared radiation in 1800. He placed a thermometer just beyond the red end of a spectrum produced by a prism and found the temperature rose -- proving there was invisible radiation beyond visible red light.

How does Herschel's discovery relate to the Sun's radiation? What are the three main regions of the electromagnetic spectrum in which the Sun radiates most of its energy?

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Identify the Physics

This is about the electromagnetic radiation emitted by the Sun -- specifically infrared, visible, and ultraviolet.

Work It Out

Herschel discovered that the Sun emits radiation beyond what our eyes can see. The Sun radiates most of its energy in three main regions: infrared (which Herschel discovered -- we feel it as heat), visible light (which we can see -- the colours of the rainbow from red to violet), and ultraviolet (which we cannot see but which causes sunburn). The peak of the Sun's emission is actually in the visible light region, which is why our eyes evolved to be sensitive to these wavelengths. But significant energy is also emitted in infrared (about 49%) and ultraviolet (about 7%).

Connect to the Syllabus

This directly covers syllabus point 6.2.1 Core (1): the Sun radiates most of its energy in infrared, visible light, and ultraviolet.

Aha! Moment

William Herschel, a German-born musician who settled in Bath, became one of Britain's greatest astronomers. His discovery of infrared radiation in 1800 was the first time anyone showed that there was light beyond what the human eye could see. He also discovered the planet Uranus in 1781 from his garden in Bath! The Herschel Museum is well worth a visit -- you can see the very garden where Uranus was spotted.

At the Culham Centre for Fusion Energy in Oxfordshire, scientists are working on the JET (Joint European Torus) tokamak, which aims to replicate the nuclear fusion process that powers the Sun. In December 2021, JET set a world record by producing 59 megajoules of fusion energy in a 5-second burst.

What nuclear reaction powers the Sun? Why is it so difficult to achieve fusion on Earth, and why are scientists trying?

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Identify the Physics

This is about nuclear fusion -- the process that powers stars.

Work It Out

The Sun is powered by the fusion of hydrogen nuclei into helium. In the Sun's core, temperatures reach 15 million degrees C and pressures are immense. Under these conditions, hydrogen nuclei overcome their electrostatic repulsion and fuse together. The resulting helium nucleus has less mass than the original hydrogen nuclei, and this mass difference is released as energy (E = mc²).

On Earth, achieving fusion is extremely difficult because we must recreate the Sun's extreme conditions -- temperatures of over 100 million degrees C (even hotter than the Sun's core, because we cannot replicate the Sun's enormous pressure). The plasma must be contained using powerful magnetic fields (since no physical container can withstand such temperatures).

Scientists are pursuing fusion because it could provide virtually limitless, clean energy. The fuel (hydrogen isotopes) is abundant, and fusion produces no greenhouse gases and very little radioactive waste compared to fission.

Connect to the Syllabus

This covers Supplement point 6.2.1 (2): stars are powered by nuclear reactions -- fusion of hydrogen into helium in stable stars.

Aha! Moment

The JET tokamak in Culham, Oxfordshire, held the world fusion energy record for over 25 years. It is being succeeded by ITER, a massive international project in France. The Sun fuses about 600 million tonnes of hydrogen into helium every second, but even at that rate, it has enough hydrogen fuel to last another 5 billion years. If we can master fusion on Earth, it would be one of the greatest achievements in human history.

ESA's Solar Orbiter mission, which includes instruments built by UK teams at UCL's Mullard Space Science Laboratory, is studying the Sun from closer than any European spacecraft before. It will get as close as 42 million km from the Sun's surface.

The Sun is described as a "medium-size star." What does this mean? Is the Sun special compared to other stars?

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Identify the Physics

This is about the Sun's classification as a medium-size star.

Work It Out

"Medium-size" means the Sun is neither one of the largest stars nor one of the smallest. Stars range enormously in size:
- Red dwarfs are much smaller and cooler than the Sun (about 0.1 to 0.5 solar masses).
- Red supergiants like Betelgeuse are hundreds of times larger than the Sun.
- Blue giants can be 10-50 times the Sun's mass.

The Sun sits comfortably in the middle. It is a yellow dwarf star (spectral type G2V), about 4.6 billion years old, and roughly halfway through its life. It is not special in terms of stellar classification -- it is an ordinary, average star. However, it is special to us because it is the only star close enough to sustain life on Earth.

Connect to the Syllabus

This covers Core point 6.2.1 (1): the Sun is a medium-size star consisting mostly of hydrogen and helium.

Aha! Moment

ESA's Solar Orbiter, launched in 2020, carries 10 instruments to study the Sun. UK teams at UCL's Mullard Space Science Laboratory and Imperial College London built key instruments. The spacecraft has a heat shield that can withstand temperatures up to 500 degrees C while keeping the instruments behind it at room temperature. It will take the closest-ever images of the Sun's poles -- regions never seen clearly before.

At the Royal Astronomical Society in London, researchers study how the Sun's ultraviolet radiation affects Earth's upper atmosphere. The ozone layer absorbs most UV radiation, but increased solar activity can temporarily deplete ozone, allowing more UV to reach the surface.

The Sun emits infrared, visible, and ultraviolet radiation. Which of these can we feel, which can we see, and which is dangerous? Why does the Sun emit radiation across these ranges?

▼

Identify the Physics

This is about the three main radiation types from the Sun and their properties.

Work It Out

Infrared: We cannot see it, but we feel it as heat (warmth on your skin on a sunny day). It has a longer wavelength than visible light.

Visible light: This is the only part we can see -- the full spectrum from red to violet. The Sun's surface temperature of about 5500 degrees C means its peak emission is in the visible range.

Ultraviolet (UV): We cannot see or feel it directly, but it causes sunburn, skin damage, and can lead to skin cancer. It has a shorter wavelength and higher energy than visible light.

The Sun emits across these ranges because it is a hot object. All hot objects emit electromagnetic radiation, and the hotter they are, the shorter the peak wavelength. At 5500 degrees C, the Sun's peak is in the visible range, but it also emits significant amounts of neighbouring infrared and ultraviolet radiation.

Connect to the Syllabus

This covers Core point 6.2.1 (1): the Sun radiates most of its energy in infrared, visible light, and ultraviolet.

Aha! Moment

The Royal Astronomical Society, founded in London in 1820, is one of the world's oldest learned societies for astronomy. Its fellows include some of the most famous astronomers in history. The Sun converts about 4 million tonnes of matter into energy every second through fusion. Despite this enormous rate, the Sun has lost only about 0.03% of its mass over its 4.6-billion-year lifetime -- it is that massive!

During Tim Peake's mission aboard the ISS, he photographed spectacular auroras from space. Auroras are caused by charged particles from the Sun (the solar wind) interacting with Earth's magnetic field and atmosphere. The Sun continuously emits these particles alongside its electromagnetic radiation.

How is the Sun's energy production related to the solar wind and auroras? What keeps the Sun stable so it does not collapse or explode?

▼

Identify the Physics

This is about nuclear fusion powering the Sun and the balance between gravity and pressure in a stable star.

Work It Out

The Sun's energy comes from nuclear fusion in its core -- hydrogen nuclei fuse into helium, releasing enormous energy. This energy heats the Sun's outer layers and some charged particles are ejected as the solar wind. When these charged particles reach Earth, they interact with Earth's magnetic field and are funnelled towards the poles, where they collide with atmospheric gases and cause them to glow -- creating the Northern Lights (aurora borealis) and Southern Lights (aurora australis).

The Sun remains stable because of a balance: gravity pulls the Sun's material inward (trying to make it collapse), while the extreme temperature and pressure from fusion in the core push outward. These two forces balance exactly, keeping the Sun at a constant size. This balance is called hydrostatic equilibrium. If fusion stopped, gravity would win and the star would collapse.

Connect to the Syllabus

This covers Supplement point 6.2.1 (2): stars are powered by nuclear fusion, and Supplement point 6.2.2 (3c): a stable star exists when gravitational inward force is balanced by outward force from high temperature.

Aha! Moment

Tim Peake described seeing auroras from the ISS as one of the most breathtaking experiences of his life. The auroras appear as shimmering curtains of green and purple light, viewed from above. The Sun's solar wind travels at about 400-800 km/s and takes 2-4 days to reach Earth. When particularly strong solar storms occur, the auroras can be visible from as far south as southern England -- a rare treat for UK observers!

Practice Questions: Section 6.2.1

Test your understanding of the Sun, its composition, and nuclear fusion.

Score 0 / 20

QUESTION 1

The Sun is best described as:

A. The largest star in the Milky Way

B. A medium-size star

C. The smallest type of star

D. A planet that produces light

The Sun is a medium-size star (yellow dwarf, spectral type G2V). Many stars are much larger (red supergiants) and many are smaller (red dwarfs). It is an ordinary, average star.

QUESTION 2

The Sun is mostly made up of:

A. Hydrogen and helium

B. Iron and nickel

C. Oxygen and nitrogen

D. Carbon and silicon

The Sun is approximately 73% hydrogen and 25% helium, with about 2% heavier elements. Hydrogen is the fuel for nuclear fusion, and helium is the product.

QUESTION 3

The Sun radiates most of its energy in which regions of the electromagnetic spectrum?

A. Radio waves, microwaves, and X-rays

B. Infrared, visible light, and ultraviolet

C. Only visible light

D. Gamma rays and X-rays

The Sun radiates most of its energy in three regions: infrared (felt as heat), visible light (seen by our eyes), and ultraviolet (causes sunburn). Its peak emission is in the visible range due to its surface temperature of about 5500 degrees C.

QUESTION 4

In the Sun's core, energy is produced by:

A. Burning hydrogen gas like a fire

B. Nuclear fission of uranium

C. Nuclear fusion of hydrogen into helium

D. Chemical reactions between gases

The Sun is powered by nuclear fusion, not combustion (burning). In the core, hydrogen nuclei fuse to form helium, releasing enormous energy. This is a nuclear reaction, not a chemical one.

QUESTION 5

A stable star maintains its size because:

A. It is spinning too fast to collapse

B. The inward gravitational force is balanced by outward pressure from high temperature

C. It has reached its maximum mass

D. Magnetic fields hold it together

A stable star is in hydrostatic equilibrium: the inward gravitational force (trying to compress it) is exactly balanced by the outward force from the high temperature and pressure of the core (produced by fusion). This balance keeps the star at a constant size.

QUESTION 6

Nuclear fusion in the Sun converts:

A. Hydrogen into helium

B. Helium into hydrogen

C. Uranium into barium

D. Carbon into oxygen

In the Sun, four hydrogen nuclei (protons) fuse together to form one helium-4 nucleus. The mass difference is released as energy. This is the primary energy source for all main-sequence stars.

QUESTION 7

Which type of radiation from the Sun causes sunburn?

A. Infrared

B. Visible light

C. Ultraviolet

D. Radio waves

Ultraviolet (UV) radiation from the Sun causes sunburn, skin damage, and can lead to skin cancer. It has higher energy than visible light. The ozone layer absorbs much of it, but some still reaches the ground.

QUESTION 8

Why can nuclear fusion only occur in the Sun's core?

A. Only the core contains hydrogen

B. Only the core has high enough temperature and pressure for fusion

C. The core contains uranium

D. The core is made of a different element

Nuclear fusion requires extremely high temperatures (about 15 million degrees C) and enormous pressures to force hydrogen nuclei close enough together to overcome their electrostatic repulsion. Only the Sun's core has these extreme conditions.

QUESTION 9

The energy released in fusion comes from:

A. Breaking chemical bonds

B. A small amount of mass being converted to energy (E = mc²)

C. Electrons being removed from atoms

D. Gravitational potential energy only

When hydrogen nuclei fuse into helium, the helium nucleus has slightly less mass than the original hydrogen nuclei. This "missing" mass is converted into energy according to Einstein's famous equation E = mc². Even a tiny amount of mass produces an enormous amount of energy because c² is so large.

QUESTION 10

If the Sun's core suddenly stopped producing energy by fusion, what would happen?

A. The outward pressure would drop and gravity would cause the Sun to contract

B. Nothing would change

C. The Sun would immediately explode

D. The Sun would start producing energy by fission instead

The Sun maintains its size through a balance between inward gravity and outward pressure from fusion. If fusion stopped, the outward pressure would decrease, and gravity would dominate, causing the Sun to contract. This is what starts to happen when a star runs out of hydrogen fuel.

QUESTION 11

Which statement about the Sun is correct?

A. The Sun produces energy by nuclear fission

B. The Sun is the largest star in the universe

C. The Sun is about 73% hydrogen and 25% helium

D. The Sun only emits visible light

The Sun is approximately 73% hydrogen and 25% helium. It uses fusion (not fission), is a medium-size star (not the largest), and emits infrared, visible, and UV radiation (not just visible light).

QUESTION 12

The difference between nuclear fusion and nuclear fission is:

A. Fusion joins small nuclei; fission splits large nuclei

B. Fusion splits large nuclei; fission joins small nuclei

C. Fusion uses uranium; fission uses hydrogen

D. There is no difference

Fusion = fusing (joining) small nuclei (e.g., hydrogen) into larger ones (e.g., helium). Fission = splitting large nuclei (e.g., uranium-235) into smaller ones. Both release energy, but through opposite processes.

QUESTION 13

The infrared radiation from the Sun is detected by our bodies as:

A. Heat (warmth)

B. Sound

C. Colour

D. Magnetic fields

We cannot see infrared radiation, but we feel it as warmth or heat. When you stand in sunlight and feel warm, much of that warming effect comes from the Sun's infrared radiation being absorbed by your skin.

QUESTION 14

The temperature at the Sun's core is approximately:

A. 5500 degrees C

B. 100 000 degrees C

C. 15 million degrees C

D. 1 billion degrees C

The Sun's core temperature is about 15 million degrees C -- hot enough and pressured enough for nuclear fusion to occur. The surface temperature is about 5500 degrees C, which is much cooler than the core.

QUESTION 15

In nuclear fusion, when four hydrogen nuclei fuse to form helium, the helium nucleus has:

A. More mass than the four hydrogen nuclei

B. Less mass than the four hydrogen nuclei

C. Exactly the same mass as the four hydrogen nuclei

D. No mass at all

The helium nucleus has slightly less mass than the four hydrogen nuclei that formed it. This "mass defect" is converted to energy (E = mc²). This is where all the Sun's energy comes from.

QUESTION 16

Which of the following is NOT true about the Sun?

A. It is powered by nuclear fusion

B. It is mostly hydrogen and helium

C. It is the only star in the Milky Way

D. It radiates infrared, visible, and ultraviolet radiation

The Sun is NOT the only star in the Milky Way -- the Milky Way contains many billions of stars. The Sun is just one ordinary star among hundreds of billions in our galaxy.

QUESTION 17

The term "nuclear reaction" in the context of the Sun refers to:

A. Atoms gaining or losing electrons

B. Chemical reactions between hydrogen and oxygen

C. Changes in atomic nuclei where hydrogen nuclei combine to form helium

D. Radioactive decay of unstable elements

Nuclear reactions involve changes to the nucleus of atoms. In the Sun, hydrogen nuclei (protons) are combined (fused) to form helium nuclei. This is different from chemical reactions (which involve electrons) and radioactive decay (which happens spontaneously).

QUESTION 18

The Sun's surface temperature of about 5500 degrees C is responsible for:

A. Making the Sun appear blue

B. The Sun's peak emission being in the visible light range

C. Making the Sun emit only radio waves

D. Causing nuclear fusion at the surface

The Sun's surface temperature determines the peak wavelength of its emission. At about 5500 degrees C, the peak falls in the visible light range, which is why the Sun appears yellow-white. Hotter stars appear blue; cooler stars appear red.

QUESTION 19

The "outward force" that prevents a stable star from collapsing is caused by:

A. The star's rotation

B. Magnetic repulsion

C. The high temperature and pressure from nuclear fusion in the core

D. Radiation pressure from nearby stars

The outward force comes from the extreme temperature and pressure generated by nuclear fusion in the core. This radiation and gas pressure pushes outward against the inward pull of gravity, maintaining the star's size.

QUESTION 20

Approximately how old is the Sun?

A. About 1 million years old

B. About 100 million years old

C. About 4.6 billion years old

D. About 13.8 billion years old

The Sun is about 4.6 billion years old and is roughly halfway through its life. It has enough hydrogen fuel to continue for another 5 billion years or so. 13.8 billion years is the estimated age of the Universe, not the Sun.

6.2.2 Stars ▼

Look up at the night sky from a dark site like the Scottish Dark Sky Observatory in Galloway Forest, and you will see thousands of stars. Each one is a distant sun, powered by nuclear fusion. In this section, we explore what galaxies are, how far away other stars are, and the fascinating life cycle of stars -- from birth in a cloud of gas to dramatic deaths as white dwarfs, neutron stars, or black holes.

Galaxies and the Milky Way

What Is a Galaxy?

A galaxy is a collection of many billions of stars held together by gravity. Galaxies also contain gas, dust, and dark matter.

Key facts:

The Sun is a star in the Milky Way galaxy.
Other stars in the Milky Way are much further from Earth than the Sun is. The Sun is about 8 light-minutes away, while the nearest other star (Proxima Centauri) is about 4.2 light-years away -- that is about 300 000 times further!
Astronomical distances are so vast that we measure them in light-years.
A light-year is the distance that light travels in one year in the vacuum of space.

Supplement

The Light-Year in Metres

One light-year = 9.5 x 10¹⁵ m

We can check this: light travels at 3.0 x 10⁸ m/s. In one year (365.25 x 24 x 3600 = 3.156 x 10⁷ s), it travels 3.0 x 10⁸ x 3.156 x 10⁷ = 9.47 x 10¹⁵ m, which rounds to 9.5 x 10¹⁵ m.

Memory Trick

A light-year is a DISTANCE, not a time! This is a very common confusion. A light-year measures how far light travels in one year. It is about 9.5 million million metres (9.5 x 10¹⁵ m). Think of it like saying "the shop is a 10-minute walk away" -- "10 minutes" is being used as a distance, not a time.

The Life Cycle of Stars

Supplement

Stage 1: Birth -- From Gas Cloud to Protostar

A star begins its life in an interstellar cloud (nebula) of gas and dust, mostly hydrogen.

Part of the cloud begins to collapse under its own gravitational attraction.
As the cloud collapses, the material compresses and its temperature increases.
This collapsing, heating cloud is called a protostar.

Stage 2: Stable Star (Main Sequence)

The protostar continues to heat up until the core reaches about 15 million degrees C. At this temperature, nuclear fusion begins -- hydrogen nuclei start fusing into helium.

The protostar becomes a stable star when the inward gravitational force is balanced by the outward force from the high temperature (radiation and gas pressure).
The star stays in this stable phase for most of its life, steadily fusing hydrogen into helium.
The Sun has been in this stage for about 4.6 billion years and will remain so for another 5 billion years.

Stage 3: Running Out of Fuel

Eventually, all stars run out of hydrogen fuel in their cores. What happens next depends on the star's mass.

Stage 4a: Less Massive Stars (like the Sun)

When the hydrogen in the core is used up, the core contracts and heats up.
The outer layers of the star expand and cool, turning red. The star becomes a red giant.
The red giant eventually sheds its outer layers, creating a planetary nebula (a glowing shell of gas).
The remaining core is left behind as a white dwarf -- a small, dense, hot remnant that gradually cools over billions of years.

Stage 4b: Very Massive Stars

Very massive stars (much more massive than the Sun) expand even more to become red supergiants.
A red supergiant eventually explodes in a catastrophic explosion called a supernova.
The supernova explosion creates a nebula containing heavier elements (elements heavier than iron are only formed in supernovae).
The remaining core collapses to form either a neutron star (if the core is not too massive) or a black hole (if the core is extremely massive).
The nebula from a supernova can eventually form new stars with planets -- recycling the heavier elements into new solar systems.

Exam Tip

You MUST know the two different pathways: Less massive star: protostar → stable star → red giant → planetary nebula + white dwarf. Very massive star: protostar → stable star → red supergiant → supernova → nebula + (neutron star or black hole). Be precise about which path each type follows.

Memory Trick

Small Stars Die Quietly, Big Stars Die Loudly.
Small/medium stars (like the Sun): gently puff off their outer layers as a planetary nebula and leave a quiet white dwarf.
Massive stars: go out with a bang -- a supernova explosion -- leaving a neutron star or black hole.
Think: Quiet white dwarf vs. Loud supernova.

Why Do Supernovae Matter?

The nebula produced by a supernova contains heavier elements that were created during the supernova explosion. This material can form new stars and planets. This means:

Every element heavier than hydrogen and helium in your body (carbon, oxygen, iron, calcium) was made inside a star or during a supernova.
The Solar System formed from a nebula that contained material from previous supernovae -- we are literally made of stardust!

🌎 Apply It: Real-World Physics

Connect stellar physics to real observatories and discoveries.

At the Scottish Dark Sky Observatory in Galloway Forest Park, one of the darkest places in Britain, visitors can observe the Milky Way stretching across the sky on clear nights. Through a telescope, they can see the Orion Nebula (M42), a stellar nursery where new stars are being born about 1344 light-years from Earth.

What is happening inside the Orion Nebula? Describe the stages from gas cloud to stable star. How far away is it in metres?

▼

Identify the Physics

This is about the birth of stars -- from interstellar cloud to protostar to stable star.

Work It Out

Inside the Orion Nebula, regions of gas and dust are collapsing under their own gravitational attraction. As the gas compresses, it heats up, forming protostars. When a protostar's core reaches about 15 million degrees C, nuclear fusion begins -- hydrogen nuclei fuse into helium. At this point, the outward force from the high temperature balances the inward gravitational force, and the protostar becomes a stable star.

Distance in metres: 1344 light-years x 9.5 x 10¹⁵ m per light-year = 1.28 x 10¹⁹ m. That is about 12.8 million million million metres!

Connect to the Syllabus

This covers Supplement 6.2.2 (3a, 3b, 3c): star formation from gas clouds, protostars increasing in temperature, and the balance of forces in a stable star.

Aha! Moment

The Scottish Dark Sky Observatory was built in 2012 and sits within the Galloway Forest Dark Sky Park -- the first in the UK. The Orion Nebula is visible to the naked eye as a fuzzy patch below Orion's Belt, and through a telescope you can see its glowing gases and the young stars embedded within it. Because the nebula is 1344 light-years away, we are seeing it as it was 1344 years ago -- the light we see tonight left the nebula during the early medieval period!

Betelgeuse, one of the brightest stars visible from Britain, is a red supergiant in the constellation Orion. It is about 700 light-years away and is roughly 20 times more massive than the Sun. Astronomers believe it could explode as a supernova at any time in the next 100 000 years.

What will happen when Betelgeuse explodes? Describe the complete end-of-life sequence for a very massive star. What will be left behind?

▼

Identify the Physics

This is about the life cycle of a very massive star -- the red supergiant to supernova pathway.

Work It Out

Betelgeuse is already a red supergiant -- it has used up the hydrogen in its core and expanded enormously. When its nuclear fuel is exhausted, the core will collapse catastrophically and the outer layers will be thrown off in a supernova explosion.

The supernova will:
1. Produce a nebula containing heavier elements (elements forged during the explosion itself).
2. Leave behind either a neutron star (an incredibly dense object made almost entirely of neutrons) or a black hole (if the remaining core is massive enough that its gravity is so strong not even light can escape).

For Betelgeuse, with about 20 solar masses, the remnant is most likely to be a neutron star, though a black hole is possible. The supernova nebula will eventually mix with other interstellar material and could form new stars and planets containing the heavy elements from the explosion.

Connect to the Syllabus

This covers Supplement 6.2.2 (3e, 3g, 3h): massive stars become red supergiants, then undergo supernova, leaving a nebula with heavier elements plus a neutron star or black hole. The nebula may form new stars with planets.

Aha! Moment

If Betelgeuse explodes as a supernova, it would be visible from Earth even in daylight for several weeks and could be as bright as the Full Moon at night. The last supernova visible to the naked eye from Britain was recorded by John of Worcester in 1181. At 700 light-years away, a Betelgeuse supernova would pose no danger to Earth -- you would just get a spectacular light show.

Astronomers at the Jodrell Bank Observatory have studied pulsars -- rapidly rotating neutron stars that emit beams of radio waves. The first pulsar was discovered by Jocelyn Bell Burnell and Antony Hewish at Cambridge in 1967. Neutron stars are so dense that a teaspoon of neutron star material would weigh about a billion tonnes.

How do neutron stars form? What type of star must it have originally been? Why are they so incredibly dense?

▼

Identify the Physics

This is about the formation of neutron stars from the supernova deaths of massive stars.

Work It Out

A neutron star forms when a very massive star (much more massive than the Sun) reaches the end of its life. The sequence is: the star uses up its hydrogen fuel → becomes a red supergiant → undergoes a supernova explosion. During the supernova, the core collapses so violently that protons and electrons are forced together to form neutrons. The remaining core is a neutron star.

Neutron stars are incredibly dense because the entire core (about 1.4 to 2.1 solar masses) is compressed into a sphere only about 10 km across. All the matter is packed into neutrons with essentially no empty space between them. A teaspoon of this material would indeed weigh about a billion tonnes -- roughly the weight of a mountain squeezed into a volume smaller than a sugar cube.

Connect to the Syllabus

This covers Supplement 6.2.2 (3g): a red supergiant undergoes supernova, leaving behind a neutron star (or black hole if even more massive).

Aha! Moment

Dame Jocelyn Bell Burnell, from Northern Ireland, was a postgraduate student at Cambridge when she discovered pulsars in 1967. Initially, the regular radio pulses were jokingly nicknamed "LGM-1" (Little Green Men), as researchers briefly wondered if they could be signals from an alien civilisation! Antony Hewish received the Nobel Prize for the discovery, though many argue Bell Burnell should have shared it. Jodrell Bank has since discovered over 800 pulsars and continues to be one of the world's leading radio observatories.

The Ring Nebula (M57) in the constellation Lyra is visible through telescopes at the Royal Observatory Greenwich. It is a planetary nebula -- the glowing shell of gas shed by a dying star about 2300 light-years away. At its centre is a white dwarf, the remnant core of the original star.

Describe the full life cycle that led to the formation of the Ring Nebula and its central white dwarf. What kind of star was the original?

▼

Identify the Physics

This is about the life cycle of a less massive star -- the red giant to planetary nebula and white dwarf pathway.

Work It Out

The full life cycle of the star that created the Ring Nebula:

1. The star began as an interstellar cloud of gas and dust (mostly hydrogen).
2. The cloud collapsed under gravity, heating up to form a protostar.
3. When the core reached about 15 million degrees C, nuclear fusion began. The outward pressure balanced gravity, and it became a stable star.
4. After billions of years, the hydrogen fuel in the core was used up. The star was not massive enough to follow the supergiant/supernova pathway.
5. The star expanded to become a red giant (when hydrogen ran out and helium burning began).
6. The red giant shed its outer layers, creating the planetary nebula (the glowing ring we see today).
7. The remaining dense, hot core was left as a white dwarf at the centre, gradually cooling over billions of years.

Connect to the Syllabus

This covers the full Supplement 6.2.2 life cycle for a less massive star: (3a) gas cloud → (3b) protostar → (3c) stable star → (3d) runs out of fuel → (3e) red giant → (3f) planetary nebula + white dwarf.

Aha! Moment

The term "planetary nebula" is misleading -- it has nothing to do with planets! William Herschel coined the term in the 1780s because these nebulae looked round and planet-like through his telescope. The Ring Nebula is expanding at about 20-30 km/s and has been doing so for about 4000 years. The central white dwarf has a surface temperature of about 120 000 degrees C -- over 20 times hotter than the Sun's surface -- but it is only about the size of the Earth.

A physics teacher at a school near Manchester asks her students: "Every atom of iron in your blood was made inside a dying star. Every atom of calcium in your bones came from a supernova." She challenges them to explain this using the stellar life cycle.

Why are heavy elements like iron, calcium, and carbon only found because of stellar processes? Where were they made?

▼

Identify the Physics

This is about supernova nucleosynthesis -- how heavier elements are created in stars and distributed by supernovae.

Work It Out

After the Big Bang, the Universe contained almost entirely hydrogen and helium -- no heavier elements. So where did carbon, oxygen, iron, calcium, and all the other elements come from?

1. Inside stable stars, hydrogen is fused into helium. In later stages, helium can fuse into carbon, and carbon into heavier elements up to iron.
2. Elements heavier than iron cannot be made by fusion in a star's core (it would require energy input rather than releasing energy). These elements are only created during the extreme conditions of a supernova explosion.
3. When a massive star explodes as a supernova, the nebula produced contains all these heavier elements.
4. This supernova debris mixes with other interstellar gas and can collapse to form new stars and planets (Supplement point 3h).
5. The Solar System formed from such enriched material about 4.6 billion years ago. So the iron in your blood, the calcium in your bones, and the carbon in your DNA all originated inside stars that lived and died before the Sun was born.

Connect to the Syllabus

This covers Supplement 6.2.2 (3g): supernova creates a nebula with heavier elements, and (3h): supernova nebula may form new stars with planets.

Aha! Moment

We are literally made of stardust. The calcium in your bones was forged in a star that died billions of years ago. The iron carrying oxygen through your bloodstream was created in a supernova. As the astronomer Carl Sagan said, we are "star stuff." The Jodrell Bank Observatory near Manchester continues to study supernovae remnants, helping us understand exactly how these elements were distributed across space to eventually form planets -- and people.

Practice Questions: Section 6.2.2

Test your understanding of galaxies, light-years, and stellar life cycles.

Score 0 / 20

QUESTION 1

A galaxy contains:

A. A few hundred stars

B. A few thousand stars

C. A few million stars

D. Many billions of stars

Galaxies are enormous collections of many billions of stars. The Milky Way alone contains an estimated 100-400 billion stars. Even "small" dwarf galaxies contain hundreds of millions of stars.

QUESTION 2

A light-year is:

A. A unit of time

B. A unit of distance

C. A unit of speed

D. A unit of energy

A light-year is a unit of distance. It is the distance light travels in one year in a vacuum. Despite having "year" in the name, it measures distance, not time. One light-year = 9.5 x 10¹⁵ m.

QUESTION 3

The nearest star to the Sun (Proxima Centauri) is about 4.2 light-years away. The Sun is about 8 light-minutes from Earth. This means Proxima Centauri is:

A. About twice as far as the Sun

B. About 500 times further than the Sun

C. About 300 000 times further than the Sun

D. About the same distance as the Sun

4.2 light-years = 4.2 x 365 x 24 x 60 light-minutes = about 2.2 million light-minutes. The Sun is 8 light-minutes away. So Proxima Centauri is about 2.2 million / 8 = 275 000, which is approximately 300 000 times further than the Sun.

QUESTION 4

A protostar forms when:

A. A white dwarf explodes

B. A cloud of gas and dust collapses under gravity and increases in temperature

C. A planet grows large enough to ignite fusion

D. Two stars collide

A protostar is a collapsing cloud of gas and dust that is increasing in temperature due to gravitational attraction. It has not yet started nuclear fusion. Once it gets hot and dense enough, fusion begins and it becomes a stable star.

QUESTION 5

A protostar becomes a stable star when:

A. It starts to cool down

B. It stops rotating

C. The inward gravitational force is balanced by the outward force from high temperature

D. It reaches maximum size

A stable star exists when the inward gravitational force is exactly balanced by the outward radiation and gas pressure from the high temperature of the core (produced by nuclear fusion). This balance is called hydrostatic equilibrium.

QUESTION 6

The correct life cycle for a star like the Sun is:

A. Gas cloud → protostar → stable star → red giant → planetary nebula + white dwarf

B. Gas cloud → protostar → stable star → red supergiant → supernova → black hole

C. Gas cloud → protostar → white dwarf → red giant → supernova

D. Supernova → protostar → stable star → red giant → white dwarf

For a star like the Sun (less massive), the sequence is: gas cloud → protostar → stable star → red giant → planetary nebula + white dwarf. The supergiant/supernova pathway only applies to very massive stars.

QUESTION 7

A supernova is:

A. A type of small, dim star

B. An enormous explosion at the end of a very massive star's life

C. The birth of a new star

D. A large planet

A supernova is a catastrophic explosion that occurs when a very massive star (red supergiant) runs out of fuel. The outer layers are blown off into space, creating a nebula with heavier elements, while the core collapses to form a neutron star or black hole.

QUESTION 8

One light-year equals approximately:

A. 3.0 x 10⁸ m

B. 3.0 x 10¹¹ m

C. 9.5 x 10¹⁵ m

D. 9.5 x 10²⁰ m

One light-year = 9.5 x 10¹⁵ m. This is calculated as: speed of light x seconds in one year = 3.0 x 10⁸ x 3.156 x 10⁷ ≈ 9.5 x 10¹⁵ m. You need to recall this value.

QUESTION 9

After a supernova, the remaining core may form:

A. A red giant or a white dwarf

B. A neutron star or a black hole

C. A new planet or asteroid

D. A protostar or comet

After a supernova, the core that remains can form either a neutron star (if it is below about 3 solar masses) or a black hole (if it is above about 3 solar masses). White dwarfs come from less massive stars that do not undergo supernovae.

QUESTION 10

Why do elements heavier than iron only form in supernovae?

A. Iron is too heavy to exist in normal stars

B. Fusion of elements heavier than iron requires energy input, which only a supernova provides

C. Iron absorbs all heavier elements

D. The Sun destroys all elements heavier than iron

Fusing elements up to iron releases energy, but fusing elements heavier than iron requires energy input. Only the extreme energy of a supernova explosion provides enough energy to create these heavier elements (gold, silver, uranium, etc.).

QUESTION 11

The Sun is in which galaxy?

A. Andromeda

B. The Milky Way

C. The Solar System Galaxy

D. The Orion Galaxy

The Sun is one star among hundreds of billions in the Milky Way galaxy. Andromeda is the nearest large galaxy to the Milky Way, about 2.5 million light-years away.

QUESTION 12

Why is a planetary nebula important in the stellar life cycle?

A. It forms new planets immediately

B. It is the outer layer shed by a dying red giant, leaving behind a white dwarf

C. It is the explosion of a massive star

D. It is where protostars form

A planetary nebula forms when a less massive red giant sheds its outer layers. The ejected gas glows and forms a beautiful shell, while the remaining core cools as a white dwarf. Planetary nebulae are NOT related to planets, and they are NOT supernova explosions.

QUESTION 13

A star 10 light-years away is seen as it was:

A. 10 years ago

B. Right now

C. 10 years in the future

D. 100 years ago

If a star is 10 light-years away, its light takes 10 years to reach us. So we see the star as it was 10 years ago, not as it is right now. The further away an object is, the further back in time we are looking.

QUESTION 14

Which of the following correctly describes a white dwarf?

A. A large, cool star

B. A small, dense, hot remnant of a dead star

C. A star that is about to explode

D. A protostar that failed to ignite fusion

A white dwarf is the remaining core of a less massive star after it has shed its outer layers as a planetary nebula. It is small (roughly Earth-sized), very dense, and initially very hot, but it gradually cools over billions of years.

QUESTION 15

The nebula produced by a supernova is important because:

A. It always forms a black hole

B. It contains heavier elements and may form new stars with planets

C. It prevents other stars from forming

D. It absorbs all light in the area

The supernova nebula contains heavier elements created during the explosion. This enriched material can collapse to form new stars and planets. Our Solar System formed from such material -- that is why Earth contains elements like iron, gold, and carbon.

QUESTION 16

Red giant stars form when:

A. A protostar gets very hot

B. A star's hydrogen is converted to helium and the outer layers expand

C. A white dwarf absorbs more gas

D. Two stars merge

When most of the hydrogen in a star's core is converted to helium, the core contracts and heats up, while the outer layers expand and cool. The expanded, cooler outer layers give the star a red colour -- hence "red giant." Very massive stars become red supergiants instead.

QUESTION 17

A black hole forms when:

A. A star cools down completely

B. Two neutron stars collide

C. The core remaining after a supernova is so massive that its gravity prevents anything, even light, from escaping

D. A galaxy runs out of stars

A black hole forms when the core remaining after a supernova explosion is extremely massive. Its gravitational pull is so strong that nothing -- not even light -- can escape from it. This is why it appears "black."

QUESTION 18

What determines whether a dying star becomes a white dwarf, a neutron star, or a black hole?

A. The mass of the star

B. The colour of the star

C. The star's distance from the centre of the galaxy

D. The number of planets orbiting the star

The mass of the original star determines its fate. Less massive stars (like the Sun) end as white dwarfs. Very massive stars undergo supernovae and leave behind neutron stars or (for the most massive) black holes.

QUESTION 19

All stars eventually:

A. Become black holes

B. Explode as supernovae

C. Run out of hydrogen fuel

D. Turn into planets

All stars eventually run out of hydrogen fuel in their cores. What happens next depends on their mass -- they may become red giants or red supergiants, but the common starting point is that they all exhaust their hydrogen supply.

QUESTION 20

The Sun is much closer to Earth than other stars in the Milky Way. The Sun is about 8 light-minutes away. How far is this in metres approximately?

A. 1.4 x 10¹¹ m

B. 1.4 x 10⁸ m

C. 9.5 x 10¹⁵ m

D. 3.0 x 10⁸ m

8 light-minutes = 8 x 60 = 480 seconds of light travel. Distance = speed x time = 3.0 x 10⁸ x 480 = 1.44 x 10¹¹ m ≈ 1.4 x 10¹¹ m (which is about 150 million km).

6.2.3 The Universe ▼

We have explored the Earth, the Solar System, and the life cycle of stars. Now let us zoom out to the biggest scale possible -- the entire Universe. How big is it? Is it expanding? How do we know? And what started it all? This final section covers the Milky Way, redshift, the expanding Universe, cosmic microwave background radiation, and the Hubble constant.

The Milky Way and Beyond

How Big Is the Milky Way?

The Milky Way galaxy:

Is one of many billions of galaxies in the observable Universe.
Has a diameter of approximately 100 000 light-years.
Contains hundreds of billions of stars.

Think about that: even travelling at the speed of light, it would take 100 000 years to cross our galaxy. And the Milky Way is just one galaxy among billions.

Redshift

What Is Redshift?

Redshift is the increase in the observed wavelength of electromagnetic radiation emitted by a source that is moving away from the observer.

When a star or galaxy moves away from us:

The light waves it emits are stretched out (their wavelength increases).
Since red light has the longest wavelength in the visible spectrum, the light is shifted towards the red end -- hence "redshift."
The faster the galaxy is moving away, the greater the redshift.

What Does Redshift Tell Us?

Observations show that light from distant galaxies is redshifted compared to the same light observed on Earth. This is crucial evidence because:

Redshift means the galaxies are moving away from us.
The more distant the galaxy, the greater the redshift -- meaning more distant galaxies are moving away faster.
This means the Universe is expanding.
This expansion supports the Big Bang Theory -- the idea that the Universe began from an extremely hot, dense point and has been expanding ever since.

Exam Tip

The redshift question is a favourite in exams. Remember the chain of logic: (1) Light from distant galaxies is redshifted → (2) This means galaxies are moving away from us → (3) More distant galaxies are moving faster → (4) Therefore the Universe is expanding → (5) This supports the Big Bang Theory. Learn this chain!

Memory Trick

"RED = RECEDING." Redshift means the light is shifted to longer (red) wavelengths because the galaxy is receding (moving away). If the galaxy were moving towards us, we would see blueshift (wavelength gets shorter/bluer).

Supplement

Cosmic Microwave Background Radiation (CMBR)

Cosmic microwave background radiation (CMBR) is microwave radiation of a specific frequency that is observed at all points in space. No matter which direction you point a radio telescope, you detect this faint microwave signal.

Key facts about CMBR:

It was produced shortly after the Universe formed (about 380 000 years after the Big Bang).
Originally, this radiation was very high-energy (short wavelength). As the Universe expanded, the radiation was stretched to longer wavelengths.
It has now been stretched into the microwave region of the electromagnetic spectrum.
CMBR is additional evidence supporting the Big Bang Theory -- it is the "afterglow" of the Big Bang.

Exam Tip

There are TWO pieces of evidence for the Big Bang Theory: (1) Redshift of distant galaxies (showing the Universe is expanding), and (2) CMBR (the remnant radiation from the early Universe). Make sure you can explain both.

The Hubble Constant

The speed at which a galaxy is moving away from us can be found from the change in wavelength due to redshift. The distance to a far galaxy can be determined by measuring the brightness of a supernova in that galaxy (since we know how bright supernovae actually are, comparing this to how bright they appear tells us the distance).

The relationship between the speed of a receding galaxy and its distance is given by the Hubble constant:

H₀ = v / d

H₀ = Hubble constant (per second, s^-1) v = speed of galaxy moving away (m/s) d = distance to the galaxy (m)

The current estimate for the Hubble constant is:

H₀ ≈ 2.2 x 10^-18 per second (s^-1)

The Age of the Universe

From the Hubble equation, we can rearrange to get:

d / v = 1 / H₀

This gives an estimate for the age of the Universe 1 / H₀ represents the time since all matter was at a single point (the Big Bang)

This is because if we know how fast galaxies are moving apart (v) and how far apart they are (d), we can calculate how long ago they were all at the same point. The equation d/v = 1/H₀ gives the estimated age of the Universe. This is evidence that all matter was once at a single point.

Worked Example A galaxy is moving away from Earth at a speed of 4.4 x 10⁶ m/s. Use the Hubble constant (H₀ = 2.2 x 10^-18 s^-1) to calculate the distance to this galaxy.

Step 1: Write the equation

H₀ = v / d, so d = v / H₀

Step 2: Substitute values

d = (4.4 x 10⁶) / (2.2 x 10^-18)

Step 3: Calculate

d = 2.0 x 10²⁴ m

Step 4: Convert to light-years

d = (2.0 x 10²⁴) / (9.5 x 10¹⁵) = 2.1 x 10⁸ light-years = about 210 million light-years

d = 2.0 x 10²⁴ m (about 210 million light-years)

Worked Example Calculate the estimated age of the Universe using H₀ = 2.2 x 10^-18 s^-1. Give your answer in years.

Step 1: Write the equation

Age of Universe ≈ 1 / H₀

Step 2: Calculate in seconds

1 / H₀ = 1 / (2.2 x 10^-18) = 4.55 x 10¹⁷ s

Step 3: Convert to years

4.55 x 10¹⁷ / (3.156 x 10⁷) = 1.44 x 10¹⁰ years = about 14.4 billion years

Age of Universe ≈ 1.4 x 10¹⁰ years (about 14 billion years)

Worked Example A galaxy is 5.0 x 10²² m from Earth. Use the Hubble constant to calculate the speed at which it is moving away.

Step 1: Write the equation

H₀ = v / d, so v = H₀ x d

Step 2: Substitute values

v = 2.2 x 10^-18 x 5.0 x 10²²

Step 3: Calculate

v = 1.1 x 10⁵ m/s = 110 000 m/s = 110 km/s

v = 1.1 x 10⁵ m/s (110 km/s)

🌎 Apply It: Real-World Physics

Connect your understanding of the expanding Universe to real discoveries and observations.

In 1929, the American astronomer Edwin Hubble used observations made at Mount Wilson Observatory to show that distant galaxies are all moving away from us, and that further galaxies are moving away faster. British astronomers at the Royal Astronomical Society in London were among the first to recognise the profound implications of this discovery.

What did Hubble's observations reveal about the Universe? How does redshift provide the evidence for this?

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Identify the Physics

This is about redshift as evidence for the expanding Universe and the Big Bang Theory.

Work It Out

Hubble measured the light from distant galaxies and found it was redshifted -- the wavelength of the light was longer than expected. This redshift showed that the galaxies were moving away from us. Crucially, Hubble found that more distant galaxies had greater redshift, meaning they were moving away faster.

This pattern -- further = faster -- meant the entire Universe is expanding. If we "rewind" this expansion, everything would have been at the same point in the past. This leads to the Big Bang Theory: the Universe began from an extremely hot, dense point and has been expanding ever since.

Hubble expressed this relationship mathematically: v = H₀ x d, where the speed of recession (v) is proportional to distance (d). The constant of proportionality (H₀) is the Hubble constant.

Connect to the Syllabus

This covers Core points: redshift of distant galaxies is evidence for the expanding Universe and supports the Big Bang Theory.

Aha! Moment

Although Hubble gets the credit, Belgian priest and physicist Georges Lemaitre had proposed the expanding Universe theory two years earlier in 1927. British astronomer Arthur Eddington championed Lemaitre's work at the Royal Astronomical Society. The key insight was transformative: the Universe is not static and eternal, as most scientists had assumed. It had a beginning, and it is still growing. Today, astronomers know the expansion is actually accelerating -- driven by mysterious "dark energy" -- a discovery that won the 2011 Nobel Prize.

In 1964, Arno Penzias and Robert Wilson accidentally discovered a faint microwave signal coming from every direction in space while working at Bell Labs. They initially thought it was caused by pigeon droppings on their antenna! British cosmologists quickly recognised this as the cosmic microwave background radiation (CMBR) predicted by Big Bang theory.

What is CMBR? Why is it observed as microwaves rather than visible light? Why is it found everywhere in space?

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Identify the Physics

This is about cosmic microwave background radiation (CMBR) as evidence for the Big Bang.

Work It Out

CMBR is microwave radiation of a specific frequency observed at all points in space. It is the remnant radiation from the early Universe.

Why microwaves, not visible light? When the radiation was first produced (about 380 000 years after the Big Bang), it was very high energy with short wavelengths (visible light and beyond). However, as the Universe has expanded over 13.8 billion years, the radiation has been stretched by the expansion of space itself, increasing its wavelength. It has now been stretched so much that it falls in the microwave region of the electromagnetic spectrum.

Why is it everywhere? The Big Bang was not an explosion at a single point in existing space. Rather, it was the expansion of space itself. The CMBR was produced everywhere in the early Universe, and since it came from all points, it is observed in all directions. There is no special centre to look towards -- the radiation fills all of space uniformly.

Connect to the Syllabus

This covers Supplement points 6.2.3 (5, 6): CMBR is microwave radiation observed at all points in space, produced shortly after the Universe formed, and expanded into the microwave region as the Universe expanded.

Aha! Moment

The CMBR has a temperature of about 2.7 Kelvin (-270.45 degrees C), making it the coldest natural temperature we can observe. Penzias and Wilson won the Nobel Prize in 1978 for their accidental discovery. ESA's Planck satellite (2009-2013) mapped the CMBR in extraordinary detail, revealing tiny temperature variations that correspond to the "seeds" from which galaxies later formed. The Planck mission had significant UK involvement from the University of Cambridge and Cardiff University.

Astronomers at the University of Cambridge's Institute of Astronomy are working to refine the Hubble constant. Different measurement methods give slightly different values, creating what scientists call the "Hubble tension." Using the current estimate of H₀ = 2.2 x 10^-18 s^-1, they calculate the age of the Universe.

Calculate the estimated age of the Universe in seconds and in years. What does this calculation assume about the expansion?

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Identify the Physics

This is about calculating the age of the Universe using 1/H₀.

Work It Out

Age of Universe ≈ 1/H₀ = 1/(2.2 x 10^-18) = 4.55 x 10¹⁷ seconds.

Converting to years: 4.55 x 10¹⁷ / (3.156 x 10⁷) = 1.44 x 10¹⁰ years ≈ 14.4 billion years.

This is close to the accepted estimate of about 13.8 billion years. The small discrepancy arises because the simple calculation assumes the expansion rate has been constant throughout the Universe's history. In reality, the expansion rate has changed -- it slowed down initially due to gravity, and is now speeding up due to dark energy. More sophisticated calculations that account for these changes give 13.8 billion years.

Connect to the Syllabus

This covers Supplement points 6.2.3 (9, 10, 11): H₀ = v/d with H₀ = 2.2 x 10^-18 s^-1, and 1/H₀ gives an estimate for the age of the Universe and is evidence that all matter was at a single point.

Aha! Moment

The "Hubble tension" is one of the biggest unsolved puzzles in modern physics. When astronomers measure H₀ using nearby supernovae, they get a slightly higher value than when they calculate it from the CMBR. This discrepancy might mean our understanding of the Universe is incomplete -- perhaps there is new physics we have not yet discovered. Cambridge astronomers are at the forefront of trying to resolve this puzzle using data from the James Webb Space Telescope.

A student watching BBC Sky at Night hears the presenter explain that a galaxy 3.0 x 10²⁴ m away is moving away from us at 6.6 x 10⁶ m/s. The student wants to verify this using the Hubble constant.

Verify the recession speed using H₀ = 2.2 x 10^-18 s^-1. Then convert the galaxy's distance to light-years. What would this distance tell you about looking into the past?

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Identify the Physics

This uses the Hubble equation and the concept of looking back in time with light.

Work It Out

Verification: v = H₀ x d = 2.2 x 10^-18 x 3.0 x 10²⁴ = 6.6 x 10⁶ m/s. This matches the stated speed.

Convert to light-years: d = 3.0 x 10²⁴ / 9.5 x 10¹⁵ = 3.16 x 10⁸ light-years ≈ about 316 million light-years.

Looking into the past: Because the light from this galaxy took 316 million years to reach us, we are seeing the galaxy as it was 316 million years ago. On Earth, 316 million years ago was the Carboniferous period, when the first reptiles were evolving. We are looking so far back in time that we see this galaxy as it existed before dinosaurs even appeared on Earth!

Connect to the Syllabus

This covers Supplement: using H₀ = v/d to calculate speed and distance, and the concept that looking at distant objects is looking back in time.

Aha! Moment

Every telescope is a time machine. The further we look into space, the further back in time we see. The James Webb Space Telescope has captured light from galaxies that formed just a few hundred million years after the Big Bang -- we are literally seeing the Universe as a baby. BBC Sky at Night regularly features these discoveries, continuing Sir Patrick Moore's tradition of making cutting-edge astronomy accessible to everyone.

ESA's Planck satellite mapped the CMBR from 2009 to 2013. Scientists at the University of Cambridge analysed the data and found tiny temperature fluctuations in the CMBR -- some spots were slightly warmer (by a few millionths of a degree) and others slightly cooler. These fluctuations are the "seeds" from which galaxies formed.

Explain how the CMBR supports the Big Bang Theory. Why are the tiny temperature variations important for understanding galaxy formation?

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Identify the Physics

This is about CMBR as evidence for the Big Bang and its connection to galaxy formation.

Work It Out

The CMBR supports the Big Bang Theory in several ways:

1. The Big Bang theory predicts that the early Universe was extremely hot and dense, filled with radiation. As the Universe expanded and cooled, this radiation should still be detectable today as a faint microwave signal. The CMBR is exactly this predicted radiation.

2. The CMBR is observed in all directions in space -- this is consistent with the Big Bang happening everywhere, not at a specific point.

3. The CMBR has the exact spectrum (pattern of frequencies) predicted for radiation from a hot body that has cooled to about 2.7 K through the expansion of the Universe.

Temperature fluctuations: The tiny variations (about 1 part in 100 000) represent regions that were slightly denser than average in the early Universe. These denser regions had slightly stronger gravity, so they attracted more matter over time. Over billions of years, these small density differences grew into the galaxies and galaxy clusters we see today. Without these tiny "seeds," the Universe would be a uniform, featureless cloud of gas with no stars or galaxies.

Connect to the Syllabus

This covers Supplement points 6.2.3 (5, 6): CMBR is microwave radiation observed everywhere, produced shortly after the Big Bang, and stretched to microwave wavelengths by the Universe's expansion.

Aha! Moment

The Planck satellite was named after Max Planck, the founder of quantum theory. The Cambridge team's analysis of Planck data confirmed the age of the Universe to unprecedented precision: 13.799 billion years, plus or minus 21 million years. The CMBR map is essentially a "baby photo" of the Universe, taken when it was only 380 000 years old. Everything we see today -- every galaxy, every star, every planet, every person -- grew from the tiny fluctuations captured in that image.

Practice Questions: Section 6.2.3

Test your understanding of the Universe, redshift, CMBR, and the Hubble constant.

Score 0 / 20

QUESTION 1

The Milky Way galaxy has a diameter of approximately:

A. 100 light-years

B. 1000 light-years

C. 100 000 light-years

D. 100 million light-years

The Milky Way has a diameter of approximately 100 000 light-years. It is a barred spiral galaxy containing hundreds of billions of stars, and our Sun is located about 26 000 light-years from the centre.

QUESTION 2

Redshift is:

A. A decrease in the wavelength of light from an approaching source

B. An increase in the observed wavelength of light from a source moving away

C. Red light emitted by all stars

D. The colour change when light enters the atmosphere

Redshift is the increase in observed wavelength of electromagnetic radiation from a source that is moving away (receding) from the observer. The light is stretched to longer wavelengths, shifting towards the red end of the spectrum.

QUESTION 3

Light from distant galaxies is redshifted. This tells us that:

A. Distant galaxies are moving towards us

B. Distant galaxies are moving away from us and the Universe is expanding

C. The galaxies are getting hotter

D. Light travels faster through space than expected

Redshift from distant galaxies means they are moving away from us. The further the galaxy, the greater the redshift. This shows the Universe is expanding, which supports the Big Bang Theory.

QUESTION 4

The two key pieces of evidence for the Big Bang Theory are:

A. The existence of the Sun and Moon

B. Redshift of distant galaxies and cosmic microwave background radiation

C. The existence of black holes and neutron stars

D. The discovery of Pluto and the asteroid belt

The two main pieces of evidence for the Big Bang are: (1) Redshift of distant galaxies (showing the Universe is expanding), and (2) Cosmic microwave background radiation (CMBR) -- the remnant radiation from the early Universe.

QUESTION 5

CMBR is observed as:

A. Visible light from all directions

B. Microwave radiation from all directions in space

C. X-rays from the centre of the Milky Way

D. Radio waves from the Sun only

CMBR is microwave radiation of a specific frequency observed at all points in space. It comes from every direction, not just from a single source. It is the stretched, cooled remnant of the radiation produced shortly after the Big Bang.

QUESTION 6

The CMBR was originally produced as high-energy radiation. It is now observed as microwaves because:

A. Microwaves are the fastest type of radiation

B. The expansion of the Universe has stretched its wavelength into the microwave region

C. It has been absorbed and re-emitted by stars

D. The Earth's atmosphere converts it to microwaves

The expansion of the Universe has stretched the CMBR radiation. Originally it was short-wavelength, high-energy radiation. Over 13.8 billion years of expansion, its wavelength has been stretched so much that it now falls in the microwave region of the electromagnetic spectrum.

QUESTION 7

Using H₀ = 2.2 x 10^-18 s^-1, the estimated age of the Universe (1/H₀) is approximately:

A. 4.5 x 10⁹ years

B. 1.4 x 10¹⁰ years

C. 2.2 x 10¹⁸ years

D. 9.5 x 10¹⁵ years

1/H₀ = 1/(2.2 x 10^-18) = 4.55 x 10¹⁷ s. Converting to years: 4.55 x 10¹⁷ / 3.156 x 10⁷ ≈ 1.4 x 10¹⁰ years ≈ 14 billion years.

QUESTION 8

A galaxy is observed to be moving away at 2.2 x 10⁶ m/s. Its distance from Earth is:

A. 1.0 x 10²⁴ m

B. 4.84 x 10^-12 m

C. 2.2 x 10⁶ m

D. 1.0 x 10¹² m

d = v / H₀ = (2.2 x 10⁶) / (2.2 x 10^-18) = 1.0 x 10²⁴ m. This is about 105 million light-years away.

QUESTION 9

The Milky Way is:

A. The only galaxy in the Universe

B. One of many billions of galaxies

C. A cluster of planets

D. A type of nebula

The Milky Way is one of many billions of galaxies in the observable Universe. Current estimates suggest there are at least 200 billion galaxies, each containing billions of stars.

QUESTION 10

The Hubble constant H₀ = v/d tells us that galaxies that are further away:

A. Are moving away faster

B. Are moving towards us

C. Are stationary

D. Are moving at the same speed as nearby galaxies

From v = H₀ x d, if d is larger, then v is larger (since H₀ is constant). This means more distant galaxies are receding faster. This is the key observation that shows the Universe is expanding uniformly.

QUESTION 11

The current accepted value of the Hubble constant is approximately:

A. 3.0 x 10⁸ s^-1

B. 2.2 x 10^-18 s^-1

C. 9.5 x 10¹⁵ s^-1

D. 6.67 x 10^-11 s^-1

The current estimate for the Hubble constant is H₀ = 2.2 x 10^-18 per second (s^-1). This is a value you need to recall for the exam.

QUESTION 12

The distance to a far galaxy can be determined by:

A. Measuring its colour

B. Counting its stars

C. Using the brightness of a supernova in that galaxy

D. Measuring how fast it rotates

The distance to a far galaxy can be determined by observing a supernova in that galaxy. Since we know the true (intrinsic) brightness of certain types of supernovae, we can compare this with how bright they appear to calculate the distance. Dimmer appearance means further away.

QUESTION 13

d/v = 1/H₀ represents:

A. The speed of light

B. An estimate for the age of the Universe

C. The size of the Milky Way

D. The distance to the nearest star

d/v = 1/H₀ gives an estimate for the age of the Universe. It represents the time since all matter was at a single point (the Big Bang). Using H₀ = 2.2 x 10^-18 s^-1, this gives about 14 billion years.

QUESTION 14

A galaxy is 8.0 x 10²⁴ m from Earth. Using H₀ = 2.2 x 10^-18 s^-1, its recession speed is:

A. 3.6 x 10⁴² m/s

B. 1.76 x 10⁷ m/s

C. 2.75 x 10^-43 m/s

D. 8.0 x 10⁶ m/s

v = H₀ x d = 2.2 x 10^-18 x 8.0 x 10²⁴ = 17.6 x 10⁶ = 1.76 x 10⁷ m/s (about 17 600 km/s).

QUESTION 15

Redshift provides evidence that the Universe is:

A. Contracting

B. Expanding

C. Staying the same size

D. Rotating

Redshift of distant galaxies shows they are all moving away from us, and further galaxies are moving away faster. This means the Universe is expanding. If it were contracting, we would see blueshift instead.

QUESTION 16

If a galaxy has a greater redshift than another galaxy, it means:

A. It is closer to us

B. It is further away and moving faster

C. It is hotter

D. It is older

Greater redshift means a larger increase in wavelength, which means the galaxy is receding faster. From the Hubble equation (v = H₀ x d), a higher speed means a greater distance. So galaxies with more redshift are further away and moving faster.

QUESTION 17

The Big Bang Theory states that:

A. The Universe has always existed in its current form

B. The Universe began from an extremely hot, dense point and has been expanding ever since

C. Galaxies are moving towards each other

D. The Universe is shrinking

The Big Bang Theory proposes that the Universe began from an extremely hot, dense state about 13.8 billion years ago and has been expanding and cooling ever since. The evidence includes redshift of distant galaxies and the CMBR.

QUESTION 18

The speed v of a galaxy moving away from us can be found from:

A. The change in wavelength of its light due to redshift

B. The colour of its brightest star

C. The number of stars it contains

D. The temperature of its centre

The speed of a receding galaxy is determined from the wavelength change due to redshift. By comparing the observed wavelength of specific spectral lines with their known wavelength on Earth, astronomers can calculate how fast the galaxy is moving away.

QUESTION 19

The fact that 1/H₀ gives an estimate for the age of the Universe is evidence that:

A. The Universe is getting younger

B. All matter was once at a single point

C. The Universe has no age

D. Stars have always existed

If we know how fast galaxies are moving apart (v) and how far apart they are (d), we can calculate when they were all at the same point: time = d/v = 1/H₀. This is evidence that all matter was once concentrated at a single point -- the starting point of the Big Bang.

QUESTION 20

A galaxy has a recession speed of 1.1 x 10⁷ m/s. Using H₀ = 2.2 x 10^-18 s^-1, what is its distance from Earth?

A. 2.42 x 10^-11 m

B. 5.0 x 10²⁰ m

C. 5.0 x 10²⁴ m

D. 5.0 x 10³⁰ m

d = v / H₀ = (1.1 x 10⁷) / (2.2 x 10^-18) = 0.5 x 10²⁵ = 5.0 x 10²⁴ m. That is about 527 million light-years away.