IGCSE Physics Stars Questions and Worked Solutions (6.2.2) | 0625

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IGCSE Physics 0625 · Topic 6.2.2

IGCSE Physics Stars Questions and Worked Solutions

50 topic-wise practice questions on Stars (Unit 6: Space Physics), written to the Cambridge IGCSE Physics 0625 syllabus for 2026–2028, each with a step-by-step worked solution.

50 questions9 Core31 Extended10 Challenge126 marksTextbook §6.2 · pp. 286–294
Questions tagged Core are for all candidates (Papers 1 & 3). Questions tagged Extended are Supplement content (Papers 2 & 4). Questions 41–50 are harder Challenge questions. Try each question first, then tap Show solution to check your working.

Topic guide: Stars

Galaxies contain billions of stars. Distances in space are so large they are measured in light-years. (Extended) Stars form from clouds of gas and dust and follow a life cycle that depends on their mass.

Key formulas

1 light-year = 9.5 × 10¹⁵ mExtendedExtended
d = c × tdistance light travels (t in seconds)Core

Prerequisite knowledge

  • Speed of light (3.3)
  • Standard form
  • Fusion (6.2.1)

Exam tips & common mistakes

  • A light-year is a DISTANCE
  • Learn both life-cycle paths in order
  • Stable star: forces balanced

Key facts from the syllabus (6.2.2)

  • Galaxies are made of many billions of stars; the Sun is in the Milky Way
  • Other stars in the Milky Way are much further away than the Sun
  • A light-year is the distance travelled by light in space in one year
  • Extended interstellar cloud → protostar (collapsing, heating up) → stable star (gravity balanced by outward force from high temperature)
  • Extended when hydrogen runs out, most stars become red giants; massive stars become red supergiants
  • Extended red giant → planetary nebula + white dwarf
  • Extended red supergiant → supernova → nebula (with heavier elements) + neutron star or black hole; nebula may form new stars and planets

Textbook: §6.2 Stars and the Universe, pp. 286–294

1
Core[1 mark]

State what a galaxy is made up of.

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Method: Billions of stars.

Many billions of stars
2
Core[1 mark]

Name the galaxy containing the Sun.

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Method: Milky Way.

The Milky Way
3
Core[2 marks]

Compare the distance to other stars in the Milky Way with the Earth–Sun distance.

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Method: Much further.

Other stars are much further from the Earth than the Sun is
4
Core[1 mark]

Define a light-year.

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Method: Distance.

The distance travelled by light in (the vacuum of) space in one year
5
Core[2 marks]

Explain why astronomers use light-years instead of metres.

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Method: Huge distances.

Astronomical distances are so huge that metres give unmanageable numbers
6
Core[2 marks]

State the approximate number of stars in a galaxy.

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Method: Billions.

Many billions
7
Core[2 marks]

State what a light-year measures.

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Method: Distance.

Distance
8
Core[2 marks]

Explain why a light-year is not a unit of time.

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Method: Distance light travels.

It is the distance light travels in one year
9
Core[2 marks]

Compare the distance from Earth to the Sun with the distance to the nearest other star.

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Method: Much further.

The nearest star is hundreds of thousands of times further away
10
Extended[1 mark]

State the value of one light-year in metres.

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Method: Syllabus.

9.5 × 10¹⁵ m
11
Extended[3 marks]

A star is 4.2 light-years away. Calculate its distance in metres.

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Method: d = 4.2 × 9.5×10¹⁵.

d = 4.0 × 10¹⁶ m
12
Extended[3 marks]

A star is 1.9 × 10¹⁷ m away. Calculate its distance in light-years.

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Method: Divide by 9.5×10¹⁵.

d = 20.0 light-years
13
Extended[3 marks]

Calculate the distance light travels in one year, using c = 3.0 × 10⁸ m/s and 1 year = 3.15 × 10⁷ s.

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Method: d = ct.

d = 3.0×10⁸ × 3.15×10⁷ = 9.4 × 10¹⁵ m
14
Extended[2 marks]

Describe how a star forms.

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Method: Gas and dust collapse.

Interstellar clouds of gas and dust containing hydrogen collapse under gravity
15
Extended[2 marks]

Describe what a protostar is.

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Method: Collapsing cloud heating up.

An interstellar cloud collapsing and increasing in temperature due to its own gravitational attraction
16
Extended[2 marks]

Explain when a protostar becomes a stable star.

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Method: Balance.

When the inward gravitational force is balanced by the outward force due to the high temperature in its centre
17
Extended[2 marks]

State what happens when most of the hydrogen in a star's core has been converted to helium.

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Method: Expands.

Most stars expand to form red giants; more massive stars form red supergiants
18
Extended[2 marks]

Describe what happens to a red giant from a less massive star.

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Method: Planetary nebula, white dwarf.

It forms a planetary nebula with a white dwarf at its centre
19
Extended[3 marks]

Describe what happens to a red supergiant.

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Method: Supernova.

It explodes as a supernova, forming a nebula containing hydrogen and new heavier elements, leaving a neutron star or black hole
20
Extended[2 marks]

State the two possible remnants at the centre of a supernova.

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Method: Neutron star, black hole.

A neutron star or a black hole
21
Extended[2 marks]

Explain why the nebula from a supernova is important for forming planets like Earth.

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Method: Heavy elements.

It contains heavier elements that can form new stars with orbiting rocky planets
22
Extended[3 marks]

Put these stages in order for a massive star: red supergiant, protostar, supernova, stable star, neutron star.

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Method: Life cycle.

Protostar → stable star → red supergiant → supernova → neutron star
23
Extended[3 marks]

Put these stages in order for a Sun-like star: white dwarf, red giant, stable star, protostar, planetary nebula.

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Method: Life cycle.

Protostar → stable star → red giant → planetary nebula → white dwarf
24
Extended[2 marks]

State what all stars eventually run out of.

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Method: Hydrogen fuel.

Hydrogen as fuel for nuclear fusion
25
Extended[3 marks]

A star is 650 light-years away. Calculate the distance in metres.

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Method: × 9.5×10¹⁵.

d = 6.2 × 10¹⁸ m
26
Extended[3 marks]

A nebula is 1.3 × 10¹⁹ m away. Calculate the distance in light-years.

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Method: ÷ 9.5×10¹⁵.

d = 1370 light-years
27
Extended[2 marks]

Light from a star 8.6 light-years away reaches us now. State when it left the star.

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Method: Years.

8.6 years ago
28
Extended[2 marks]

State the force that causes a cloud of gas and dust to collapse to form a protostar.

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Method: Gravity.

Gravitational attraction
29
Extended[2 marks]

State why a protostar heats up.

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Method: Collapse.

Gravitational potential energy is converted to thermal energy as it collapses
30
Extended[2 marks]

State the process that starts in the core when a protostar becomes a star.

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Method: Fusion.

Nuclear fusion of hydrogen
31
Extended[2 marks]

Describe what happens to a star's size when it becomes a red giant.

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Method: Expands.

It expands greatly and its surface cools (becomes red)
32
Extended[2 marks]

State the end stage of a star like the Sun.

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Method: White dwarf.

A white dwarf (surrounded by a planetary nebula)
33
Extended[2 marks]

State what decides whether a star becomes a red giant or a red supergiant.

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Method: Mass.

Its mass (more massive stars form red supergiants)
34
Extended[2 marks]

Describe a supernova.

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Method: Explosion.

A huge explosion of a red supergiant
35
Extended[2 marks]

State the remnant formed by the most massive stars after a supernova.

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Method: Black hole.

A black hole
36
Extended[2 marks]

Explain how elements heavier than iron came to be in the Earth.

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Method: Supernovae.

They were made in supernova explosions and spread into nebulae from which the Solar System formed
37
Extended[3 marks]

Describe the life cycle of a star much more massive than the Sun.

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Method: Sequence.

Nebula → protostar → stable star → red supergiant → supernova → neutron star or black hole
38
Extended[2 marks]

State what a planetary nebula is.

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Method: Outer layers.

The shell of gas thrown off by a red giant, with a white dwarf at its centre
39
Extended[2 marks]

Explain why a stable star does not change size for billions of years.

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Method: Balance.

Gravity inwards is balanced by the outward force from the high core temperature
40
Extended[3 marks]

Calculate the time for light to cross the Milky Way (diameter 100 000 light-years).

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Method: By definition.

100 000 years

Challenge questions

Harder, multi-step questions at the top end of the exam. Attempt these once you are confident with the questions above.

41
ChallengeCore[3 marks]

Explain why astronomers measure distances to stars in light-years rather than kilometres.

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Method: Huge numbers.

Distances in km are enormous and unwieldy; light-years give manageable numbers
42
ChallengeExtended[4 marks]

The star Betelgeuse is about 640 light-years away. Calculate its distance in metres and how long ago the light we see left it.

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Method: × 9.5×10¹⁵.

d = 6.1 × 10¹⁸ m; 640 years ago
43
ChallengeExtended[4 marks]

Show that one light-year is approximately 9.5 × 10¹⁵ m, using c = 3.0 × 10⁸ m/s and 1 year = 365 days.

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Method: d = ct.

d = 3.0×10⁸ × 365 × 24 × 3600 = 9.46 × 10¹⁵ m ≈ 9.5×10¹⁵ m
44
ChallengeExtended[5 marks]

Describe the life cycle of a star with a mass similar to the Sun, from nebula to final stage.

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Method: Sequence.

A cloud of gas and dust (hydrogen) collapses under gravity to form a protostar that heats up; fusion starts and it becomes a stable star when gravity balances outward pressure; when core hydrogen runs out it expands into a red giant; it then forms a planetary nebula with a white dwarf at the centre
45
ChallengeExtended[5 marks]

Describe the life cycle of a star much more massive than the Sun, including the possible end points.

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Method: Sequence.

Protostar → stable star → red supergiant when hydrogen runs out → supernova explosion → nebula of hydrogen and heavier elements, leaving a neutron star or a black hole
46
ChallengeExtended[4 marks]

Explain the forces acting on a stable star and why it stays the same size for billions of years.

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Method: Equilibrium.

Gravitational attraction acts inwards; the high temperature (thermal pressure) from fusion pushes outwards; while these balance, the star is stable
47
ChallengeExtended[4 marks]

Explain how the elements in your body that are heavier than helium were formed.

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Method: Stellar fusion and supernovae.

They were made by fusion in earlier stars and in supernova explosions, then spread into nebulae from which the Sun and Earth formed
48
ChallengeExtended[4 marks]

Explain why a protostar's temperature increases as it collapses.

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Method: Energy transfer.

Gravitational potential energy is converted to kinetic energy of particles as the cloud contracts, raising the temperature
49
ChallengeExtended[4 marks]

A star is 2.85 × 10¹⁷ m away. Calculate its distance in light-years and the time taken for its light to reach Earth.

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Method: ÷ 9.5×10¹⁵.

30.0 light-years; 30.0 years
50
ChallengeExtended[3 marks]

Explain why the nebula left by a supernova may form new stars with planets.

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Method: Gravity collapse.

The nebula contains hydrogen and heavier elements; regions can collapse under gravity to form new stars and orbiting planets
All IGCSE Physics topics← 6.2.1 The Sun as a Star6.2.3 The Universe →

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