IGCSE Physics Sun as a Star Questions and Worked Solutions (6.2.1) | 0625

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IGCSE Physics 0625 · Topic 6.2.1

IGCSE Physics Sun as a Star Questions and Worked Solutions

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

50 questions22 Core18 Extended10 Challenge114 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: The Sun as a Star

The Sun is a medium-sized star made mostly of hydrogen and helium. It radiates most of its energy as infrared, visible light and ultraviolet. (Extended) Stars are powered by nuclear fusion of hydrogen into helium.

Key formulas

No equations to learn in this subtopic — fusion: hydrogen → helium.

Prerequisite knowledge

  • EM spectrum (3.3)
  • Fusion (5.1.2, Extended)

Exam tips & common mistakes

  • Fusion, not fission, powers stars
  • Name all three EM regions
  • 'Medium-sized' is the expected description

Key facts from the syllabus (6.2.1)

  • The Sun is a medium-sized star
  • It consists mostly of hydrogen and helium
  • It radiates most energy in the infrared, visible and ultraviolet regions
  • Extended stars are powered by nuclear reactions that release energy
  • Extended in stable stars the reactions are fusion of hydrogen into helium

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

1
Core[1 mark]

State the type of object the Sun is.

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Method: Star.

A star of medium size
2
Core[2 marks]

State the two main elements in the Sun.

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Method: H and He.

Hydrogen and helium
3
Core[2 marks]

State the three regions of the electromagnetic spectrum in which the Sun radiates most energy.

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Method: IR, visible, UV.

Infrared, visible light and ultraviolet
4
Core[1 mark]

State the size of the Sun compared with other stars.

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Method: Medium.

Medium-sized
5
Core[2 marks]

Explain why the Sun appears much brighter than other stars.

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Method: Closer.

It is much closer to the Earth than any other star
6
Core[2 marks]

State which type of radiation from the Sun causes sunburn.

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Method: UV.

Ultraviolet
7
Core[2 marks]

State one type of radiation from the Sun that we feel as warmth.

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Method: Infrared.

Infrared
8
Core[1 mark]

State what proportion of the Solar System's mass is in the Sun.

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Method: Most.

Most of it (over 99%)
9
Core[3 marks]

Light takes about 500 s to travel from the Sun to the Earth. Calculate the distance (c = 3.0 × 10⁸ m/s).

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Method: d = ct.

d = 3.0×10⁸ × 500 = 1.5 × 10¹¹ m
10
Core[2 marks]

Explain why the Earth's atmosphere is important in protecting us from solar radiation.

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Method: Absorbs UV.

It absorbs much of the harmful ultraviolet (and other) radiation
11
Core[2 marks]

State one way energy from the Sun reaches the Earth.

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Method: Radiation.

By electromagnetic radiation (light and infrared) through space
12
Core[1 mark]

State whether the Sun is a small, medium or large star.

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Method: Medium.

Medium
13
Core[2 marks]

State the approximate composition of the Sun.

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Method: H and He.

Mostly hydrogen with helium
14
Core[2 marks]

Name the region of the electromagnetic spectrum in which the Sun emits most strongly that we can see.

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Method: Visible.

Visible light
15
Core[2 marks]

State why sunlight can damage the eyes.

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Method: Intense light, UV.

It is very intense and includes ultraviolet
16
Core[2 marks]

State the type of energy transfer by which the Sun warms the Earth.

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Method: Radiation.

Radiation (electromagnetic waves)
17
Core[2 marks]

State how the Sun's energy is used by plants.

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Method: Photosynthesis.

For photosynthesis
18
Core[2 marks]

State why we see the Sun as a disc and other stars as points of light.

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Method: Distance.

The Sun is much closer
19
Core[3 marks]

The Sun is 1.5 × 10¹¹ m away. Calculate how long ago the light we see left the Sun, in minutes.

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Method: t = d/c.

t = 500 s = 8.33 min
20
Core[2 marks]

Explain why ultraviolet from the Sun is harmful.

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Method: Damages cells.

It damages surface cells and eyes, leading to skin cancer
21
Core[2 marks]

State the Sun's position in the Solar System.

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Method: Centre.

At the centre, with planets orbiting it
22
Core[2 marks]

State one reason infrared from the Sun is important for life on Earth.

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Method: Warmth.

It warms the Earth's surface to temperatures suitable for life
23
Extended[1 mark]

State what powers stars.

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Method: Nuclear reactions.

Nuclear reactions (fusion) that release energy
24
Extended[2 marks]

Describe the nuclear reaction in stable stars.

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Method: H → He fusion.

Fusion of hydrogen nuclei into helium
25
Extended[2 marks]

Explain why fusion releases energy in the Sun.

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Method: Mass decrease.

The mass of the products is slightly less than the reactants; the lost mass is released as energy
26
Extended[2 marks]

Explain why fusion happens only in the Sun's core.

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Method: High T and pressure.

Only the core has the very high temperature and pressure needed for nuclei to collide fast enough to fuse
27
Extended[2 marks]

Explain why the Sun is a stable star.

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Method: Balance of forces.

Inward gravitational force is balanced by outward force from the high temperature (pressure) at its centre
28
Extended[2 marks]

State what will eventually happen to the hydrogen fuel in the Sun.

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Method: Runs out.

It will eventually run out
29
Extended[2 marks]

Distinguish between fusion in the Sun and fission in a nuclear power station.

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Method: Joining vs splitting.

Fusion joins light nuclei (hydrogen → helium); fission splits heavy nuclei (uranium)
30
Extended[2 marks]

State the element produced by hydrogen fusion in the Sun.

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Method: Helium.

Helium
31
Extended[2 marks]

Explain why the Sun does not collapse under its own gravity.

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Method: Outward pressure.

The high temperature in the core creates an outward force balancing gravity
32
Extended[2 marks]

State the particles that fuse in the core of a stable star.

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Method: Hydrogen nuclei.

Hydrogen nuclei
33
Extended[2 marks]

Explain why nuclear fusion in the Sun does not cause it to explode.

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Method: Gravity balances.

The inward gravitational force balances the outward force from the hot core
34
Extended[2 marks]

State the main product of fusion in the Sun.

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Method: Helium.

Helium
35
Extended[2 marks]

Explain why the Sun's core must be extremely hot for fusion.

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Method: Overcome repulsion.

Nuclei are positive and repel; they need very high speeds to collide and fuse
36
Extended[2 marks]

Explain why the Sun loses mass every second.

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Method: Mass → energy.

Fusion converts some mass into energy, which is radiated away
37
Extended[2 marks]

State what will happen to the Sun when its core hydrogen runs out.

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Method: Red giant.

It will expand to become a red giant
38
Extended[2 marks]

Compare the energy source of the Sun with that of a nuclear power station.

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Method: Fusion vs fission.

The Sun uses fusion; power stations use fission
39
Extended[2 marks]

State why the Sun is described as a stable star.

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Method: Balanced forces.

Its inward gravity and outward thermal pressure are balanced
40
Extended[2 marks]

Explain why the Sun emits most of its energy in three particular regions of the spectrum.

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Method: Surface temperature.

Its surface temperature (about 5800 K) makes it emit mainly infrared, visible and ultraviolet

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[4 marks]

Describe the Sun's size, composition and the regions of the electromagnetic spectrum in which it emits most energy.

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Method: Facts.

Medium-sized star; mostly hydrogen and helium; most energy in infrared, visible and ultraviolet
42
ChallengeCore[3 marks]

Explain why most of the Sun's ultraviolet does not reach the Earth's surface and why some protection is still needed.

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Method: Atmosphere absorbs.

The ozone layer absorbs much of the UV, but some still reaches the surface and can damage skin and eyes
43
ChallengeCore[4 marks]

Light takes 500 s to travel from the Sun to the Earth. Calculate the distance and explain why we would not know immediately if the Sun stopped shining.

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Method: d = ct.

d = 1.5 × 10¹¹ m; light already on its way would keep arriving for about 8.3 minutes
44
ChallengeCore[3 marks]

Explain why infrared and visible light from the Sun are both essential for life on Earth.

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Method: Warmth and photosynthesis.

Infrared warms the Earth to a suitable temperature; visible light drives photosynthesis and vision
45
ChallengeExtended[4 marks]

Explain how the Sun produces energy, including the nuclei involved and why the reaction releases energy.

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Method: Fusion.

Hydrogen nuclei fuse to form helium in the core; the mass of the products is less than the reactants, and the difference is released as energy
46
ChallengeExtended[4 marks]

Explain why the Sun is a stable star and what will eventually upset this balance.

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Method: Gravity vs pressure.

Inward gravity is balanced by the outward force due to the high temperature at its centre; when the core hydrogen runs out fusion slows and the balance changes
47
ChallengeExtended[4 marks]

Explain why fusion can occur in the Sun's core but not near its surface.

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Method: Conditions.

The core has extremely high temperature and pressure so nuclei move fast enough to overcome repulsion; the outer layers are too cool
48
ChallengeExtended[4 marks]

The Sun loses about 4 × 10⁹ kg of mass every second. Explain where this mass goes and why the Sun will still last billions of years.

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Method: Mass → energy.

It is converted into energy radiated into space; the Sun's mass (about 2 × 10³⁰ kg) is so enormous that this loss is tiny
49
ChallengeExtended[3 marks]

Compare the energy-releasing reactions in the Sun and in a nuclear power station.

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Method: Fusion vs fission.

The Sun joins light nuclei (fusion); power stations split heavy nuclei (fission)
50
ChallengeExtended[4 marks]

Explain why the Sun will become a red giant rather than a supernova.

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Method: Mass.

It is a medium-mass star; only much more massive stars become red supergiants and explode as supernovae
All IGCSE Physics topics← 6.1.2 The Solar System6.2.2 Stars →

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