Physics 0625 · syllabus 6.2
Cambridge IGCSE Physics 0625 — Stars and the Universe topical past papers
The Sun as a star, light-years and galaxies, the life cycle of stars, redshift and the expanding Universe, the cosmic microwave background and the Hubble constant.
- Questions
- 206
- Years
- 2023–2026
- With mark scheme
- 100%
- Questions 2022–2026
- 206
Anyone can browse these pages. Opening the questions and their mark schemes takes a free Momentum account.
What Stars and the Universe covers
The Sun is a medium-sized star made mostly of hydrogen and helium, and most of the energy it radiates is infrared, visible light and ultraviolet. Extended candidates know that a star's energy comes from nuclear fusion: in a stable star, hydrogen nuclei fuse into helium. The Sun is one of many billions of stars in our galaxy, the Milky Way, and the other stars in it are far further from us than the Sun is. Astronomical distances are measured in light-years, the distance light travels through space in a year, which Extended candidates know is 9.5 × 10¹⁵ m. The Milky Way is about 100 000 light-years across, and the Universe holds billions of other galaxies besides.
Extended candidates follow a star through its whole life. A cloud of gas and dust containing hydrogen collapses under its own gravity and heats up, forming a protostar, which becomes a stable star once the inward pull of gravity is balanced by the outward push produced by the very high temperature at its centre. When most of the hydrogen in its core has become helium, a star like the Sun swells into a red giant, then throws off its outer layers as a planetary nebula, leaving a white dwarf. A much more massive star swells into a red supergiant and explodes as a supernova, leaving a neutron star or a black hole inside a nebula of hydrogen and newly made heavier elements, from which new stars and planets can form.
Light from distant galaxies is redshifted: its wavelengths are longer than those of the same light produced on Earth, because the galaxies are moving away from us. That shows the Universe is expanding, and it is evidence for the Big Bang theory. Extended candidates add the cosmic microwave background radiation, microwaves arriving from every direction in space, produced shortly after the Universe began and stretched into the microwave region as the Universe expanded. They find a galaxy's speed from its redshift and its distance from the brightness of a supernova in it, define the Hubble constant as speed ÷ distance, H₀ = v/d, currently estimated at 2.2 × 10⁻¹⁸ per second, and use d/v = 1/H₀ to estimate the age of the Universe.
In the syllabus
The headings this topic has in the Cambridge International 2026–2028 syllabus, as the board prints them.
- 6.2 Stars and the Universe
What you should be able to do
- Describe the Sun as a star and, for Extended, explain where its energy comes from
- Use light-years as a unit of distance, converting with 1 light-year = 9.5 × 10¹⁵ m (Extended)
- Trace the life of a star like the Sun and of a much more massive star, stage by stage (Extended)
- Explain redshift and why it is evidence for an expanding Universe and the Big Bang
- Explain how the cosmic microwave background radiation was produced (Extended)
- Use H₀ = v/d, and estimate the age of the Universe from 1/H₀ (Extended)
Key ideas
- A light-year is a distance: about 9.5 × 10¹⁵ m (the value for Extended)
- Stable star: the inward pull of gravity balanced by an outward force from the hot core; energy from hydrogen fusing into helium (Extended)
- Redshift: wavelengths observed longer than those emitted, because the source is moving away
- Hubble constant H₀ = v/d ≈ 2.2 × 10⁻¹⁸ per second (Extended)
- Age of the Universe ≈ d/v = 1/H₀ (Extended)
How Stars and the Universe has been examined
From 2022 to 2026, Stars and the Universe was on Paper 4 · Theory (Extended) in 11 of 14 exam series, worth about 10% of that paper’s marks.
| Paper | Questions | Share of marks | Series |
|---|---|---|---|
| Paper 1 · Multiple Choice (Core) | 72 | 6% | 11 of 14 |
| Paper 2 · Multiple Choice (Extended) | 76 | 6% | 11 of 14 |
| Paper 3 · Theory (Core) | 25 | 7% | 11 of 14 |
| Paper 4 · Theory (Extended) | 33 | 10% | 11 of 14 |
Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 1. The bank files this topic together with neighbouring ones, so these figures count every question in its chapters, including some on those topics.
Filed in Momentum under: Space Physics.
A worked example
Taking the Hubble constant as 2.2 × 10⁻¹⁸ per second, find how fast a galaxy 1.0 × 10⁹ light-years away is receding from the Earth, and estimate how old the Universe is, in years. (1 light-year = 9.5 × 10¹⁵ m)
- Distance in metres: d = (1.0 × 10⁹) × (9.5 × 10¹⁵) = 9.5 × 10²⁴ m
- H₀ = v/d, so v = H₀d = (2.2 × 10⁻¹⁸) × (9.5 × 10²⁴) = 2.1 × 10⁷ m/s
- Age ≈ 1/H₀ = 1 / (2.2 × 10⁻¹⁸) = 4.5 × 10¹⁷ s
- One year = 365 × 24 × 3600 = 3.15 × 10⁷ s, so the age ≈ (4.5 × 10¹⁷) / (3.15 × 10⁷) = 1.4 × 10¹⁰ years
- Check: 2.1 × 10⁷ m/s is about 7% of the speed of light, a sensible speed for a galaxy this far away
Answer: About 2.1 × 10⁷ m/s; the Universe is about 1.4 × 10¹⁰ years (14 billion years) old
Written by Momentum for this page, with its own numbers; it is not taken from a past paper.
Mistakes that cost marks
- Treating a light-year as a time — it is the distance light travels in one year
- Saying redshift means the light turns red — the wavelengths are stretched towards the red end, and even invisible radiation such as radio waves can be redshifted
- Mixing up the two life cycles — a star like the Sun goes red giant, planetary nebula, white dwarf; only a much more massive star goes red supergiant, supernova, then neutron star or black hole
- Mixing units in H₀ = v/d — with H₀ in per second, the speed must be in m/s and the distance in metres, so light-years have to be converted first
- Saying the cosmic microwave background was given out as microwaves — it was produced at much shorter wavelengths and has been stretched into the microwave region as the Universe expanded
Exam technique
- In life-cycle answers, name each stage in order and say what moves the star on to the next one
- When explaining the evidence for the Big Bang, state each observation — redshift and the microwave background — and then what it shows
- Keep every number in standard form in cosmology calculations, and check the order of magnitude: the age of the Universe comes out at about 10¹⁰ years
Practising in Momentum
- Every question in a chapter as one list, from every paper and year in the bank, newest first or shuffled.
- Each question turns over to its own part of the mark scheme, where the board published one.
- Narrow by paper, year, session, zone and marks, and star the questions worth keeping.
- Pick chapters, a number of questions and a time, and the exam builder draws a mock to sit and mark.
Past-paper questions and mark schemes are the copyright of Cambridge International. Momentum shows them to signed-in learners for revision and does not publish them on these pages.