IGCSE Physics: Stars and the Universe Practice Questions
Stars are powered by nuclear fusion, in which hydrogen nuclei join to form helium and release energy. Galaxies are vast collections of stars, and the observed redshift of distant galaxies is evidence that the Universe is expanding.
Sub-topic 6.2 of Cambridge IGCSE Physics 0625 closes the syllabus and is almost pure explanation. The life cycle question is marked stage by stage and depends on the star's mass, so branching answers score better than linear ones. The questions below cover fusion, the life cycle, light-years and redshift.
What you need to know for Stars and the Universe
- Nuclear fusion in starsHydrogen nuclei fuse to form helium nuclei, releasing enormous amounts of energy. This is the process powering the Sun and every other stable star.
- A stable starA star is stable when the outward pressure from fusion balances the inward gravitational attraction of its own mass. This balance can last billions of years.
- Galaxy and the Milky WayA galaxy is a huge collection of stars held together by gravity. The Sun is one star among many billions in the Milky Way, which is itself one of many billions of galaxies.
- Light-yearThe distance travelled by light in one year, roughly 9.5 x 1015 m. It is a unit of distance, not of time.
- Life cycle of a starA nebula collapses to form a protostar, which becomes a stable star. A star like the Sun then becomes a red giant and ends as a white dwarf. A much more massive star becomes a red supergiant, explodes as a supernova, and leaves a neutron star or a black hole.
- Redshift and the Big BangLight from distant galaxies is shifted towards longer wavelengths, and the shift is greater for more distant galaxies. This shows the Universe is expanding, which supports the Big Bang theory.
IGCSE Physics Stars and the Universe questions and answers
4 exam-style questions written to the 0625 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Describe the process that releases energy in the Sun, and explain what keeps the Sun stable over billions of years.
Show the worked answer
- The core of the Sun is extremely hot and dense, so hydrogen nuclei move fast enough to overcome their mutual repulsion and collide.
- They undergo nuclear fusion, joining to form helium nuclei and releasing an enormous amount of energy.
- That energy creates an outward pressure pushing the outer layers of the Sun away from the core.
- The Sun's own very large mass produces an inward gravitational attraction. While the outward pressure from fusion balances that inward gravitational pull, the star's size stays constant and it remains stable.
Describe the stages in the life cycle of a star with a mass much greater than that of the Sun, from its formation to its final state.
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- A cloud of dust and gas called a nebula is pulled together by gravitational attraction.
- As it collapses it heats up and forms a protostar. When the core is hot and dense enough, fusion begins and it becomes a stable star.
- When the hydrogen in the core runs low, the star expands and cools at the surface to become a red supergiant.
- It then explodes as a supernova, throwing its outer layers out into space and forming heavier elements.
- The remaining core collapses under gravity to form a neutron star, or a black hole if the mass is great enough. The material ejected by the supernova can later form new stars and planets.
A star is 4.2 light-years from Earth. Explain what a light-year is and state what this tells you about the light we see from that star.
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- A light-year is a unit of distance, not of time. It is the distance that light travels in one year, roughly 9.5 x 1015 m.
- Light travels at a finite speed of approximately 3 x 108 m/s.
- Light from a star 4.2 light-years away therefore takes 4.2 years to reach us.
- We are seeing the star as it was 4.2 years ago, not as it is now. For very distant objects this delay can be millions or billions of years.
Light from distant galaxies is observed to be shifted towards the red end of the spectrum, and the shift is larger for galaxies that are further away. Explain what this shows.
Show the worked answer
- A redshift means the observed wavelength of the light is longer than the wavelength emitted by the source.
- This happens when the source is moving away from the observer, so the galaxies are receding from us.
- The shift is greater for more distant galaxies, which means the more distant a galaxy is, the faster it is moving away.
- This pattern is what would be seen if the whole of space were expanding, and it supports the Big Bang theory, in which the Universe began from a very small, hot, dense point and has been expanding ever since.
Common mistakes in this topic
- Describing a light-year as a time.
- Saying stars burn fuel. They fuse nuclei.
- Giving the low mass life cycle when the question specifies a massive star, or the reverse.
- Saying redshift means the light gets redder in colour rather than shifting to longer wavelengths.
- Confusing a galaxy with the Universe. A galaxy is a collection of stars, and the Universe contains billions of galaxies.
Exam tips
- Learn the two life cycle routes side by side, branching at the point where hydrogen in the core runs low.
- In every stability question, use the phrase outward pressure from fusion balances inward gravitational attraction.
- State explicitly that a light-year is a distance. That statement alone is often worth a mark.
- For redshift, describe the wavelength change first, then the motion, then the conclusion.
- Remember that supernovae produce and scatter the heavier elements, which is where the material for planets comes from.
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Stars and the Universe FAQs
What is nuclear fusion in stars?
Nuclear fusion is the joining of light nuclei to form a heavier one, releasing energy. In a stable star, hydrogen nuclei fuse to form helium nuclei in the extremely hot and dense core. This is the source of all the energy a star radiates, including the Sun's.
What is a light-year?
A light-year is a unit of distance, not of time. It is the distance light travels in one year, approximately 9.5 x 10 to the power 15 metres. If a star is 4.2 light-years away, the light reaching us now left it 4.2 years ago.
What are the stages in the life cycle of a star?
A nebula collapses under gravity into a protostar, which becomes a stable star once fusion begins. A star like the Sun then becomes a red giant and ends as a white dwarf. A much more massive star becomes a red supergiant, explodes as a supernova, and leaves a neutron star or a black hole.
What does redshift tell us about the Universe?
Light from distant galaxies arrives with longer wavelengths than it was emitted with, showing those galaxies are moving away from us. The shift is larger for more distant galaxies, meaning they recede faster. This pattern indicates the Universe is expanding and supports the Big Bang theory.
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Written to the published Cambridge IGCSE Physics (0625) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.