IGCSE Physics 6.2 Stars and the Universe MCQs – 30 Practice Questions with Answers (Cambridge 0625)
Practise Cambridge IGCSE Physics (0625) Section 6.2, Stars and the Universe, with 30 original multiple-choice questions covering how stars form and shine, the life cycles of low-mass and high-mass stars, galaxies, light-years and the expanding universe. Tap Show Answer under each question to reveal the correct answer with a full explanation.
Formula recap: one light-year is the distance light travels in one year: distance = speed × time = (3.0 × 108 m/s) × (365.25 × 24 × 3600 s) ≈ 9.5 × 1015 m. Keep every key equation for 0625 in one place with our IGCSE Physics Formula Sheet.
How Stars Form & Shine
Q1. A nebula is best described as:
- (A) a huge cloud of gas and dust in space
- (B) a group of stars orbiting the centre of a galaxy
- (C) a planet that has not yet cooled down
- (D) a small moon made of ice and rock
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Answer: A – a nebula is an enormous cloud of gas (mostly hydrogen) and dust in space. Stars are born inside nebulae when parts of the cloud collapse inwards under their own gravity.
Q2. As a region of a nebula slowly contracts to form a new star, the force mainly responsible for pulling the gas and dust particles together is:
- (A) friction between the particles
- (B) magnetism
- (C) gravity
- (D) electrostatic repulsion
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Answer: C – every particle in the cloud attracts every other particle gravitationally. Once a region starts to contract, gravity pulls it tighter and tighter, squeezing the gas into a shrinking, heating ball.
Q3. A contracting cloud of gas and dust that is getting hotter and denser, but in which nuclear fusion has not yet begun, is called a:
- (A) white dwarf
- (B) protostar
- (C) red giant
- (D) neutron star
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Answer: B – the collapsing, heating cloud is a protostar. It only becomes a true star when its core gets hot and dense enough for nuclear fusion to start; white dwarfs and neutron stars are end-of-life stages, not beginnings.
Q4. Why does the temperature of a protostar rise as gravity makes it contract?
- (A) It drifts closer to other, hotter stars
- (B) Chemical burning starts in its core
- (C) It absorbs light from the whole galaxy
- (D) Particles falling inwards gain kinetic energy, so gravitational potential energy is transferred to internal (thermal) energy
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Answer: D – as the cloud shrinks, the particles speed up as they fall towards the centre and collide more violently. Their rising kinetic energy is what we measure as a rising temperature, until the core reaches millions of degrees.
Q5. The energy radiated by a stable, main-sequence star is released by:
- (A) nuclear fusion in the core
- (B) hydrogen gas burning in oxygen
- (C) friction between the layers of the star
- (D) the star’s gravitational contraction alone
Show Answer
Answer: A – in the core, the temperature and pressure are so extreme that atomic nuclei fuse together, releasing huge amounts of energy. This energy travels outwards through the star and is radiated from the surface as light and heat.
Q6. In the core of the Sun, the main fusion reaction converts:
- (A) helium nuclei into hydrogen nuclei
- (B) hydrogen nuclei into helium nuclei
- (C) carbon nuclei into oxygen nuclei
- (D) protons into neutrons, forming no new element
Show Answer
Answer: B – hydrogen nuclei (protons) are forced together to form helium nuclei. The helium nucleus has slightly less mass than the hydrogen nuclei that formed it, and that lost mass is released as energy.
Q7. During nuclear fusion in a star’s core:
- (A) energy is absorbed from the surroundings
- (B) helium nuclei are split apart into hydrogen
- (C) a small amount of mass is converted into energy, which is eventually radiated away
- (D) the total mass of the star gradually increases
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Answer: C – fusion is an energy-releasing process: the products have slightly less mass than the starting nuclei, and the difference appears as energy. This is why a star very slowly loses mass as it shines.
Q8. A star on the main sequence keeps the same size for billions of years because:
- (A) gravity stops acting while fusion is taking place
- (B) the star spins fast enough to hold itself up
- (C) there is no force pulling the star inwards
- (D) the outward pressure generated by fusion energy balances the inward pull of gravity
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Answer: D – gravity constantly tries to crush the star, while the energy released by core fusion produces an outward pressure. On the main sequence these two effects balance, so the star is stable.
Q9. Our Sun is best described as:
- (A) a medium-mass star on the main sequence
- (B) a high-mass star near the end of its life
- (C) a white dwarf
- (D) a red supergiant
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Answer: A – the Sun is a fairly ordinary, medium (low-to-medium) mass star about halfway through its main-sequence life, steadily fusing hydrogen into helium in its core.
Q10. Nuclear fusion in the Sun takes place:
- (A) evenly throughout the whole star
- (B) in the core, where the temperature and pressure are highest
- (C) only at the visible surface
- (D) in a thin shell just below the surface
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Answer: B – only the core is hot enough (about 15 million °C) and dense enough for nuclei to overcome their mutual repulsion and fuse. The outer layers simply transport the energy outwards.
Life Cycles of Stars
Q11. When the hydrogen fuel in the Sun’s core is largely used up, the Sun will next become a:
- (A) neutron star
- (B) black hole
- (C) red giant
- (D) protostar
Show Answer
Answer: C – with less fusion in the core, the balance is upset, the core contracts and heats, and the outer layers expand and cool, turning the Sun into a red giant. Stars of the Sun’s mass never become neutron stars or black holes.
Q12. Near the end of its life, a star like the Sun blows off its outer layers. The expanding shell of glowing gas left around the core is called a:
- (A) comet tail
- (B) asteroid belt
- (C) solar flare
- (D) planetary nebula
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Answer: D – the expelled outer layers form a planetary nebula (the name is historical; it has nothing to do with planets). The shell slowly drifts away into space, recycling its material.
Q13. After a Sun-like star has shed its outer layers as a planetary nebula, the hot, dense core that remains behind is a:
- (A) red giant
- (B) white dwarf
- (C) black hole
- (D) main-sequence star
Show Answer
Answer: B – the leftover core is a white dwarf: about the size of the Earth but with roughly the mass of the Sun, so it is extremely dense. It is not massive enough to collapse further into a neutron star or black hole.
Q14. A white dwarf carries out no nuclear fusion, yet it still shines. This is because:
- (A) it is still very hot and gradually radiates away its stored thermal energy as it cools
- (B) it reflects light from nearby bright stars
- (C) chemical reactions on its surface release light
- (D) it is constantly being heated by falling comets
Show Answer
Answer: A – a white dwarf is the exposed, intensely hot core of a dead star. With no fusion to replace the energy it radiates, it slowly cools down over billions of years, becoming dimmer and redder.
Q15. A star with a mass much greater than the Sun becomes, after its main-sequence stage, a:
- (A) white dwarf
- (B) planetary nebula
- (C) red supergiant
- (D) protostar
Show Answer
Answer: C – high-mass stars swell into red supergiants, far larger and brighter than the red giants formed by Sun-like stars. Their much greater mass means their later stages are far more violent.
Q16. The life of a high-mass star ends when its core suddenly collapses and the outer layers are blasted into space in an enormous explosion called a:
- (A) planetary nebula
- (B) solar eclipse
- (C) red shift
- (D) supernova
Show Answer
Answer: D – a supernova can briefly outshine an entire galaxy. Unlike the gentle shedding of a planetary nebula, it is a catastrophic explosion that scatters most of the star’s material into space.
Q17. After a supernova, the collapsed core of a massive star may remain as an extremely dense object made almost entirely of neutrons. This object is a:
- (A) white dwarf
- (B) neutron star
- (C) red giant
- (D) comet
Show Answer
Answer: B – in a neutron star, gravity has crushed protons and electrons together to form neutrons. A teaspoon of neutron-star material would have a mass of about a billion tonnes.
Q18. If the core left behind after a supernova is massive enough, it collapses completely to form a:
- (A) black hole, whose gravity is so strong that not even light can escape from it
- (B) new main-sequence star
- (C) rocky planet
- (D) nebula that immediately forms planets
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Answer: A – when the remnant core is above a certain mass, nothing can stop the collapse. The result is a black hole, an object with an escape velocity greater than the speed of light.
Q19. High-mass stars have much shorter lives than the Sun, even though they start with far more fuel. This is because:
- (A) they blow most of their fuel away before using it
- (B) no fusion takes place inside them
- (C) their cores are cooler, so reactions run more slowly
- (D) their higher core temperature and pressure make them burn through their fuel at a much faster rate
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Answer: D – a massive star’s core is squeezed to much higher temperatures, so fusion runs extremely fast. The star is far more luminous and exhausts its hydrogen in millions, rather than billions, of years.
Q20. Elements heavier than iron are formed:
- (A) in the cores of all stars throughout the main sequence
- (B) on the surfaces of rocky planets
- (C) in the extreme temperatures and pressures of a supernova explosion
- (D) inside white dwarfs as they cool
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Answer: C – building elements heavier than iron by fusion absorbs energy rather than releasing it, so it cannot happen in a stable core. Only the immense energy of a supernova can create them and scatter them into space.
Galaxies, Light-Years & the Expanding Universe
Q21. A galaxy is:
- (A) a single very large star
- (B) a huge collection of stars, gas and dust held together by gravity
- (C) a group of planets orbiting one star
- (D) the region of space inside the Solar System
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Answer: B – a typical galaxy contains hundreds of billions of stars, together with the gas and dust from which new stars form, all bound together by gravity.
Q22. The Sun is one star in the galaxy called:
- (A) the Milky Way
- (B) Andromeda
- (C) the Large Magellanic Cloud
- (D) Orion
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Answer: A – our home galaxy is the Milky Way, a spiral galaxy. Andromeda is a neighbouring galaxy, and the Large Magellanic Cloud is a small satellite galaxy of the Milky Way.
Q23. One light-year is:
- (A) the time light takes to travel from the Sun to the Earth
- (B) the speed of light multiplied by one year, measured in m/s
- (C) a unit of distance: the distance light travels in one year
- (D) a unit of time used to measure the ages of stars
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Answer: C – despite its name, a light-year measures distance, not time. Because light travels extremely fast, one light-year is an enormous distance, which makes it convenient for measuring the separations of stars.
Q24. Taking the speed of light as 3.0 × 108 m/s, one light-year is approximately:
- (A) 3.0 × 108 m
- (B) 3.6 × 1011 m
- (C) 9.5 × 1012 m
- (D) 9.5 × 1015 m
Show Answer
Answer: D – working: number of seconds in one year = 365.25 × 24 × 3600 ≈ 3.16 × 107 s. Distance = speed × time = 3.0 × 108 × 3.16 × 107 ≈ 9.5 × 1015 m (about 9.5 trillion km).
Q25. The nearest star to the Sun, Proxima Centauri, is about 4.2 light-years away. This means that:
- (A) the light we see from it left the star about 4.2 years ago, so we see the star as it was then
- (B) a spacecraft travelling at any speed would take 4.2 years to reach it
- (C) the star is 4.2 years old
- (D) we see the star exactly as it is now, with no delay
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Answer: A – light travels at a finite speed, so it takes 4.2 years to cross 4.2 light-years. Looking at distant objects is always looking back in time; for the most distant galaxies, we see light that left billions of years ago.
Q26. The light from a distant galaxy shows a red shift. [Supplement] This tells us that:
- (A) the galaxy is moving towards us, squashing its light waves together
- (B) the galaxy is moving away from us, so the wavelengths of its light are stretched to longer values
- (C) the galaxy is cooling down and turning red
- (D) the galaxy is made mainly of red giant stars
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Answer: B – when a light source moves away, each wave is stretched out, increasing the wavelength and shifting the spectrum towards the red end. Motion towards us would produce a blue shift instead.
Q27. Observations show that the more distant a galaxy is, the greater its red shift. [Supplement] This suggests that:
- (A) all galaxies are moving towards the Milky Way
- (B) distant galaxies contain only old, red stars
- (C) more distant galaxies are receding faster, which means the universe is expanding
- (D) light gradually slows down as it travels through space
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Answer: C – a bigger red shift means a faster recession. Because recession speed increases with distance in every direction, the pattern is explained by space itself expanding, carrying the galaxies apart.
Q28. Which observation is key evidence that the universe began in an extremely hot, dense state, as described by the Big Bang theory?
- (A) The existence of black holes in galactic centres
- (B) Planets orbiting the Sun in ellipses
- (C) The Moon raising tides on the Earth
- (D) The red shift of distant galaxies, showing that the universe is expanding
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Answer: D – if the universe is expanding now, it must have been smaller, hotter and denser in the past. Running the expansion backwards leads to the universe starting from a very hot, dense state about 13.8 billion years ago.
Q29. The cosmic microwave background radiation is:
- (A) faint microwave radiation arriving from all directions in space, left over from the very early, hot universe
- (B) radiation given out by the Sun’s outer atmosphere
- (C) the visible glow of the Milky Way on a dark night
- (D) radio waves reflected from planets in the Solar System
Show Answer
Answer: A – this background radiation is the cooled remnant of the intense radiation that filled the universe just after the Big Bang. Its detection in every direction is strong supporting evidence for the Big Bang theory.
Q30. Hubble’s law states, in simple terms, that for distant galaxies: [Supplement]
- (A) brightness increases as distance increases
- (B) the speed at which a galaxy recedes from us increases with its distance from us
- (C) all galaxies recede at exactly the same speed
- (D) a galaxy’s size decreases as its distance increases
Show Answer
Answer: B – Hubble’s law links recession speed directly to distance: double the distance, double the recession speed. It is the quantitative form of the red-shift observations and a cornerstone of the expanding-universe model.
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