Edexcel IGCSE Physics: Astrophysics — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Astrophysics revision notes

---

Section A

1. Place these in order of increasing size: galaxy, planet, solar system, universe, star. [2]

2. Explain why a planet in a circular orbit is accelerating even though its speed is constant. [2]

Section B

3. A satellite orbits at radius r with speed v.

(a) State the equation linking orbital speed, radius and period. [1] (b) Explain what happens to the orbital speed as the orbital radius increases. [2] (c) Explain why gravity causes a satellite to move in a circular orbit, and use the equation v = 2πr ÷ T together with g = v² ÷ r to explain why a satellite’s orbital period increases as its orbital radius increases. [3]

4. Describe the life cycle of a star with a mass similar to the Sun, from nebula to its final state. [5]

5. Describe how the life cycle differs for a star much more massive than the Sun. [4]

6. Light from distant galaxies is red-shifted.

(a) Explain what red shift means and what causes it. [3] (b) State the relationship between a galaxy’s distance and its speed of recession. [2] (c) Explain how red shift and CMB radiation support the Big Bang theory. [4]

---

Section C

7. A satellite orbits at radius 4.2 × 10⁷ m with a period of 8.64 × 10⁴ s (24 hours).

(a) Calculate its orbital speed. [2] (b) State one way in which a comet’s orbit typically differs from a planet’s orbit. [1]

8. A galaxy’s light shows a wavelength shift of 15 nm from a reference (laboratory) wavelength of 500 nm.

(a) Calculate the galaxy’s recession speed as a fraction of the speed of light, and as a value in m s⁻¹. [2] (b) A second galaxy is twice as far away. State, with a reason, what this implies about its red shift. [1]

9. Two stars appear equally bright when observed from Earth, but one is much further away than the other.

(a) Distinguish between absolute magnitude and apparent brightness. [2] (b) State what is plotted on each axis of a Hertzsprung-Russell (HR) diagram. [2] (c) Explain why comparing the two stars’ apparent brightness alone would not fairly compare their true luminosities. [2]

---

Answers

1. Planet, star, solar system, galaxy, universe [2 — 1 mark if one is misplaced].

2. Its direction is constantly changing, so its velocity changes even though the speed does not [1]; acceleration is the rate of change of velocity, and the gravitational force provides a centripetal acceleration towards the centre [1].

3. (a) v = 2πr ÷ T [1]. (b) The orbital speed decreases [1], because the gravitational field strength is weaker further out, so a smaller centripetal force and a slower speed are needed to maintain the orbit [1]. (c) Gravity provides the centripetal force needed to keep the satellite moving in a circle rather than travelling in a straight line [1]; combining v = 2πr ÷ T with g = v² ÷ r gives g = 4π²r ÷ T², so at a larger orbital radius the gravitational field strength g is smaller, meaning a lower orbital speed is needed to maintain the orbit [1]; since a larger r is now paired with a smaller v in v = 2πr ÷ T, the period T must be longer — more distant satellites take longer to orbit [1].

4. A nebula of dust and gas is pulled together by gravity [1]; as it contracts it heats up and nuclear fusion of hydrogen begins, forming a main sequence star, stable while the outward pressure from fusion balances gravity [1]. When the hydrogen in the core runs out it expands into a red giant [1]. The outer layers are then shed as a planetary nebula [1], leaving a hot dense core — a white dwarf — which cools to a black dwarf [1].

5. A massive star becomes a red supergiant rather than a red giant [1]. It then explodes as a supernova [1], during which the elements heavier than iron are formed and scattered into space [1]. The remnant core becomes a neutron star, or a black hole if the star is massive enough [1].

6. (a) The wavelength of light received from the galaxy is longer than the wavelength it was emitted at, shifted towards the red end of the spectrum [1]; this occurs because the galaxy is moving away from us [1]; the greater the recession speed, the greater the shift [1]. (b) The further away a galaxy is, the greater its red shift and so the faster it is moving away [1] — the speed is proportional to the distance [1]. (c) Red shift shows that all distant galaxies are receding and that the universe is expanding [1]; running the expansion backwards implies everything began from a single point [1]. The cosmic microwave background radiation fills the whole sky uniformly [1] and is the cooled remnant of the intense radiation from that early hot dense state — something the steady state theory cannot explain [1].

7. (a) v = 2πr ÷ T = 2π × 4.2 × 10⁷ ÷ 8.64 × 10⁴ [1] ≈ 3,050 m s⁻¹ [1]. (b) A comet’s orbit is typically much more elongated (elliptical) than the roughly circular orbits of planets and moons [1].

8. (a) v ÷ c = Δλ ÷ λ₀ = 15 ÷ 500 = 0.03 [1], so v = 0.03 × 3.00 × 10⁸ = 9.0 × 10⁶ m s⁻¹ [1]. (b) Because recession speed (and hence red shift) is proportional to distance, a galaxy twice as far away would show roughly twice the red shift [1].

9. (a) Absolute magnitude is how bright a star would appear at a fixed standard distance, allowing fair brightness comparisons [1]; apparent brightness is how bright it actually appears as observed from Earth, which depends on its real distance [1]. (b) Temperature (or colour) on one axis and luminosity (or absolute magnitude) on the other [2]. (c) Two stars can appear equally bright from Earth even with very different true luminosities, simply because the more luminous one is much further away [1]; only correcting for distance, as absolute magnitude does, allows their true luminosities to be compared fairly [1].

---

Where marks are usually lost

• Saying an orbiting body is not accelerating because its speed is constant.
• Confusing the fates of low-mass and high-mass stars.
• Thinking red shift shows galaxies moving away from a fixed centre (with Earth at that centre) — in fact, space itself is expanding, so every observer sees the same pattern with no privileged centre.
• Forgetting the CMB as separate evidence from red shift.
• Confusing absolute magnitude (standard-distance brightness) with apparent brightness (brightness as observed).
• Forgetting that Δλ/λ₀ = v/c requires the reference (laboratory) wavelength, not the observed wavelength, as the denominator.