Edexcel IAL Physics: Astrophysics and Cosmology — 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 and Cosmology revision notes

---

Questions

1. Distinguish between luminosity and radiant flux intensity. [2]

2. Describe the axes of a Hertzsprung–Russell diagram, noting anything unusual. [3]

3. A star appears in the top right of the HR diagram.

(a) State what this tells you about its temperature and luminosity. [2] (b) Deduce what this implies about its size, explaining your reasoning. [3] (c) Name this class of star. [1]

4. A star has peak emission at 720 nm and luminosity 8.4 × 10²⁸ W. (Wien constant = 2.898 × 10⁻³ m K; σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴)

(a) Calculate its surface temperature. [2] (b) Calculate its radius. [3]

5. Describe the life cycle of (a) a star of about one solar mass, (b) a star of twenty solar masses. [6]

6. Explain why more massive main-sequence stars have shorter lifetimes despite having more fuel. [3]

7. State what the Chandrasekhar limit is and its significance. [2]

8. Explain why Type Ia supernovae are described as “standard candles” and how they are used to measure distances to remote galaxies. [3]

9. A galaxy’s hydrogen-alpha absorption line, normally at 656 nm in the laboratory, is observed at 660 nm.

(a) Calculate the galaxy’s recession velocity. [3] (b) Given a Hubble constant of 70 km s⁻¹ Mpc⁻¹, calculate the galaxy’s distance. [2]

10. State three independent pieces of observational evidence for the Big Bang model, briefly explaining what each shows. [6]

11. A planet of mass 6.4 × 10²³ kg has a moon orbiting at a radius of 9.4 × 10⁶ m. (G = 6.67 × 10⁻¹¹ N m² kg⁻²)

(a) State the equation for gravitational field strength due to a point mass, and give one similarity and one difference between gravitational and electric fields. [3] (b) Show that the moon’s orbital speed is about 2.1 × 10³ m s⁻¹, using the fact that gravity provides the centripetal force. [3]

---

Answers

1. Luminosity is the total power radiated by the star [1]; radiant flux intensity is the power received per unit area at the observer [1].

2. Luminosity on the vertical axis, temperature on the horizontal [1]. Temperature increases to the left — the axis is reversed [1]. Both axes are usually logarithmic [1].

3. (a) Low temperature (it is on the right) but high luminosity (it is at the top) [1] [1]. (b) Luminosity depends on both temperature and surface area (L = 4πr²σT⁴) [1]. For a low temperature to produce a high luminosity, the surface area must be very large [1], so the radius is very large [1]. (c) A red giant (or supergiant) [1].

4. (a) T = 2.898 × 10⁻³ ÷ (720 × 10⁻⁹) [1] = 4025 K [1]. (b) r = √(L ÷ (4πσT⁴)) [1] T⁴ = 4025⁴ = 2.625 × 10¹⁴ r = √(8.4 × 10²⁸ ÷ (4π × 5.67 × 10⁻⁸ × 2.625 × 10¹⁴)) [1] = 2.1 × 10¹⁰ m [1].

5. (a) Nebula → protostar → main sequence → red giant → planetary nebula → white dwarf [1] [1] [1]. (b) Nebula → protostar → main sequence → red supergiant → supernova → neutron star or black hole [1] [1] [1].

6. A more massive star has a much greater luminosity [1]. Luminosity rises far faster than mass — roughly as the cube or fourth power [1], so it consumes its larger fuel supply disproportionately quickly [1].

7. The maximum mass of a white dwarf, about 1.4 solar masses [1]. Above it, electron degeneracy pressure cannot resist gravitational collapse, so the core collapses further to a neutron star or black hole [1].

8. Type Ia supernovae detonate at a fixed critical mass (the Chandrasekhar limit), so every one reaches a consistent, known peak luminosity [1] — this is what “standard candle” means [1]. Measuring the received flux from a distant supernova and comparing it with this known luminosity, using F = L ÷ (4πd²), allows the distance d to be calculated [1].

9. (a) Δλ = 660 − 656 = 4 nm [1]. v = c × (Δλ ÷ λ) = 3.0 × 10⁸ × (4 ÷ 656) [1] = 1.83 × 10⁶ m s⁻¹ (1830 km s⁻¹) [1]. (b) d = v ÷ H₀ = 1830 ÷ 70 [1] = 26.1 Mpc [1].

10. Any three: Galactic redshift — almost all distant galaxies show redshifted light, and redshift increases with distance, showing the universe is expanding and was once smaller and denser [2]. Cosmic microwave background radiation — a near-uniform black-body radiation at about 2.7 K found in all directions, the cooled remnant of the hot, dense early universe [2]. Hydrogen and helium abundances — the observed ratio of these elements in the universe matches the ratio predicted by Big Bang nucleosynthesis calculations [2].

11. (a) g = Gm/r² [1]. Both fields are inverse-square and have radial and uniform forms [1], but gravitational forces are always attractive whereas electric forces can be attractive or repulsive [1]. (b) Gravity provides the centripetal force: GMm/r² = mv²/r, so v = √(GM/r) [1] = √(6.67 × 10⁻¹¹ × 6.4 × 10²³ ÷ 9.4 × 10⁶) [1] = 2.1 × 10³ m s⁻¹ [1].

---

Where marks are usually lost

• Confusing luminosity with flux.
• Forgetting the HR diagram’s reversed temperature axis.
• Applying Stefan’s law before Wien’s, leaving two unknowns.
• Saying massive stars live longer because they have more fuel.
• Forgetting to convert the Hubble constant’s units (km s⁻¹ Mpc⁻¹) consistently with the velocity used, or forgetting that the resulting distance comes out in Mpc, not metres, unless converted.
• Claiming galactic redshift proves Earth is at the centre of the universe — space itself is expanding, so every observer anywhere would see the same pattern.
• Citing only one piece of Big Bang evidence when a question asks for several independent strands.

Redshift, Hubble’s law and the age of the universe

Astronomers use the same Doppler-shift logic behind question 9 to build the Hubble law, v = H₀d, relating a galaxy’s recession velocity to its distance. Because essentially every distant galaxy is redshifted, and more distant galaxies are redshifted by more, the universe must be expanding — and running that expansion backwards gives a rough estimate for the universe’s age, of order 1/H₀. Dark matter and dark energy are each inferred from separate lines of evidence: dark matter from galactic rotation curves, where outer stars orbit faster than the visible mass alone can explain, and dark energy from supernova observations showing the rate of expansion is itself accelerating, not just continuing at a constant rate. For the full treatment of cosmology alongside stellar evolution, see the Astrophysics and Cosmology revision notes.