Astronomy & cosmology: A Level Physics questions with answers
Free Astronomy & cosmology questions for A Level Physics (9702), part of Thermal & modern physics. Each question has the answer and the marking points an examiner would look for.
The spectrum of a star has peak intensity at a wavelength of 380 nm. The luminosity of the star is 1.0 × 1027 W.
(a)
Use Wien’s displacement law to calculate the surface temperature.
[2]
(b)
Calculate the radius of the star.
[3]
(c)
A spectral line of wavelength 656 nm measured in the laboratory is observed in light from a distant galaxy at 676 nm. Calculate the recession speed of the galaxy and its distance, using H0 = 2.2 × 10−18 s−1.
[3]
(d)
State what is meant by a standard candle.
[1]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
7630 K
λmaxT = 2.9 × 10−3 m K C1 7630 A1
2
(b)
6.43 × 108 m
L = 4πσr2T4C1 rearranged C1 6.43 × 108A1
3
(c)
9.15 × 106 m s−1, 4.16 × 1024 m
Δλ/λ = v/c C1 9.15 × 106A1 4.16 × 1024A1
3
(d)
an object of known luminosity
known luminosity B1
1
Question 2Structured · 4 marksA* challenge
A star has surface temperature 12000 K and luminosity 3.8 × 1026 W.
(a)
Use Wien’s displacement law to find the peak wavelength.
[2]
(b)
Calculate the radius of the star.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
2.42 × 10−7 m
λmaxT = 2.9 × 10−3 m K C1 2.42 × 10−7 m A1
2
(b)
1.60 × 108 m
L = 4πσr2T4C1 1.60 × 108 m A1
2
Question 3Structured · 7 marks
A distant galaxy is receding at 3.0 × 106 m s−1. Take the Hubble constant as 2.2 × 10−18 s−1.
(a)
State Hubble’s law.
[1]
(b)
Calculate the distance to the galaxy.
[2]
(c)
Calculate the observed wavelength of a spectral line emitted at 486 nm.
[2]
(d)
Estimate the age of the Universe in years.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
recession speed proportional to distance
the recession speed of a galaxy is proportional to its distance from us (v = H₀d) B1
1
(b)
1.36 × 1024 m
d = v/H₀ C1 1.36 × 1024 m A1
2
(c)
491 nm
Δλ/λ = v/c C1 491 nm A1
2
(d)
1.44 × 1010 years
t = 1/H₀ = 4.55 × 1017 s C1 1.44 × 1010 years A1
2
Question 4Structured · 3 marks
Standard candles are used to find distances to galaxies.
(a)
State what is meant by a standard candle.
[1]
(b)
Explain how the distance to a standard candle is found.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
object of known luminosity
an astronomical object of known luminosity B1
1
(b)
measure flux, use F = L/4πd2
measure the radiant flux intensity at Earth B1 use F = L/4πd2 to calculate d B1
2
Question 5Structured · 11 marks
A star has a surface temperature of 10000 K and a luminosity of 25 times that of the Sun (luminosity of the Sun = 3.83 × 1026 W).
(a)
Define luminosity of a star.
[1]
(b)
Use Wien’s displacement law to calculate the wavelength at which the star’s emission is a maximum. (λmaxT = 2.9 × 10−3 m K)
[2]
(c)
Use the Stefan–Boltzmann law to calculate the radius of the star.
[3]
(d)
Define radiant flux intensity and explain how astronomers use a standard candle to find the distance to a galaxy.
[3]
(e)
Explain how the redshift of light from distant galaxies supports the idea of an expanding Universe.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
total power emitted
the total radiant power emitted by the star B1
1
(b)
2.90 × 10−7 m
λmax = 2.9 × 10−3/10000 C1 2.90 × 10−7 m A1
2
(c)
1.16 × 109 m
L = 4πσr2T4C1 r2 = L/(4πσT4) C1 r = 1.16 × 109 m A1
3
(d)
power per unit area; known luminosity; measure flux, d = √(L/4πF)
radiant flux intensity is power received per unit area B1 a standard candle has a known luminosity B1 measuring F and using F = L/(4πd2) gives d B1
3
(e)
almost all galaxies redshifted; greater distance, greater recession speed
light from nearly all distant galaxies is redshifted so they are moving away B1 recession speed is proportional to distance (Hubble’s law), consistent with expansion B1
2
Question 6Structured · 7 marks
The radiant flux intensity received from a star is 2.5 × 10−9 W m−2. The star is 25.0 light-years away (1 ly = 9.46 × 1015 m).
(a)
State the inverse-square law for radiant flux intensity.
[1]
(b)
Calculate the luminosity of the star.
[2]
(c)
Use your answer to (b) to compare the star with the Sun (luminosity 3.83 × 1026 W).
[1]
(d)
Explain why the star’s spectrum shows dark absorption lines and what they reveal.
[3]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
F = L/(4πd²)
F = L/(4πd2): flux intensity is inversely proportional to the square of the distance B1
1
(b)
1.76 × 1027 W
L = 4πd2F C1 1.76 × 1027 W A1
2
(c)
4.59 times
4.59 × the Sun’s luminosity B1
1
(d)
cooler gas in outer layers absorbs specific wavelengths; identifies elements; shifts give velocity
light from the hot core passes through cooler gas in the star’s atmosphere B1 atoms absorb photons of specific energies matching transitions, giving dark lines B1 the lines identify the elements present (and their shift gives velocity) B1
Riyaz Shaikh has mentored IGCSE, IB and A Level students for 15+ years, one-on-one, online and in Mumbai. The hardest questions on this site are the ones he uses to find a student's weak points.
Original Cambridge-style practice questions written for mentorriyaz.in. These are not Cambridge past paper questions, and this site is not affiliated with or endorsed by Cambridge University Press & Assessment. Updated 2026-11-04. Paper generator · mentorriyaz.in