CCEA A Level Physics
CCEA A Level Physics 2.7 Astronomy Exam Questions
Free CCEA A Level Physics revision for 2.7 Astronomy: what you need to know, common mistakes and exam-style questions with answers. Covers doppler shift, red shift parameter z, cosmological red shift, hubble's law and the age of the universe.
Practise 2.7 Astronomy with past papers
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About CCEA A Level Physics 2.7 Astronomy
Astronomy closes Unit AS 2: Waves, Photons and Astronomy by applying wave ideas to the expanding universe. You need to explain the Doppler effect for light, calculate the red shift parameter z and recession velocity for speeds much less than the speed of light, and distinguish Doppler red shift from cosmological red shift. Hubble's law links recession velocity to distance, and the Hubble constant gives an estimate of the age of the universe. Examiners tend to set a calculation chain from wavelengths to z to velocity to distance, a unit conversion of the Hubble constant into s-1 and a short explanation of what red shift tells us.
What you need to know
- Doppler effect When a source moves relative to an observer, the observed wavelength changes. A source moving away gives a longer wavelength (red shift); one moving towards gives a shorter wavelength (blue shift).
- Red shift parameter z = Δλ/λ, where Δλ is the change in wavelength and λ the emitted (laboratory) wavelength. For v much less than c, z = v/c.
- Cosmological red shift Doppler red shift is caused by a source moving through space. Cosmological red shift is caused by space itself expanding while the light travels, stretching its wavelength.
- Hubble's law Recession velocity is proportional to distance: v = H0 d. The Hubble constant is often quoted around 70 km s-1 Mpc-1; distant galaxies receding faster supports an expanding universe.
- Age of the universe If galaxies have moved apart at constant speed since the Big Bang, the age is about 1/H0. H0 must be in s-1 first, giving an age of roughly 1.4 x 10^10 years for H0 = 70 km s-1 Mpc-1.
Common mistakes in 2.7 Astronomy
- Dividing Δλ by the observed wavelength; z uses the emitted (rest) wavelength.
- Forgetting to convert km s-1 to m s-1, or Mpc to m, before calculating 1/H0.
- Saying red shift means a galaxy is red; it means the spectral lines are shifted to longer wavelengths.
- Treating cosmological red shift as galaxies flying through space; it is due to the expansion of space itself.
Try three 2.7 Astronomy exam-style questions
1. A hydrogen line with laboratory wavelength 656.3 nm is observed at 669.4 nm in a galaxy's spectrum. Calculate z and the recession velocity. (3 marks)
Show the mark scheme answer
Δλ = 13.1 nm; z = 13.1/656.3 = 0.020; v = zc = 0.020 x 3.0 x 10^8 = 6.0 x 10^6 m s-1.
2. Using H0 = 70 km s-1 Mpc-1, calculate the distance to the galaxy in the previous question in Mpc. (2 marks)
Show the mark scheme answer
v = 6.0 x 10^3 km s-1; d = v/H0 = 6000/70 = 86 Mpc.
3. Estimate the age of the universe for H0 = 70 km s-1 Mpc-1. 1 Mpc = 3.09 x 10^22 m. (3 marks)
Show the mark scheme answer
H0 = 70 x 10^3 / 3.09 x 10^22 = 2.27 x 10^-18 s-1; age = 1/H0 = 4.4 x 10^17 s; = 4.4 x 10^17 / 3.16 x 10^7 = 1.4 x 10^10 years.
2.7 Astronomy FAQs
What does CCEA A Level Physics 2.7 Astronomy cover?
Doppler shift of light, red shift and the parameter z, the difference between Doppler and cosmological red shift, Hubble's law and estimating the age of the universe.
Which unit is 2.7 examined in?
It is in Unit AS 2: Waves, Photons and Astronomy.
What equations do I need for Astronomy?
z = Δλ/λ, z = v/c for v much less than c, v = H0 d and age of the universe approximately 1/H0, with care over unit conversions.
