Doppler effect (A-level only): Galaxies and quasars.

The idea

Galaxies are vast gravitationally bound systems of stars, gas, dust and dark matter; our Milky Way is one example. Their spectra contain absorption and emission lines, so the observed wavelengths identify chemical composition and reveal motion through the Doppler effect. Quasars are extremely luminous active galactic nuclei powered by supermassive black holes accreting matter; they can outshine their host galaxy and are observed at great distances, so their light was emitted when the universe was younger. In this section you interpret galactic and quasar spectra, recognise large redshifts, and relate observed wavelength shifts to recession speed using Δλ/λ₀ = v/c for speeds much less than c. You should distinguish a galaxy from a quasar by scale, luminosity and spectral appearance, and explain why quasars are useful probes of the distant universe.

What earns the marks

• A galaxy is a gravitationally bound system containing stars, gas, dust and dark matter; the Milky Way is the galaxy containing the Solar System.
• A quasar is an extremely luminous active galactic nucleus powered by accretion onto a supermassive black hole, often outshining its host galaxy.
• Spectral lines from galaxies and quasars are compared with laboratory reference wavelengths to identify redshift or blueshift.
• For speeds much less than c, the Doppler shift is approximated by Δλ/λ₀ = v/c, where Δλ is the observed minus rest wavelength and λ₀ is the rest wavelength.
• Large redshifts of quasars indicate high recession speeds and great distances, so they are observed as they were when the universe was younger.
• Quasars are not simply distant galaxies; they are compact, exceptionally luminous active nuclei, whereas galaxies are extended systems of stars and gas.

Common mistakes and how to put them right

• Treating a quasar as just a very distant galaxy.

  Put it right by stating that a quasar is an active galactic nucleus powered by a supermassive black hole and can outshine its host galaxy.

• Using the relativistic Doppler formula when the non-relativistic approximation Δλ/λ₀ = v/c is intended.

  Put it right by checking that the recession speed is much less than c before using the simple formula.

• Confusing redshift with blueshift.

  Put it right by remembering that redshift means observed wavelengths are longer than rest wavelengths, which corresponds to recession.

• Forgetting to convert wavelength units consistently.

  Put it right by converting all wavelengths to the same unit, such as nanometres or metres, before calculating Δλ/λ₀.

Exam tips

• When comparing spectra, state clearly whether lines are shifted to longer or shorter wavelengths and link this to motion away from or towards the observer.
• Show the substitution into Δλ/λ₀ = v/c, including the rest wavelength λ₀, before calculating the speed.
• Use the term active galactic nucleus when explaining quasar energy sources, and avoid saying that a quasar is simply a bright star.
• If a question asks why quasars are useful, refer to their extreme luminosity allowing them to be observed at large distances and their large redshifts indicating early cosmic times.