The measurement of the gravitational form of this phenomenon was one of the three classical tests of general relativity. For 10 points each:
[10e] Name this phenomenon, which causes galaxies travelling away from Earth to emit higher-wavelength photons.
ANSWER: redshift [prompt on Doppler effect]
[10h] Colour excess is a measure of the reddening due to this astronomical phenomenon, the absorption and scattering of light by dust and gas between the object and the observer. This phenomenon causes reddening because blue light is more strongly attenuated than red light.
ANSWER: extinction
[10m] This is the name given to the gas and dust between stars. It consists mainly of hydrogen and helium.
ANSWER: interstellar medium