Doppler red-shift

• As discussed in section 3 (see pages 115-16), the Doppler effect is the apparent change in frequency of a wave caused by the relative motion between the source of the wave and the observer.
• If a distant star and the Earth were stationary objects, the frequency of light observed on the Earth from the star would appear the same as the frequency from a similar source on Earth.
• Visible light is dispersed to form a spectrum of colours (see page 112). When astronomers view the light from a star or a galaxy, they see black lines in the spectrum. These black lines are caused by various elements absorbing particular wavelengths of light; for example, our Sun contains helium and hydrogen and the (simplified) spectrum from the Sun is shown below. This is known as an absorption spectrum.

• If a star or galaxy containing hydrogen and helium were moving away from the Earth, the frequency of these black lines would appear lower (longer wavelength). This is known as red-shift. The visible light observed is shifting towards the red end of the spectrum.
• If a star or galaxy containing hydrogen and helium were moving towards the Earth, the frequency of these black lines would appear higher (shorter wavelength). This is known as blue-shift. The visible light observed is shifting towards the blue end of the spectrum.