Absorption spectroscopy

An absorption spectrum is made up of a continues spectrum with lines missing. Compare the emission spectrum and the absorption spectrum of helium below. You will notice that the absorption spectrum contains black lines at exactly the same position as the lines in the emission spectrum appear.

The spectrum that originates from stars is usually an absorption spectrum. Just like an emission spectra, absorption spectra can be used to identify elements present in stars.

Absorption spectra can also reveal another aspect of a distant star, its velocity. When we look at the absorption spectra of distant stars we may see that the lines are shifted in the red or blue direction of the spectrum. A shift in the red direction, as shown below, known as redshift, indicates that the star is moving away from us at a velocity that is proportional to the amount the spectrum has shifted towards the red. The greater the shift the greater the velocity of the star. A shift in the blue direction, blueshift,indicates that the star is moving towards us at a velocity proportional to the amount of shift in the blue direction.

Being able to measure the velocity of an astronomical object gives us a deeper understanding of our Universe. For example we know how galaxies rotate by looking at their spectra. Knowing the velocity of a star, within a galaxy, we can use it to :
- calculate the mass of the star;
- infer the existence of dark matter. Astronomers have calculated the velocity of stars from the centre of galaxies and have suggested that there is more matter in the galaxy than is visible. When astronomers do their calculations they find that, by some estimates, 90% of the mass in the universe is missing. Atronomers have therefoire suggested the existence of dark matter.
- show expansion of the Universe. Edwin Hubble observed that more distant galaxies tended to have more redshifted spectra.
  How are spectra formed?