Galaxy Spectra

When you look at the spectrum of a galaxy, you are really looking at the combination of spectra from the millions of stars in the galaxy. So studying the features of a galaxy spectrum tells you about the types of stars the galaxy contains, and the relative abundances of each type of star.

M51 Courtesy of The Hubble Heritage Project

Galaxy spectra also clearly show you whether a galaxy contains star-forming regions called HII regions. HII is a spectral emission line that corresponds to ionized hydrogen - a hydrogen atom that has lost its electron. HII regions are areas of a galaxy where hydrogen nuclei and electrons are recombining to form neutral hydrogen.

When an electron recombines with a hydrogen nucleus, it loses energy and gives off a photon (you might recognize this as the opposite of what happens in absorption lines from the Spectral Types project). The HII spectral line is in the red part of the spectrum, so HII regions in galaxies have a beautiful red or pink tint in visible photographs (NOTE: SDSS uses the red filter for the green picture in a tri-color image. Therefore, HII regions do no appear red in SDSS images). You can clearly see HII regions in the photograph of M51 at the right.

Now, let's examine several galaxies both visually and spectroscopically.

Galaxies do change as they age. Older galaxies have few young blue stars. Younger galaxies have lots of HII regions where stars are forming.