Massive Star Formation and the Interstellar Medium in Nearby Galaxies Four edge-on spiral galaxies, in the light of H-alpha, which traces ionized Hydrogen. This image shows the wide range of properties that galaxies exhibit. The top galaxy, NGC 891, is very actively forming stars, and has a bright H-alpha halo. The second galaxy, NGC 4631, is also actively forming stars, and is distorted by gravitational interactions with another galaxy. The third galaxy, UGC 9242, has a more quiescent disk, but there is very violent star formation occuring in the bright nucleus. The bottom galaxy, NGC 4244, is a small, inactive galaxy.