Other galaxy classifications

The classification of galaxies by shape relies upon images made with visible light. But galaxies observed in other wavelengths, or with longer exposure times, or at earlier cosmic epochs, show the need for additional categories. Many such categories exist. They may indicate different ways that galaxies form, or may show various stages through which galaxies evolve. These videos are visualizations of these other galaxy types.

The center of a galaxy can be a source of stupendous energy. Such active galactic nuclei are among the most luminous objects in the universe. Radio observations sometimes reveal jets extending well beyond the visible galaxy. The emitted energy comes from matter falling into supermassive black holes.

In starburst galaxies, stars are born at prodigious rates. These galaxies typically have many pockets of intense star formation that glow with light from hot blue stars. An intriguing subclass called “faint blue galaxies” reveals an energetic episode, about five billion years ago, that has no known counterpart in nearby galaxies.

"Ghostly" is how one might describe low-surface-brightness galaxies. Though many of them are large, these galaxies have so few stars that they are hard to detect. Surveys using long exposures or contrast enhancement have revealed many more low-surface-brightness galaxies than galaxies of normal surface brightness.