Galaxies with shapes that fall between the elliptical and spiral categories—such as NGC 1201 in Fornax—are called lenticular galaxies. Lenticulars have a central nucleus as spiral galaxies do, but, like the ellipticals, show little or no evidence of spiral structure. They contain mostly old stars and very little gas.
A fifth category contains those galaxies with no regular structure—irregular galaxies.
Only 5 percent of bright galaxies can be called irregular.
These sprawling conglomerations of stars may have conspicuous bars, or even hints of spiral structure. Intense bursts of star formation dominate the appearance of some irregulars. Look for "hotspots" of bright young clusters and ionized gas. The Magellanic
Clouds, easily seen with the naked eye from the Southern Hemisphere, are irregular galaxies that contain many star-forming regions.
A galaxy that appears to have suffered a severe disturbance is known as a peculiar galaxy. One member of this class is the elliptical NGC 5128.
Active galaxies are those that have unusually energetic cores. Quasars are extremely distant objects believed to be the cores of active galaxies. They stand out only because they are exceptionally bright.