These are the irregulars. These galaxies have no identifiable form. Their stars, gas, and dust spread randomly. These are the smallest galaxies, and may contain as few as one million stars. Many small irregular galaxies orbit the Milky Way.
Astronomers also categorize galaxies by how much energy they produce in their cores.
The most powerful active galaxies are quasars. They are the brightest and most distant objects in the universe. A quasar may emit more energy than an entire galaxy of stars from a region no bigger than our own solar system. Astronomers believe these objects contain supermassive black holes at their hearts, which are encircled by disks of gas. A black hole is an object that is squeezed together so tightly that it has extremely strong gravity.