GALAXY
A galaxy is a gravitationally bound group of stars, stellar remnants, interstellar gas, dust, and dark matter; most have a supermassive black hole at their center. They can range in size from a dwarf with a few million stars and a diameter of just a couple thousand light-years, to a bona fide giant; hundreds of thousands of light-years across, with trillions of stars. It is believed there are more than two trillion galaxies in the universe; they are organized into groups that form a vast intergalactic web of galaxies known as superclusters.
CLASS E: ELLIPTICAL
Elliptical galaxies are shaped like a spheroid. They are typically brightest in the center and grow dimmer toward the edges. Most elliptical galaxies contain older, low mass stars and can range from a few thousand light-years across, to several hundred thousand. Most elliptical galaxies are formed as the result of mergers between other galaxies. Elliptical galaxies are classified on a scale of E0 to E7, where E0 is nearly spherical, and E7 is shaped more like an oval.
.png)
CLASS I: IRREGULAR
Irregular galaxies are most notable for their structure--or lack thereof. While some may feature a bar formation and areas of active star formation, they generally do not have enough structure to characterize them as spiral or elliptical.
CLASS L: LENTICULAR
A lenticular galaxy falls somewhere between a spiral and an elliptical galaxy, often demonstrating features of both, such as a spherical central bulge surrounded by indistinct arm-like structures.
CLASS R: RING
Ring galaxies are defined by a core of older stars encircled by a ring of luminous, young stars. They are formed when a smaller galaxy passes through a larger one.
CLASS S: SPIRAL
Spiral galaxies are among the most common in the universe and feature a large central bulge (often with a super-massive black hole in the center), surrounded by several spiraled bands. Subgroups include Sa, Sb, and Sc, where Sa galaxies have several tightly wound arms, Sb have a few loosely wound arms, and Sc have one or two arms that do not wrap around the galactic core.
CLASS SB: SPIRAL BARRED
A barred-spiral galaxy is very similar to its spiraled counterpart, with one notable difference--the arms spiral out from a bar of stars in the center. About a third of all spiral galaxies have a bar. Subgroups include SBa, SBb, and SBc. An SBa has a short bar of stars at the center, while an SBc has a long, well-defined bar.
