Galaxies are generally classified by their shape or morphology, with some other relevant features, if exist. When galaxies were classified originally, there were too many types, like spiral galaxies, barred-spiral galaxies, peculiar galaxies etc. Astronomers have grouped all these types into three types:
In last few years, astronomers have learnt too many new things about morphology and other properties of galaxies. Galactic content, stellar content and gas and dust masses varies with galaxy morphology.
Disc galaxies have a pancake like disc as one of their components. Disc galaxies have basically 4 parts, a disc, a central bulge, halo and arms. The disk of a galaxy contains stars, dust and gas. In the center of disc, is the bulge. Bulge is almost sperical in shape and is mainly composed of stars with some gas and dust. Arms are long, bright features that originate from central bulge and extend upto edge of disc galaxy in the shape of an arc. Halo is the remaining part of the galaxy. It is spheroidal in shape and contains gas, dust and stars, but contains dark matter mainly.
Our own galaxy, Milku Way is a disc galaxy. On a clear night, Milky Way appears as a thin band of light. Viewing from southern hemisphere of earth, its central bulge becomes visible.
Disc galaxies can be further categorized into following types:
Spiral galaxies have spiral arms starting from central bulge to the end of galaxy disc. This is why these galaxies are called spiral galaxies. Central bulge of the spiral galaxy can be small or too large. Spiral arms are the most prominent part of the spiral galaxy. Like the constituents of galaxy, spiral arms also contain stars, gas and dust. Stars in spiral arms are brightest and youngest than stars that are between the arms. Due these brightest stars, spiral arms are prominent.
Barred spiral galaxies are like normal spiral galaxies with one main difference. Instead of having a round bulge in the center, as in the case of normal spiral galaxies, barred spiral galaxies have a barred or elongated bulge (hence the name barred spiral galaxies) in the center. Spiral arms are same as that of normal spiral galaxies. Another noticeable difference is that in normal spiral galaxies, spiral arms originate from all sizes of the rounded bulge, but in barred spiral galaxies, spiral arms originate only from elongated ends of the bulge.
Milky way is a barred spiral galaxy having central bulge of the shape of peanut.
Name of these galaxies is self describing, galaxies of huge size. These galaxies can contain trillion of stars as compared to normal spiral and barred spiral galaxies, which have a few billion of stars. Most interesting feature of these galaxies is that they have satellite galaxies, i.e; small galaxies that orbit these gaint galaxies and gravitationally bound to galaxies they orbit.
Spheroidal galaxies are other dominating galaxies in the universe. These galaxies are spherical in shape.
Difference between spheroidal and disc galaxies is that spheroidal galaxies, as a group containg less gas and dust than disk galaxies, and some spheroidal galaxies do not contain gas and dust at all. Spheroidal galaxies are both largest and smallest in universe and these are the most common type of galaxies in the universe, since most dwarf galaxies are spheroidal and number of dwarf galaxies is much more than large or average sized galaxies.
Spheroidal galaxies can be further categorized into following categories:
As the name suggests, these galaxies are small in size and spheroidal in shape. These are a few parsecs in diamter and contain a few hundred millions o stars. Very few of these galaxies exist alone. Most of these galaxies orbit large or giant galaxies.
Astronomers have found even small galaxies containing about 10 million of stars and a few hundred parsecs across. Due high density of stars, these galaxies are called ultra compact dwarf spheroidal galaxies.
Elliptical galaxies are ellipsoidal in shapet. Astronomers hypothesized these three dimensional ellipsoidal galaxies. Elliptical galaxies are composed of older low mass stars and their star formation rate is slow. These galaxies are surrounded by globular clusters an they mainly find close to the center of galaxy cluster.
Some galaxies are neither perfectly disc nor spheroidal. These galaxies come in category of other galaxies.
Lenticular galaxies are intermediate between disc and spheroidal galaxies. These galaxies have very less interstellar matter, and thus star formation is slow. These galaxies mostly contain aging an dying stars.
Irregular galaxies do not have any regular shape like disc or spheroidal galaxies. Irregular galaxies contain abundant amounts of gas and dust. These galaxies considered to be once disc or elliptical galaxies but deformed due to disorders in gravitational pull.
Peculiar galaxy is not a single galaxy, but multiple galaxies that are in the process of merging into a single galaxy. Peculiar galaxies are much irregular in shape due to strong gravitational fields from other galaxies. Peculiar galaxies can have irregular amounts of dust and gas and their brightness may be less or more than normal galaxies.