For centuries astronomers peered into the night sky and observed galaxies outside of our own. However, they didn't really understand what they saw.
Until the early 1900s, scientists believed that the entire Universe was contained in the Milky Way Galaxy. The prevailing wisdom was that we lived in a static Universe, one in which the Universe was neither expanding or contracting.
Einstein's revelation of General relativity suggested, from a purely theoretical standpoint that the Universe should be in some state of expansion, but since this conflicted with known observation, a cosmological constant was added to "offset" the expansion.
Then, a young astronomer by the name of Edwin Hubble, for whom the Hubble Space Telescope is named, made a startling discovery. He found a cephid variable star in the object known as the Andromeda Nebula.
Cephid variable stars are used to measure distances in the Universe. Since their oscillation period is proportional to their absolute luminosity, we can calculate their distance from us by measuring their apparent brightness.
According to Hubble's calculations the Andromeda Nebula was far outside the bounds of the Milky Way and he therefore drew the conclusion that Andromeda was not a nebula at all, but rather an entire other galaxy.
Often when we imagine galaxies we picture spiral shapes. Most likely, this is due to our own Milky Way which we believed to be a barred spiral galaxy. However, there are several different types of galaxies, and even then there is considerable variation within each type.
- Spiral Galaxies: Most of the galaxies in the Universe are known as Spiral Galaxies, many of which are actually of the Barred Spiral type. These galaxies are, naturally, characterized by their spiral shape, with arms that sweep around the rest of the galaxy. The "tightness" of the arms is wide ranging, so much so that it can be difficult to tell from optical images that they are even present. There is also a distinctive central bulge. It contains a very high density of stars and is typically the brightest region of the galaxy. Finally, there is a dark matter halo that surrounds spiral galaxies. The mass of the dark matter dwarfs the amount of gas and dust that is present in the rest of the system.
- Ellipticals: Elliptical galaxies, as the name suggests, are egg shaped objects that range from nearly spherical (E0) to football shaped (E7). They lack the highly complex structure found in Spiral or Irregular galaxies. Elliptical galaxies contain mostly older stars. There contains very little interstellar gas, meaning that there is a distinct lack of new star formation. One theory contends that elliptical galaxies are actually the result of mergers of two or more spiral galaxies. In fact new research suggests that in a couple billion years the Andromeda Galaxy will merge with the Milky Way and become a single elliptical galaxy.
- Peculiar Galaxies: Some galaxies have non-uniform size shape or composition. These galaxies are collectively known as peculiar galaxies. They have roughly the same size of spiral or elliptical galaxies. Peculiar galaxies are also thought to result from mergers between galaxies. Eventually, peculiar galaxies may from elliptical galaxies under gravitational interactions.
- Irregular Galaxies: These galaxies are also "peculiar" in shape in that they are non-uniform. However, they are considered different from Peculiar galaxies because they are a couple of orders of magnitude smaller. They usually get their shape from gravitational interactions with other larger, nearby galaxies.
- Dwarf Galaxies: Simply, Dwarf Galaxies are smaller versions of the normal galaxies listed above. In fact, some dwarf galaxies are similar in size to star clusters, however there is one distinct difference: dwarf galaxies contain dark matter.
Galaxy Mergers and Collisions
Galaxies seem to appear in groups and clusters. With galaxies in such close proximity gravitational interactions are inevitable.
Some of these interactions can produce distortions in the galaxies, creating peculiar or irregular galaxies.
Also, galaxies will sometimes merge, forming a new single galaxy. Initially, these mergers may produce a Peculiar galaxy, but ultimately, collisions between two spiral galaxies are expected to collide and form an elliptical galaxy.
Scientists now predict that the Andromeda galaxy and the Milky Way galaxy will merger in several billion years to form a new galaxy. Eventually, it is believed, that the new galaxy could eventually become an elliptical type galaxy.