Scientists and astronomers believe that galaxies were formed within milliseconds after the cosmic “big bang” around 10-20 billion years ago.
The explosion cased gas clouds to collapse, coalesce, mix, and compress under gravitational forces to help to build the materials for galaxies.
There are a few theories on how the galaxies themselves formed. One theory is that smaller groups of around a million stars called “globular clusters” began to form and then gather into what we see as galaxies.
Another theory is that the galaxies were formed first and then later the stars started to move into the clusters.
Edwin P. Hubble created the galaxy classifications and then they were given more detail later by Gerard de Vaucouleurs, another famous astronomer.
There are three major galaxy types and then additional details based on the characteristics of the galaxy including the size and extent of bars, the spiral openness, and the galactic bulge size.
More characteristics were given based on the age spectrum of a galaxy’s stars and the star-formation rate.
Spiral Galaxies are the most common type of known galaxy in our universe. They are large disks of nebulae and stars that rotate and are surrounded by a dark matter shell.
The middle area at the center of the galaxy is brightly lit and called the “galactic bulge.” A lot of spiral galaxies have star clusters and halos of stars below and above the disk.
Those spiral galaxies that have a bright band or bar of material and stars around the center sections are called “barred spirals,” and these make up a majority of galaxy types.
Other spiral galaxies may also have supermassive black holes at the center. The subgroups of spiral galaxies are classified by the spiral arms, how tight or loose the arms are wound, and the bulges.
Elliptical Galaxies are oval or elliptical, also called egg-shaped. These are mostly in smaller groups and galaxy clusters, and contain lower mass, older stars.
There is usually very little new star formations in elliptical galaxies because they don’t contain the required dust clouds and gases. Elliptical galaxies vary in the number of stars that they can have; from a few hundred million to potentially a hundred trillion.
They also come in a variety of sizes with some a few thousand light-years across to others that are a few hundred thousand light-years across.
The subgroups of elliptical galaxies include those that have characteristics that are between globular clusters and standard ellipticals. These are called “dwarf ellipticals.”
Irregular Galaxies are kind of a catchall for those that don’t have a shape that is common but also is irregular in shape.
The best example of an irregular galaxy can be seen in our night sky: the Small Magellanic Cloud. They may have a bar structure, can have some regions for star nurseries, and some that are smaller are called “dwarf irregulars.”
The lack of structure is what characterizes irregular galaxies.
Galaxies can exist as a single galaxy or in pairs. Galaxies are usually part of a larger group known as a supercluster, cluster, or just group.
When in a group, galaxies interact with each other and even collide and merge. Galaxy mergers are an interactivity of gravity that makes the gases move to the centers and cause fast star formations.
Our Milky Way Galaxy is on a collision course with the Andromeda Galaxy, which is two million light-years away.
Scientists have a theory that the processes of intergalactic interaction may be part of a natural occurrence allowing irregular galaxies to become other types.
The word “galaxy” comes from “galaxias,” a Greek word that means “milky.” So far, we have observed 170 million galaxies.
Star nurseries are in the spiral arms of spiral galaxies that contain dense clouds of dust and hydrogen gas.
The Hubble classification scheme for spiral galaxies start with the letter “S” and the details are followed with how tight the spiral arms are with letters “a, b, or c” and if it’s a barred spiral with the letters “SB.”
Elliptical galaxies are thought to be older than spiral galaxies due to spiral galaxies quickly burning their dust and gas content, eventually becoming elliptical galaxies.