A galaxy is a massive, gravitationally bound system consisting of stars, an interstellar medium of gas and dust, and dark matter all orbiting a center of mass that can have anywhere between 10 million and 1 trillion stars.  We live in the Milky Way galaxy.

Formation of Galaxies

     The leading theory on the formation of galaxies revolves around the big bang theory. According to the standard Big Bang theory, our universe sprang into existence as a "singularity" around 13.7 billion years ago.   Singularities are zones which defy our current understanding of physics. They are thought to exist at the core of black holes. Black holes are areas of intense gravitational pressure. The pressure is thought to be so intense that finite matter is actually squished into infinite density. These zones of infinite density are called "singularities." Our universe is thought to have begun as a small, hot, dense, singularity. Where did it come from? We don't know?  It began to inflate, the Big Bang, it expanded and cooled, going from very, small and very, hot, to the size and temperature of our current universe. It continues to expand and cool to this day and we exist inside of it.  One common misconception about the Big Bang theory is that it involves an explosion but it doesn’t.  Instead the galaxies are just expanding and moving outward.

Types of Galaxies

There are many types of galaxies but the main ones are the elliptical, dwarf, ring, and spiral galaxies:

The elliptical galaxy resembles an egg; it has an elliptical shape from every angle you view it from. This type of galaxy has the most volume making it the largest type of galaxy.

Dwarf galaxies are around 1/100 the size of the Milky Way and they actually orbit around larger galaxies. They are by far the most numerous type of galaxy. They can also be classified again as elliptical, irregular, ring, and spiral galaxies.

Ring galaxies form when one galaxy passes through the center of another galaxy. It causes rings of stars to form around a bare middle.

There are two different types of spiral galaxies, an unbarred spiral galaxy and a barred spiral galaxy. An unbarred spiral galaxy has no distinct arms and looks like a solid disk with a bulge in the middle. A barred spiral galaxy has distinct arms that rotate around the core. The Milky Way galaxy is a barred spiral galaxy.

 

View a video that explains the Big Bang Theory in detail here!