What Is A Galaxy?

A Galaxy is a large structure in the universe composed of a multitude of stars. All are bound together in cyclic motion within the gravitational forces of the galaxy — which often have a massive black hole at their center.

(Whether black holes form the galaxy, or the galaxy forms the black hole, is not clear. It is possible that both can be true.)

The Milky Way Galaxy (our home galaxy where our star, the Sun, was born) contains about 200 billion stars. Nor is the Milky Way especially large. Massive galaxies might hold trillions of stars, while small galaxies only in the hundreds of thousands.

Galaxies are formed from vast, spinning clouds of gas and dust (in essentially the same process by which stars are formed, but on a higher order of magnitude where mass and energy are concerned). It is from this massive gravitational collapse that many black holes are formed as well, at the center.

The greatest density at the center might form a black hole, that is the heart of the galaxy, while less dense regions in the cloud further from the center form stars and planets.

As these clouds collapse, they spin, creating great angular momentum, and increasing thermal and kinetic energy. They continue to spin, which effects the speed of rotation of the galaxy, as well as its shape.

The Four Basic Types of Galaxy

Elliptical Galaxy

Elliptical galaxies range from being spherical to slightly elongated spheres (egg-shaped).

These galaxies contain mostly old stars, indicating that they are probably the oldest type of galaxy too — the first formed after the Big Bang.

Elliptical galaxies are the most common type of galaxy that we see in the universe.

Spiral Galaxy

Spiral Galaxies are different from elliptical galaxies, in that they are shaped like a disc, with great spiral arms reaching out from the center when viewed from above (compared to the roughly spherical shapes of elliptical galaxies).

The older stars exist in the nucleus of the spiral Galaxy, it’s center (near an active black hole that is probably at the center of most spiral galaxies). The great clouds that formed the black hole, also formed the old stars long ago (billions of years in the past).

Most material that remains in the form of stellar nebulae exists in the spiral arms, where the younger stars live, and where new stars are formed. (The Sun resides about 2/3 of the way out from the center of the Milky Way on a spiral arm.)

Barred-Spiral Galaxy

Barred-Spiral Galaxies are a type of spiral Galaxy, but instead of the spiral arms extending from the center of the galaxy, there is a bar shape across the nucleus which the spiral arms extend from.

Irregular Galaxy

Some Irregular Galaxies have the hint of a spiral structure (like the diffuse remnants of a whirlpool as it begins to fade) like the galaxy was once spinning, but has since ceased.

Other Irregular galaxies have no hint of a spiral structure.

These are the rarest types of galaxy.

Classifying Galaxies By Their Structure

Elliptical Galaxies are classified from E0 (spherical) to E7 (very flattened) according to how spherical or elongated the galaxy is.

Likewise Spiral (S) and Barred-Spiral (SB) are classified from a to c depending on how compact the central nucleus of the galaxy is, and how tight the arms are.

Irregular Galaxies (Irr) can also be divided into two types, I and II.

Life Cycle of a Galaxy

Quasars — quasi-stellar objects — are incredibly active and turbulent objects in space. These are some of the brightest objects in the universe, and also some of the most distant things that we can see from Earth.

It is believed that Quasars are the dying heart of dead galaxies, the cores of the first galaxies of the universe, which have since dispersed, leaving only the quasar remaining.

I’m the same way that a White Dwarf is the final stage of life for a small-to-average sized star, a quasar is probably the final stage of life of some galaxies.

Notes

The Andromeda Galaxy is very far away from us, at about 2.2 million light-years distance. Meaning that it takes 2.2 million years for the light from Andromeda to reach us.

Our image of the Andromeda Galaxy when we see it, is actually the galaxy as it appeared 2.2 million years ago. A giant space monster could have eaten it by now. Who knows?