What is a Galaxy?

A close-up picture of Messier 31, better known as the Andromeda Galaxy. M31 is the closest spiral galaxy to us at 2.5 million light years away and is slightly larger than the Milky Way.
Credit: Adam Evans

Galaxies are gravitationallybound systems of stars, nebulae, interstellar dust, gas, and dark matter. There are countless galaxies in the universe, all different ages, sizes, and types. And, on average, each of these galaxies contains billions upon billions of stars and planets. At this immense scale, the universe seems unfathomable and infinite. However, scientists have learned a lot about galaxies, from the closest ones to those at the edge of the observable universe. Despite their vast knowledge, like many scientific endeavors, scientists have discovered more questions than answers about the cosmos. 

How were galaxies discovered?

The term “galaxy” comes from the Greek word galaxías, which means “milky”. Since galaxies were thought to be similar to our own, the Milky Way, it made sense to name them accordingly. The Milky Way’s name comes from its appearance in the night sky, as if someone spilled milk across the sky. 

Up until the turn of the 20th century, scientists did not know that galaxies existed. Although some of the larger and closer galaxies were charted by astronomers, they were classified as nebulae, clouds of interstellar gas in which stars are born. In fact, in the 18th century, Charles Messier, known for his catalogue of these types of objects, called the first galaxies he saw in his telescope “spiral nebulae.” Unbeknownst to him, these mysterious objects were much larger and farther away than regular nebulae.

By the early 1900’s, developments in astronomical techniques enabled astronomers to determine the distances to these “nebulae.” Because of this, the idea of what we now know as a galaxy was born. Due to the uncertainties in the calculations of these distances, some still argued that these spiral nebulae were inside our own galaxy, the Milky Way. However, an American astronomer, Edwin Hubble,  accurately determined the distances to a number of nearby galaxies, thus solidifying the idea that they were well beyond the Milky Way. 

What are the different types of galaxies?

Galaxies are classified by what is known as their morphology. There are three main morphological types: elliptical, irregular, and spiral. Each of these classifications has sub-classes, such as “barred” for spiral galaxies. In addition to the four main types, there are dwarf galaxies, which actually account for most of the galaxies in the universe. Dwarf galaxies can be classified as elliptical (sometimes called spheroidal), irregular, and spiral. Other types of galaxies include lenticular, peculiar, and ultra-diffuse, among others. 

A section of the Hubble Legacy Field, a deep-field image of galaxies from the Hubble Space Telescope. Every single point of light in this picture is a galaxy, each of a different type and at a different distance.
Credit: NASA

The types, as you might expect, were originally developed to correspond with the appearances of the galaxies. Irregular galaxies look like a randomly distributed cloud of stars and gas. Elliptical galaxies, unsurprisingly, look like ellipsoids. Spiral galaxies, probably the most well-known type, have spiral arms and a central nucleus. Dwarf galaxies are similar to the other types, but, as their name suggests, they are much smaller in overall size. Lenticular galaxies are often classified as a cross between elliptical and spiral galaxies.

In terms of differences between morphological types, there is more than just shape and size. One of the main differences between the classes is their overall stellar composition and age. In general, elliptical galaxies have older, redder stellar populations, whereas irregular galaxies usually have younger, bluer stellar populations. Spiral and lenticular galaxies lie somewhere in between; spiral arms are sources of star formation, whereas nuclei contain older stars. 

How do galaxies evolve?

Another important part of morphology is galactic evolution. Initially, astronomers thought that elliptical galaxies evolved into spiral and lenticular galaxies. We now know that this is not correct. Today, there are two opposing theories of galactic evolution. The “top-down” or monolithic collapse model, suggests that galaxies as they appear now formed directly from dust and gas. The “bottom-up” or hierarchical clustering model suggests that there were intermediate phases in the size and morphology of galaxies.

The top-down model suggests that all galaxies formed from a universe-wide cloud of dust and gas, just as stars and planets form in star systems. This would explain why stars and globular clusters (large spherical clusters of stars) in the halo of spiral galaxies are older and contain fewer heavy elements than those in the disk of the galaxy. One of the main problems with this is that developments in cosmology have shown that the existence of such a cloud of dust is highly improbable. In essence, this theory does not work with the current model of cosmology.

A collage of galaxies colliding, a process commonly referred to as galactic cannibalism. Galaxies go through a back-and-forth gravitational “dance,” in which countless stars are thrown out into empty space.
Credit: NASA

The bottom-up model suggests that larger galaxies form through the collisions of smaller dwarf galaxies. Essentially, sometime after the Big Bang, the universe formed stars and globular clusters, which coalesced into the first dwarf galaxies. These dwarf galaxies eventually followed suit and formed some of the larger galaxies we see today, even our own. Current evidence points to this model of galactic evolution as being correct, or at least more so than the previous one.

A key principle of astronomy is “look-back time,” which essentially says that the farther you look in space, the farther back in time you are looking. This is a factor of the speed of light being finite; it takes time for the light to get from the object to you. With this in mind, astronomers can see how galaxies evolved over time by simply looking farther away. What they discovered was that galaxies are smaller and less developed the farther they look. This suggests that the bottom-up model is right. The theory also incorporates dark matter, which makes it more compatible with modern cosmology. The theory suggests that the first galaxies began to form about 13 billion years ago. Due to this fact, the bottom-up model is the predominant theory of galactic evolution. 

How do galaxies interact with each other?

As mentioned in our discussion of the bottom-up model of galaxy formation, galaxies collide with one another. We have observational proof of this process, which astronomers often refer to as galactic cannibalism. In fact, evidence suggests that our galactic neighbor, the Andromeda Galaxy, is on a collision course with the Milky Way. Since Andromeda is about 2.5 million light years away, the event won’t start for at least a few billion years—so I wouldn’t worry about it too much! In this type of galactic dance, some stars from the galaxies involved are thrown out into intergalactic space. Fortunately, the odds of stars colliding with one another are slim because of the vast amount of space in between them. 

Galaxies organize themselves according to the large-scale structure of the universe and local gravitational interactions. We know that the Milky Way is part of a local neighborhood of about a few dozen galaxies, succinctly called the Local Group. This group of galaxies is located in the Virgo Cluster, a larger group of around 1000 galaxies, the majority of which lie in the direction of the constellation Virgo. This cluster is on the outskirts of an even larger cluster called the Virgo Supercluster, which is also found in Virgo. The supercluster contains thousands of galaxies and is about 110 million light years across. 

A model of the Local Group containing most of the galaxies that we know exist in it. The Milky Way and Andromeda (M31) are the largest galaxies in the group and each have a number of satellite galaxies.
Credit: Antonio Ciccolella

It does not stop there. The Virgo Supercluster is part of a larger galaxy cluster, the Laniakea Supercluster, which is part of the Pisces-Cetus Supercluster Complex, a filament of galaxies. This is one of many filaments that make up the observable universe. These filaments represent the largest type of organized structure of matter that we know of. Each of them contain billions of galaxies, which together form the large-scale structure of the universe, the “cosmic web.” 

Galaxies are a fundamental structure in the universe, just as stars are fundamental in galaxies, and planets in star systems. Overall, the observable universe is about 92 billion light years across, and, as modern estimates suggest, contains more than a trillion galaxies. In other words, there are more galaxies in the cosmos than there are grains of sand on Earth.

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