Before looking into what are the different types of galaxies and finding out if there are 3 types of galaxies or 4 types of galaxies, one must be aware and in touch with how are galaxies classified and came into existence to be called as galaxies.
Precisely, there are two theories that explain the origins of galaxies, the bottom-up theory, and the top-down theory.
If we analyze what the bottom-up theory has to say we can put it as when after the big bang several different clusters came into existence which then combined together as larger units to form different types of galaxies. The top-down theory however suggests that different types of galaxies came into formation first and subsequently the stars and other celestial bodies came into existence.
Others also include lenticular galaxies and ultra-diffuse galaxies, there are also starburst galaxies and as technological advancements occur, we will discover deeper and deeper into space where we would find more than what we know now.
The Scientific Background
This classification into 3 types of galaxies was in part because of the system that the scientists used to group the types of the galaxy based merely on their shapes. Out of these sequences, the one that was most widely accepted was by Edwin Hubble where the galaxy classification was under 4 types of galaxies as mentioned above.
However, out of these kinds of galaxies, spiral galaxy has been the most common to date where if we want to know what type of galaxy is the milky way, then that too is a spiral. However, the answer to what are the different types of galaxies were studied in detail by Vaucouleurs where they categorized it into mainly 3 types of galaxies, as mentioned above.
As we now know about how and what are the three types of galaxies, we would look into all the different types of spiral galaxies, elliptical galaxies, and irregular galaxies.
1. Spiral Galaxies
The three main components that make up a spiral galaxy are the center called bulge, the disk that makes the arms and the last part is a halo. Now, what are galaxies that are spiral, they are basically a mesh of older stars where different components of the spiral hold some form of celestial bodies.
They even include dust, gases, and even younger stars. If we were to look at our milky way, that too is a spiral where our Sun is in the arm of it. As far as the halo is concerned, it consists of stars that are old and is in a loose structural form.
How are galaxies classified under spirals?
As there two-thirds of all the galaxy types are spirals there are further classifications under the spiral galaxies that are ordinary and barred. In ordinary spirals, the arms are connected directly to the bulge or nucleus whereas in barred, there we see a bar running through the bulge after which we see arms in a spiral from that.
The different classes of galaxies have been classified by “a”, “b”, “c”, “d” and so on where these numbers are given according to the tightness of the spiral where the Sa is the tightest spiral, almost like a circle.
3. Irregular Galaxies
As for irregular galaxies, and to simply put it, as their name suggests are irregular with no definite shape of symmetry. The shape they take is more of a cloud than a recognizable one. Now since these galaxies do not show any form of a pattern or continuity, these classes of galaxies are the smallest of the galaxies where because of no definitive start or end are difficult to classify.
Some of the astronomers are even of the view that they may serve as more of a building block for other types of galaxies to form. There is also a notion that these galaxies were once in some sort of a spiral or elliptical shape but due to strong or uneven external gravity, deformed to be as scattered as they are.
These galaxies are mainly a mesh of large amounts of gas, dust, black holes, formations of nebulous, stars, and various different elements that are similar or common with other galaxies. The explanation for this to happen is more likely galaxy cannibalism or two galaxies colliding to form an irregularly shaped galaxy. These types of galaxy are very few in number.