Types of Galaxies

 4 Main types of Galaxies

1. Spiral Galaxies

A spiral galaxy contains a central bulge surrounded by a rotating flattened disc of young and bright stars in an interstellar medium divided into two or more spiraling arms. The central bulge of these galaxies is made of an extremely dense amount of dim, old stars. In most cases though, it's a supermassive black hole. There are new stars created at the end of each arm of a spiral galaxy. 

Spiral galaxies are divided into three main types depending on how tightly wound their spiral arms are: Sa, Sb and Sc. Sa galaxies have very tightly wound arms around a larger central nucleus. Sc galaxies have very loosely wound arms around a smaller nucleus. Sb's are between, having moderately wound arms around an average sized nucleus. 

Spiral galaxies have a lot of gas, dust and newly forming stars. Since they have a lot of hot, young stars, they are often among the brightest galaxies in the universe. About 20% of all galaxies are spirals. The Milky Way and Andromeda are examples of Spiral Galaxy.

2. Elliptical Galaxies

Elliptical Galaxies are the most abundant type of galaxies found in the universe. But due to their age and dim qualities, they are outshone by younger and brighter collection of stars. Elliptical galaxies lack swirling arms like their counterparts, Spiral Galaxies. Instead they bear the rounded shape of an ellipse, an elongated circle. 

'Hubble tuning-fork' is a system that is used to classify the different types of galaxies found. It organizes galaxies based on their shape. Here, elliptical galaxies are classified by how stretched out they are. There are two main types of elliptical galaxies: E0 and E7. Galaxies classified as E0 appear to be almost perfect circles (a circle is an ellipse) while those listed as E7 seem much longer than they are wide. If viewed from the Earth, sometimes, a galaxy having the E7 shape might appear as an E0 when seen head on. 

Elliptical galaxies have a broader range in size than any other types of galaxies. The smallest are dwarf elliptical galaxies, which can be less than 10 percent the size of the Milky Way. But elliptical galaxies can also stretch to more than a million light-years across, and contain more than ten trillion stars. Elliptical galaxies are found in a large number across the universe because when specific regions of space are studied in depth, more elliptical galaxies are found. Astronomers think such counts are consistent throughout the universe.

3. Lenticular Galaxies

Lenticular galaxies are somewhere between elliptical and spiral galaxies. They exhibit characteristics of both the galaxies combined. These galaxies will have a central bulge and disks but will not have arms like you would find in a spiral galaxy. 'Lenticular' comes from the word 'lens' because there is a bulge in the middle but no extending arms outwards. Lenticular galaxies are unique in the way that they have a visible disk component as well as a prominent bulge component. They have much higher bulge-to-disk ratios than typical spirals. When the galaxies are view head on, they can be mis-identified as being elliptical galaxies.

There are two types of Lenticular galaxies based on the 'Hubble tuning-fork' system: Barred lenticulars and Non-barred lenticulars. Barred lenticular galaxies are classified by the prominence of a central bar. Unfortunately, the properties of bars in lenticular galaxies have not been researched in great detail. Understanding these properties, as well as understanding the formation mechanism for bars, would help clarify the formation or evolution history of lenticular galaxies. Non-barred galaxies basically don't have a bar.

Lenticular galaxies tend to be old galaxies that have used up most of their gas and dust and are just living the twilight years. The twilight years tend to be very long. Its not to say that star creation has stopped totally in the galaxy but the amount is very small compared to other galaxies. The way astronomers determine whether it is an old or a young galaxy is by looking at the colors of the stars. If the stars are mainly red, then the galaxy is assumed to be old whereas blue stars are considered to be of young galaxies. 

4. Irregular Galaxies

Irregular galaxies have no particular shape. They are among the smallest galaxies and are full of gas and dust. Having a lot of gas and dust means that these galaxies have a lot of star formation going on within them. This can make them very bright. They are two small galaxies which orbit around our own Milky Way Galaxy. About 25% of all galaxies are irregulars. Irregular galaxies do not fall into any of the regular classes of the 'Hubble tuning-fork' system and are often chaotic in appearance.

Some irregular galaxies were once spiral or elliptical galaxies but were deformed by an uneven external gravitational force. Irregular galaxies are commonly small, about one tenth the mass of the Milky Way galaxy. Due to their small sizes, they are prone to environmental effects like crashing with large galaxies and intergalactic clouds. They are usually fainter than spiral galaxies. The best known examples of irregular galaxies are the small and large magellanic clouds. These are companion galaxies to our own Milky Way, and can be easily seen at dark sites in the Southern Hemisphere.