A galaxy is a cluster of stars, gas, and dust that are kept together by the force of gravity. Within galaxies, there are also planets, black holes, moons, comets, and dark matter.

For example, the Milky Way, the solar system in which we reside, is classified as a barred spiral galaxy because of its general spiral shape accompanied by a bar and disk of stars at the center. There are many kinds of galaxies based on number of classificatiosn.

1. Spiral Galaxies

Named for their shape, spiral galaxies are the most common type of galaxy in the universe. Of all the known galaxies in the universe, 77% of them are classified as spiral galaxies.

These types of galaxies are classified by their 3 main components – a disk, bulge, and halo. The bulge is housed at the center of the galaxy. It is spherically shaped, and only contains old stars.

2. Lenticular Galaxy

Briefly elaborating, S0 galaxies are like a combination between a spiral galaxy and elliptical galaxy because of the bulge and disk that they do have, like spiral galaxies, but they do not have a spiral-like structure.

A lenticular galaxy is a type of galaxy intermediate between an elliptical galaxy and a spiral galaxy. This designation is according to the galaxy morphological classification scheme.

3. Barred Spiral Galaxies

Although similar to spiral galaxies, barred spiral galaxies differ in their shape slightly. Rather than just being spiraled from the nucleus like ordinary spiral galaxies, these have a ‘bar’ of matter running through them2. From the ends of this ‘bar’ the arms emerge.

4. Elliptical Galaxies

These elliptical galaxies are classified by their ovular shape and lack of central bulge. In contrast to their name, however, sometimes these galaxies are rather circular.

The stars and gasses in these types of galaxies are spread fairly evenly throughout, however, the center is still the brightest area. Additionally, elliptical galaxies do not contain many new stars. Rather, these galaxies are comprised of old, low mass stars.

5. Irregular Galaxies

As the name implies, irregular galaxies are all quite unique. Each irregular galaxy does not have a size or shape that is what we know as ‘regular’ or ‘normal’. These do not have any previously discussed components, such as a bulge, disk, or arms, that other galaxies are known to have.

According to NASA, the uniqueness of irregular galaxies may be because of interactions they could have had in the past with other, neighboring galaxies. Source: https://astrobackyard.com/types-of-galaxies/