Draft1

Why are galaxies flat?
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. Galaxies may contain many multiple star systems, star clusters, and various interstellar clouds. The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun. Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual morphology). A common form is the elliptical galaxy, which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped assemblages with dusty, curving arms. Galaxies with irregular or unusual shapes are known as irregular galaxies, and typically result from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy. Small galaxies that lack a coherent structure could also be referred to as irregular galaxies. There are probably more than 170 billion (1.7 × 1011) galaxies in the observable universe. It is known that at the center of a galaxy exists at least one supermassive black hole. These black holes generally gives to each galaxy a rotational movement depending of its size and gravity, and they also clean galaxies of spacial trash such as dust, asteroids, meteorites, and other things. **Galaxies types**
 * Definition**


 * Spiral galaxies**: onsist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the bulge are relatively bright arms. has tightly wound, poorly defined arms and possesses a relatively large core region. At the other extreme, also this galaxies has open, well-defined arms and a small core region.


 * Ellipticals galaxies**: These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter. They are dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions.

A lenticular galaxy is an intermediate form that has properties of both elliptical and spiral galaxies, they possess ill-defined spiral arms with an elliptical halo of stars. In addition to the classifications mentioned above, there are a number of galaxies that can not be readily classified into an elliptical or spiral morphology. These are categorized as irregular galaxies.
 * Other types**: Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies. An example of this is the ring galaxy, which possesses a ring-like structure of stars and interstellar medium surrounding a bare core.

**Why are galaxies flat?** To understand the reason of the flat form of the majority of galaxies first we need to know the definition of the angular momentum.

Angular momentum: is a conserved vector quantity that can be used to describe the overall state of a physical system. The angular momentum L of a particle with respect to some point of origin is

R = position, p= linear momentum, m = mass, v = velocity

So, the best explanation to our question is that galaxies formed from a spinning cloud (well, actually, probably many spinning clouds, but all with about the same spin). As the cloud contracted, it started spin faster, due to the law of conservation of angular momentum. This spin flattened out the shape of the galaxy due to centripedal force. The same thing happens to stars. As they contract, their spin increases, and gas and dust from their formation makes a disk around the star. From this disk, planets are formed. That is why the solar system displays this feature as well.

Now elliptical galaxies are a different story. They are formed from galactic collisions. When two disk galaxies collide and merge, their star's orbits are mixed up and scrambled, thus resulting in a disorderly, sphereical shape.

A very extreme case of spin-up through contraction is the neutron star, which may spin as fast as 1000 times a second!