Disk galaxies or spiral galaxies are a type of galaxies which were first discovered by the American astronomer Edwin Hubble. Spiral galaxies are flat and disk-shaped. They contain stars, dust, asteroids and gas and serve as a residence to a group of stars located in the center of the galaxy. This center is known as the bulge.
Spiral galaxy is the most common type of galaxy, as almost 60% of observed galaxies in the universe are spiral galaxies. There is no surprise here that spiral galaxies look like spirals, but if you are able to get a distant clear view from the side then galaxy would look like a vast circle. A clear view of this galaxy is called the “Face on Spiral”, and from the side it is called “Edge on Spiral”. Spiral galaxies usually have arms (cluster of stars) that are joined very closely to them, while other galaxies are not so much close to their exterior clusters of stars.
In early 20th century astronomy, Edwin Hubble formed an idea about the expansion of universe. People have estimated the approximate age of the universe based on its rate of expansion. Some existing galaxies are a billion light years away from us, which means that they are the oldest.
In 1962, astronomers Olin Eggen, Donald Lynden-Bell and Alln Sandage suggested that spiral galaxies form due to the collapsing of large gas clouds. When the cloud falls it settles and the erupting gas takes a form of a rotating disk. This theory was very popular at some point, but is no longer accepted. Spiral galaxies are quite fragile and thin so they merge with other galaxies very easily, this merger makes it more difficult to understand their formation.
Astronomers still don’t have a definite answer on the formation and merger of galaxies, however, in recent years a great amount of research has been done to understand this concept. Galaxies travel the universe and collide with each other, sometimes this collision destroys them and sometimes they merge and become a bigger galaxy.