A tightly packed group of stars is called a bulge. It was previously thought to be as elliptical galaxies that have group of stars around them. However, high resolution images provided by space telescopes show that bulges can be spiral galaxies. Nowadays, there are two types of bulges. The first one is the elliptical or classical bulges and the other are same as the spiral galaxies.
The elliptical or classical bulges are composed of stars of the older population. They have very little dust and gas. They can be found in random orbits. Astronomers say that they are products of the merging of galaxies. During the merge of small structures, the path of the gas clouds that form stars are disrupted that cause the randomness of the bulge orbits.
The remaining gas and dust that were not part of the merging process settle around the bulge that makes the outer disk.
The second type is the same as the spiral galaxies which are called pseudobulges or disky-bulges. This type of bulges orbit in an orderly fashion compared to classical ones. Studies say that they are not mainly composed of dust instead; they are made of complex structures. They also believe that this type is not a product of a merging process. It is formed through secular evolution wherein the stars in a galaxy rearrange.
The evolution of bulges is still being studied so that a more concrete detail of its properties can be determined.