The Hubble sequence is a classification for galaxies that Edwin Hubble invented in 1926. The sequence divides regular galaxies into three different types, elliptical, spiral, and lenticular galaxies. Today, the Hubble sequence is widely used by both professional and amateur astronomers to classify galaxies.
To the left side of the diagram lie the elliptical galaxies. These galaxies are pretty much featureless. In fact, it's been said that if you've seen one elliptical galaxy then you've seen them all. This is because nearly every elliptical galaxy is very similar in structure and appearance. The largest galaxies in the universe, however, are elliptical galaxies and can contain more than 1 trillion stars.
Moving to the right of the diagram, we see two parallel branches, both containing spiral galaxies. Unlike elliptical galaxies, spirals have amazing, stunning features. Spiral galaxies are basically flattened disks with huge spiral arms composed of stars, gas and dust. Spiral galaxies have a central region known as the bulge. For most spiral galaxies, a supermassive black hole exists in this region. The upper branch contains the regular spiral galaxies, which are denoted by the letter S. The lower branch contains the barred spiral galaxies, which are denoted by the letters SB. These galaxies can be further divided into other groups shown in the diagram.
Just before the diagram breaks off into the two branches which contain the spiral galaxies, we find the intermediate galaxies between elliptical and spiral galaxies, known as lenticular galaxies. Lenticular galaxies are given the symbol S0. Like spiral galaxies, lenticular galaxies have a disk like structure and a central bulge. Unlike spiral galaxies. the disks of lenticular galaxies have no visible spiral structure. Like elliptical galaxies, lenticular galaxies galaxies have very little star formation. Well, to know more follow me.