In the vast space around us, there are numerous galaxies present, which are known as large groups filled with stars, dust, and gases all held together due to gravity. All the galaxies are likely to contain stars, planets, moons, comets, asteroids, black holes, nebulae, dust, neutron stars etc.
Just like our solar system that lies in the Milky Way galaxy, there are many galaxies in the space. Every galaxy moves at certain speed and exists in a certain shape. Our milky way galaxy is in the spiral shape with a bright bulge at its center.
The famous astronomer Edwin Hubble studied the galaxies for many years and found that there are four shapes in which the galaxies expand. This classification system is known as Hubble sequence and the four types are
Spiral galaxy is the Galaxy that was formed in a spiral shape with a distinct flattened disk-like structure with a bright center called a nucleus. Our Milky Way galaxy is an example of the spiral galaxy. The young stars are formed in the outer arms of the galaxy while the old stars are formed in the inner arms. There are four subgroups of spiral galaxies, such as
The Barred spiral galaxies which are one-third of the universe are the ones that have their arms spiral out from a straight bar of stars instead of coming from a nucleus. The barred spiral galaxies are represented by the letters SB are divided into barred subgroups as SBA, SBb and SBc which differ from having a short bar of stars to long and well-defined bar of stars extending out.
The Elliptical galaxies vary in shape from round to extremely elongated oval but have no bright nucleus. These galaxies are represented by the alphabet E and are divided into seven sub-groups from E0 to E7 which range from nearly circular to extremely elongated ones.
As the name implies, these galaxies have no discernible shape or structure. These are divided into two classes such as Im and I0. Im class galaxies show some kind of structure which are faint remnants of spiral ones, whereas I0 class galaxies are chaotic in shape.