What is a galaxy ?

A galaxy is a group of stars, dark matter, nebulae and other astronomical objects. Most of them contain billions of stars and are tens of thousands of light years in diameter.

Galaxies are divided according to its shapes – elliptical galaxies (uniform), spiral galaxies (mostly thin disks with spiral arms) and irregular galaxies (with no definite structure).

Spiral galaxy centers around a focal point, which is generally spherical and bulges externally from the disks. The gravitational center of a galaxy is the hub, which is characterized by a massive black hole. Spiral arms surround the hub, which are grouped-up waves of gases and stars orbiting the center.

An elliptical galaxy has a large hub on the center but without the arms similar to the spiral galaxy. These galaxies feature yellowish to reddish stars. Irregular galaxies mostly have no distinct structure, but they often have distorted elliptical or spiral shape.

There are approximately a hundred billion galaxies in the universe. Most of these galaxies occur in huge agglomeration called “galaxy superclusters”. With the study of the spiral galaxies’ rotation, astronomers have discovered that most of the universal matter is not made up of gas or stars. Instead, it is made up of a dark matter, an invisible matter that still exerts gravity.

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