What is a galaxy? What is Milky Way? « Back to Questions List

A galaxy is a group of stars and other astronomical bodies including dust and gases held together by gravitational force. The Universe is constituted from everything that exists. The constituents of the Universe vary from sub atomic particles to huge structures. The Big Bang Theory is the widely accepted concept of formation of the Universe.  According to this theory, the Universe is created from the explosion of a large ball of compressed mass of very high temperature. This explosion resulted in formation of billions of galaxies including our galaxy called the Milky Way. It is estimated that there are 100 billion galaxies with each one having average 100 billion stars. 

There are different kinds of galaxies. Each one is unique in its own way. Some are bright, some are dim. Scientists have, however, categorized galaxies into three galaxy groups based on their shape. They are spiral, elliptical and irregular galaxy groups. The spiral one looks like spiraling arms from a central bulge while elliptical one is oval shaped. The irregular galaxy does not have an apparent shape.
 Sometimes, the shape of a galaxy is distorted due to collusion of two galaxies. The changes that occur in the nuclei form supermassive objects like black holes. Such changes happened billions of years ago and are now appearing as compact, highly luminous objects called Quasars. The exact natures of Quasars are not known as they are very far away. Some active galaxies emit extra-large amount of radio energy and is thought to be caused by the presence of black holes. 

The sun is just one of the billions of stars in our galaxy named the Milky Way. It is a spiral shaped galaxy. The Milky Way has four spiral arms, named Orion arm, Sagittarius arm, Perseus arm and Crux-Centaurus arm, surrounding the central bulge. The spiral arms contain clouds of dust and gas in which hot, young blue stars exists. The Solar system is in the Orion arm and is at a distance of 25,000 light years from the centre of the Milky Way. A less dense halo borders the Milky Way. The Milky Way is rotating in space. The speed of rotation of the outer stars is less compared to the inner stars. The galaxy nearest to Milky Way is Andromeda. 

The galaxy is different from the solar system. A galaxy contains billions of stars. But in a solar system there is only one star. In our solar system, the sun is the centre and other bodies like planets, moons and asteroids surround and revolve round the sun. 
 

Saraswaty Supercluster- Supercluster Discovered by Indian Scientists

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