Galaxy is the system of gravitational bounding of stars, interstellar gas, stellar remnants, dust and dark matter. The Galaxies range with few million stars to hundred trillion stars. All objects of galaxy are orbiting its centre. Many galaxies have super massive black hole at their centre. Tentative numbers of galaxies in universe are from 200 million to two trillion or more having more than 1X 1024 stars. Size of the galaxies is from 3000 light years (1000 parsecs) to 300000 light years (100000 parsecs) in diameter and they all separated from each other by distance of millions of parsecs. Our milkyway galaxy is in diameter of 30000 parsecs or 100000 light years.
Formation of Galaxy
How the galaxies were formed is the challenging task for astronomers till date. It is assumed that the first galaxy was formed as little as one billion years after the Big Bang which took place before 13 to 14 billion years.
There are two main theories to explain how first galaxy was formed.
The Top-down theory suggests that the galaxy was formed by collapse of a large cloud of gases. The gas clusters are mainly consists of dark matter which gravitationally interacted. Due to gravity force, they are started to get angular momentum and matter at the centre of it speeds up its rotation. After that matter forms the disk but gases in disk are not gravitationally stable and clouds of gas starts to convert into stars. Remaining dark matter remains outside the disk is known as dark halo.
According to Bottom-up theory small clusters of dark matter merge to each other to form galaxies. According to this theory, all early galaxies are spiral and after merging to each other they become elliptical.
In the early 1900’s, many astronomers believed that entire universe is the part of the milkyway galaxy. This belief was end when Edwin Hubble discovered many stars outside the milkyway galaxy. He found that other galaxies are moving away from us with very faster rate and from that he was able to find that universe is expanding.
The galaxy formation has not stopped till today, small galaxies are gobbled up by larger galaxies. Galaxies are too massive and the gravity is also very high. When two galaxies reach near to each other, due to gravity attraction they merged into each other and form new single giant crowd of new stars. Merger between two galaxies is so violent and heavy frictional interaction between two gases causes gravitational shockwaves which creates new stars in new born galaxy.
The milkyway and Andromeda galaxy are gravitationally bounded and they are moving towards each other with tremendous speed and will be expected to collide to each other after less than five billion years. During this collision the Sun and solar system will be ejected from its current orbital path.
According colour magnitude diagram, the galaxies fall into two different locations. One is “Red Sequence” and other is “Blue Cloud”. Red Sequence galaxy is non star forming galaxy with small amount of gas and dust while Blue Cloud galaxy is star forming dusty spiral galaxy.
Classification of Galaxy
Edwin Hubble classified the galaxies into four main groups.
- Spiral Galaxies: It is most common type of galaxy in universe. About 77% of galaxies observed are spiral. This galaxy is flat rotating disk containing stars, gas and dust having high concentration of star in centre is known as bulge. Bulge is the house of the old stars. It has spiral arms outgoing from centre which is the area where new stars are born. Arms have most of the gas of whole galaxy and full of blue stars. Andromeda is perfect spiral galaxy.
- Barred Spiral Galaxies: Barred spiral galaxy has same feature like spiral galaxy. It consists bar of bright stars which lies along the bulge and extend into disk. Our Milkyway is example of barred spiral galaxy.
- Elliptical Galaxies: Elliptical galaxies vary from size and mass by huge amount. The smallest elliptical galaxy size is about one tenth of milkyway galaxy.Stars motion in this galaxy is radial and star formation is very low due to lack of gas compared to spiral galaxy. Supermassive black hole is present in the centre of each massive elliptical galaxy. Elliptical galaxies are visible yellow-red in colour.
- Irregular Galaxies: Irregular galaxies have no specific shape as like spiral or elliptical galaxy. Some of the irregular galaxies were spiral or elliptical in early stage but due to uneven internal gravitational force they become irregular in shape. They have no any bulge at centre or any arms.
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Authored by BK Suthar.
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