GALAXY AND STARS

Our universe contains billions and billions of stars. These stars are emit disturbed uniformed in space but occur in huge bunches or cluster. Such bunches or cluster contains billions of stars called galaxy. In other words, the galaxy is a city of star. Spaces between the stars are filled by clouds of dusts and gases.

Thus, galaxy is a giant collection of billions of star, dusts and gases. A galaxy can be considered as the building blocks of the universe. There are more than 10 to the power 10 galaxies in the universe. Each galaxy on an average contains 100 billion stars. The galaxies are not stationary. They are continuously moving. On the basis of shapes, galaxies can be classified into the following three types: elliptical galaxies, spiral galaxies and irregular galaxies

Elliptical galaxies look like a flat elliptical disc. Spiral galaxy is a shaped like a disc with small spherical bright central region bulging out in the center. The stars the spiral galaxies are concentrating at the center. About 70% of the galaxies are of spiral type. The two important spiral galaxies are Milky Way and Andromeda galaxy. Irregular galaxies are the galaxies which is neither elliptical shape nor spiral. These are generally smaller than the elliptical or spiral galaxies. 

It is discovered that almost all the galaxies are moving away from each other at the tremendous speeds. The galaxies farthest from the earth appear to travel to the farthest. Due to this fact scientist believe the universe is expanding. 

Milky Way galaxy is the galaxy which contains 100 billion stars of which our sun us a member in the Milky Way galaxy. It is of spiral type. All the members of the Milky Way galaxy are revolving around the center core of a galaxy called galactic center. The velocity of the sun around the galactic center is 250 km/sec. it completes one rotation in 200 million years. This time taken by the sun to complete its rotation around the galactic center is known as one cosmic year. The distance between the heavenly bodies are measured in the unit of light years. One light year represents a distance equal to 9,500,000,000,000km.

Nothing in the universe stays the same forever. There is cycle of birth and death for everything. Likewise, a star born at a certain point lives for certain period of time and finally dies. The interstellar space is filled with clouds of dusts and gases mainly hydrogen and helium. Whenever the mass of such a cloud exceeds a certain amount the clouds begins to contract due to weak gravitational attraction between them. Since there are very large masses of clouds, the gravitational pull between the clouds become so large that the clouds begin to contract due to its gravity. Ultimately the clouds compress to a highly condensed mass. This dense condensed mass is called Protostar.

Due to the contraction, the cloud heats up. The pressure inside the cloud also rises. At certain stage, the cloud explodes into large number of fragments. Each fragment continues to contract further under its own weight. This results in the increase in temperature. When the temperature of each fragment reaches about 10 to the power 7 K, the nuclear fusion reaction starts converting hydrogen into helium with the release of tremendous energy. Thus, each fragment becomes a star and a cluster of stars is born. 

When the hydrogen is completely converts into helium in the central core of the star, there is no more production of heat. Thus, the outward pressure will become to zero. As a result, there is only inward gravitational pull. Due to this, the star contracts continuously and the temperature of the core increases further. The increase in the temperature causes the expansion of the outer layer and the size of the star. There is considerable increase in the size is the star. Due to the expansion the temperature, the outer layer starts falling. Finally, a stage is reached when the star becomes very large in size and red in color. This red star is called red giant.

Out sun will become red giant about 5 billion years from now. In this stage, it will expand about 48 to 64 million km and consume mercury and Venus. The earth’s temperature will increase so high that all forms of life will be burnt. A star may remain as a red giant for several million years, at the end of the stage the rate of emission of energy increases enormously and a violent explosion called nova or supernova occurs in the star. This explosion throws a large portion of the star into space. The brightness of the star increases sharply for some time and diminishes at this stage. This is called the death of the star.