Galaxies

The universe:

 Air, soil, rocks, stars, planets and all the objects present in space are called the Universe. The universe is so vast that its vastness could not be measured so far. Our earth is just like a small particle in it. 

 Big bang theory: 

 The scientists present the big bang theory for explaining the beginning of the universe. 
According to the theory: 
The whole matter of the universe was condensed at one place about 15 billion years ago. It was in the form of extremely huge and dense ball of hydrogen. A tremendous explosion occurred in this ball. This explosion is called “Big Bang”. Due to this explosion, the whole condensed matter scattered far away in the.
 After this explosion (Big bang), the universe assumed the form of gigantic clouds of extremely hot, expanding and contracting gases, which, with the passage of time, scattered far away from each other. After about one million years, these clouds of gases got frozen and started shrinking resulting in the formation of galaxies and stars. These galaxies are moving away from the center and the universe is continuously expanding. 

 Nebulae:

 If we look at the sky during night without telescope, we see numerous stars. During night, some patches of dim light on the sky are also seen to us. These patches are called nebula. Nebula is a Latin word which means “cloud”. Actually the huge clouds of gases and dust in space are called nebula. When these gases reflect the light of stars, then nebula looks illuminated. 
The contracting and freezing of nebula forms stars.

Nebulae

Galaxy
A nebula consists of several billion stars.The family of stars is called “galaxy”. 
 Number of galaxies
 There are at least one billion galaxies in the universe and the universe is so vast that most of the galaxies are several hundred thousands light years away from each other, Hence, these galaxies look like dim bulbs of light in the sky. 

Milky way Galaxy: 

 Our solar system is also a part of a galaxy, called “Milky Way Galaxy” There are about 100 billion stars and numerous planets, which also include our earth, in the Milky Way galaxy! What so ever a star we see without telescope is a part of this galaxy. The diameter of Milky Way galaxy is about one hundred thousand light years. 

 Blue Giant: 

The temperature of stars is extremely high. A star whose temperature is greater than 15000°C is called blue giant. Such a star is extremely hot and it emits intense light. For Example: Deneb is a blue giant. Its temperature is usually is usually 15000°C. 

Red Giant:

 A star whose temperature is less than 5000°C is called red giant. It is comparatively a cold star. For Example: . Bernard star is a red giant. Its temperature is usually 3200°C. Arcturus is also a red giant whose temperature is about 4800°C. 

One light year

 One light year is a distance which light covers within one year. The speed of light is 3xl05 Km per second. Light covers 9.48 x 10l2 Km in one years. 

 Small stars: 

 In terms of size, stars ranging from one and half to three times bigger than the sun are called small stars. In the core of stars, there is an often hydrogen and helium gas. The quantity of hydrogen is much more than helium. By burning these gases, light emits from stars. In the core of star, hydrogen gas is continuously converted into helium till the complete discharge of hydrogen gas from the core, the core of star after contraction is collapsed. 

Red Giant: 

 After the collapse previous one and then the star becomes red giant. For this happening, several billion years are spent. 

White dwarfs: 

The outer layers of star while gradually expanding are converted into gases, and its size become small. This small star is called white dwarf. Its size is often equal to our Earth. 

 Red dwarf:

 After several hundred thousands years, white dwarf after gradually cooling down goes to dim. Meanwhile red light is emitted from the core the star. Such star is called red dwarf. The diameter of red dwarf ranges from 3000 kilometers to 50000 kilometers.

 Black Dwarf: 

 When red dwarf after having become frozen stops shining completely then such a dead star is called black dwarf. 
Massive stars: 
 The stars three times bigger than the sun are called “massive stars”. In the core of these stars, hydrogen gas is continuously converted into helium. This process in massive stars is usually completed within 10 million years. 

 Red supergiant: 

At the complete discharge of hydrogen gas, massive stars become “red supergiant” and their size become 1000 times enlarger than the previous one. 
Supernova: 
 In the next few hundred thousand years, the core of red supergaint suddenly collapses which creates a huge explosion, called “supernova”. And for the time being its light becomes much more than all other stars of the galaxy. Neutron star: If during supernova, the core of star is some how the sun, then the core of star contracts after freezing. And it converts into “neutron star”.

 Characteristics of neutron star: ' 

Neutron stars are very dense and their core contains neutrons only. These stars speedily revolve and emit light whereas the black holes are known by their extreme gravity. Their gravity is so much that even light cannot escape from it. Diameter of neutron star: The diameter of neutron star is usually 10 kilometer. Its core is extremely dense.

 Mass of neutron atom: 

. One spoon mass of the material of its core is equivalent to one billion metric tones or even more than this. Black hole: If the mass of the core of star is three times bigger than the mass of the sun, then the core of star contracts such a way that too much density comes on one point. 
It is called “black hole”. There are several hundred thousands black holes in the Milky Way galaxy. Black hole is actually a last stage of the life of star. The diameter of the sun is 1392000 Kilometres. According to astronomers, at any stage if it is converted into a black hole, then its diameter will only remain 6 kilometre.