Origin of the Universe

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Origin of Universe

Geocentric Theory: According to this theory Earth is the centre of the Universe. Ptolemy proposed this theory in AD 140.
Heliocentric Theory: According to this theory Sun is the centre of the Universe. Copernicus proposed this theory in AD 1543.
Our Solar system occupies a very small part of galaxy. And there are numerous galaxies in the Universe.

Universe

The infinite space which contains Solar system, numerous stars and Galaxies etc is called Universe.

Evolution of Universe

Big Bang Theory: E George Lamantor proposed this theory in 1960-70. According to this theory Universe was once compact dense and hot. After Big bang explosion the Universe started expanding and cooling. This explosion which happened before 15 billion years ago is the main reason behind the formation of galaxies of stars and other heavenly bodies. Some of the theory associated with the evolution of Universe is discussed below.
Steady State Theory: This theory states that Universe is continuously expanding and number of galaxies available in the observable Universe remains constant and the free space created by the galaxies which move away from the observable Universe is taken over by new galaxies. This new galaxies are continuously being created out of free space and fill the free space. Formation of new Universe doesn’t disturb overall mass and size of the observable universe it remains constant. Hence the state of the Universe remains steady.
Pulsating Theory: This theory is proposed by Albert Einstein. According to this theory the universe is pulsating i.e., universe goes through cycle of expansion and contraction. Presently Universe is expanding. This theory is also known as oscillating or cyclic universe theory. Hence according to this theory universe starts with “Big Bang” (Expansion) and ends with “Big Crunch” (Contraction).

Galaxy

Galaxy is the collection of infinite number of stars, gas, dust, planets asteroids, comets etc. Due to gravitational attraction this infinite numbers of objects are held together in the galaxy.
Galaxy in which our solar system exists is called Milky Way or Akash Ganga.
Galaxies can be divided into:

Normal Galaxies: Galaxies are differentiated from one another on the basis of radio radiation. Galaxies which radiates less radio radiation are normal galaxies. These are normally bright at centre and started dimming towards edges.

Normal galaxies are further differentiated from one another on the basis of their shape. These are:

In this 18% of the Galaxies are elliptical, 80% of the Galaxies are spiral and only 2% of the Galaxies are irregular Galaxy.
If we differentiated normal galaxies according to their age then irregular Galaxies are youngest, spiral Galaxies are middle aged and elliptical Galaxies are oldest.

Radio galaxies: These Galaxies emit many million times more radio radiations than normal Galaxy.

Milky Way or Akash Ganga

Our Solar system occupies a part of Milky Way. Hence we belong to Milky Way galaxy.
It contains millions of stars and glows in such a way that it seems stream of milk is flowing in the sky that’s why named Milky Way.

Stars

Star is a bright and shiny heavenly body in the sky which emits its own light.
Sun is also a star and hence is the nearest start of the earth.
Infinite numbers of stars are present in the sky. However only 2500 stars can be seen at time.
Star Alpha Centauri occupies second position after sun if we measure distance from the earth.

The life cycle of a star

It should be noted that initial mass of stars decides its last stage. A massive star in its last stage will end up as black hole and neutron star where as a low & intermediate mass star will end up as white dwarf. Process of formation of stars are discussed below:

Stars form in nebulae. Cloud of dust and gas mainly hydrogen is called nebulae. This cloud of dust and gas contracts under gravitation.
This contraction and then collapse of dust and gas under gravitational pull form protostars. Protostars again collapses due to gravitation and forms main sequence stars.
The fuel of stars is hydrogen. As the core runs out of fuel, the core contracts and the outer layer expands and becomes cool and less bright. At this stage of stars life cycle it is called a red giant or red super giant. This red or red or yellow super giant collapse and explodes. (It should be noted that initial mass of a star will decide whether a star in its last stage will becomes red giant or red or yellow super giant.)
This red giant in next stage of their life will become white dwarf and red or yellow super giant will become Supernova. Supernova in next stage will split into Black hole and Neutron Star.

White Dwarf

In the above figure we can see that a low & intermediate mass star in its last stage of life becomes white dwarf. White dwarf have mass which is equal to sun but there diameter are only 1 % of sun’s diameter. Now you can imagine there density and hence there gravitational pull.

Supernova

As in the above figure we can see that massive star in one of its last but one stage becomes supernova. After becoming supernova, stars explode and obtain short time brightness which is equal to the brightness of 100 millions suns. Types of supernova:-
TYPE I: This Supernova explosion occurs in binary star systems in which gas falls from one star to a white dwarf, causing it to explode.
Type II: These types of supernova explosion occur in massive stars, in which fuels i.e., hydrogen and helium ended. These type of supernova explosion produces neutron stars and black hole. It has been said that these type of supernova explosion produces elements which are heavier than hydrogen and helium.

Neutron stars

These type of stars are formed after supernova explosion of massive stars. These stars contains mainly neutrons, as protons and electrons combine by force to produce a neutron star.
These stars are very dense and compact as compared to other stars. Their mass are three times that of sun but having diameter of 20 km only. Now imagine their density and their gravitational pull.
Pulsar which are spinning very rapidly are believed to be neutron stars.

Black Holes

These are stars that are formed after supernova explosion of massive stars. These are the last stage of a massive star.
It should be noted that after supernova explosion of massive stars Black hole and Neutron stars both are formed. We have also seen in the fig above.
These black holes are having infinite density and hence infinite gravitational pull. Hence it has been said that Black holes attract all things which comes in their way even stars and light rays cannot escape from them.
The gravitational pull in a black hole is so great that nothing can escape from it, not even light.

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