Evolution of Universe 

The three main theories put forward to explain the origin and evolution of the universe are:

Big Bang Theory:

It was an explosion that occurred 12.8 billion years ago leading to the formation of galaxies of stars and other heavenly bodies.

Steady State Theory:

Bondi, Gold and Fred Hoyle developed this theory and state that although the universe is expanding. It nevertheless does not changes its appearances over time, it has no beginning and no end.

The Pulsating Theory:

The universe is supposed to be expanding and contracting alternately. At present the universe is expanding.

According to Nebular Hypothesis,

  • The planets were formed out of a cloud of material associated with a youthful sun.
  • Galaxy is a group of stars. Galaxies normally spread over vast distances, which are measured in thousands of light-years.
  • A galaxy starts to form by the accumulation of hydrogen gas in the form of a very large cloud called nebula.
  • One light year is the distance travelled by light in one year, which is equal to 9.4611012 km.
  • Light travels at the speed of 300,000 km/second.
  • The mean distance between the sun and the earth is about 149,598,000 km. And, in terms of light years, it is 8.311 minutes.

Milky Way

  • Our Solar system namely “Milky Way” consists of eight planets, the sun, 63 moons, millions of smaller bodies like asteroids and comets and huge quantity of dust-particles and gases.
  • The planets of Milky Way were formed about 4.6 billion years ago.
  • The planets, Mercury, Venus, Earth, and Mars are called the inner planets as they lie between the sun and the belt of asteroids and the rest four planets i.e. Jupiter, Saturn, Uranus, and Neptune are known as outer planets.
  • The moon is the only natural satellite of the planet earth.
  • The earth’s oceans were formed within 500 million years from the evolution of the earth.
  • About 3,800 million years ago, life began to evolve and the process of photosynthesis got evolved about 2,500-3,000 million years ago.

Stars 

  • Stars are heavenly bodies made up of hot burning gases this shining by their own light.
  • The stars is bigger than the sun but bit more than twice as big it will turn into a Neutron star or Pulsar. They are formed due to novae or super novae explosion.

Evolutionary stages of a Star:

  • Proto Star 
  • It is the stage where the helium core become increasingly heavy accompanied with expanding outer layers.
  • Red Giant 
  • This stage results into the swelling and reddening of the outer region of the stars.
  • Such stars of gigantic dimension is called Red Star.
  • White Dwarf 
  • If the mass of the star is relatively small like that of our Sun the gases that reach the outer layer are expelled. As these expelled gases cool and contract the star becomes a White Dwarf.
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