1. Big Bang Theory
 Georges LeMaitre and George Gamow Gammow proposed this theory.
 According to this theory, at the beginning of the universe, the whole matter of the universe
   was once concentrated in an extremely dense and hot (~1012K) fireball.
 Then about 20 billion years ago a vast explosion (big bang) occurred.
 The matter was broken into pieces, which were thrown out with high speed in all directions
   forming stars and galaxies; which are still moving away from one another.
2. Steady State Theory
 It states that the universe is always expanding though it maintains a constant average
   density.
 This theory was developed in 1948 by Fred Hoyle, Thomas Gold and Hermann Bondi
   stating that there was continuous creation of matter to ensure that the cosmological principle
   is adhered to.
 It was created as an alternative to the Big Bang theory.
3. Pulsating Theory
 It states that the universe is actually expanding and contracting alternately.
 At present, the universe is expanding.
GALAXY
  It is a system of millions or billions of stars, together with gas and dust, held together by
   gravitational attraction.
  The Milky Way (or in hindi Akash Ganga) is the galaxy that contains our Solar System.
  The Milky Way Galaxy is roughly 100,000 light years in diameter.
   1 light-year = 9.4605284 × 1015 meters
  The Andromeda Galaxy is the nearest spiral galaxy to our Milky Way galaxy.
  The galaxies are made up of billions of stars.
  There are three main types of galaxies
   1. Elliptical Galaxies shaped like a spheroid, or elongated sphere.
   2. Spiral Galaxies have three main components: a bulge, disk, and halo.
   3. Irregular Galaxies have no regular or symmetrical structure.
STAR
  It is a massive, luminous sphere of plasma held together by its own gravity.
  The nearest star to Earth is the Sun.
  A star"s life begins with the gravitational collapse of a gaseous nebula of material composed
   primarily of hydrogen, along with helium and trace amounts of heavier elements.
  A star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing
   energy that traverses the star"s interior and then radiates into outer space.
  The oldest star HE 1523-0901, is an estimated 13.2 billion years old.
  The most massive star known is Eta Carinae, which, with 100 - 150 times as much mass as
   the Sun.
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