Theories of Origin 

  • Various theories have been proposed to explain the way the Universe was possibly formed. Some such theories are the astronomical phenomenon called the Red Shift or the Doppler Effect, The Big Bang Theory, etc. 

Doppler Effect and Red Shift 

  • The shifting of light coming from the galaxies to the red end of the spectrum is known as the Red Shift. The red colour band has the lowest frequency and the longest wavelength in the visible spectrum. 
  • This proportionate increase shows that matter in the Universe is in a state of rapid expansion and this idea of rapid expansion is the basis of all the modem theories about the origin of the Universe. 

Big Bang Theory 

  • According to the Big Bang Theory, about 10 to 20 billion years ago, cosmic matter (universe) was confined in a super dense ball in a state of extremely high temperature and pressure. 
  • A primordial explosion hurled the broken material far into outer space, where the fragmented material is still travelling around at thousands of miles per second. 
  • The sudden cooling which accompanied the expansion led to the trans-formation of atomic particles into atoms of lighter elements, which then condensed into gaseous clouds. 
  • Eventually, galaxies and other celestial bodies were formed, without disrupting the expansion of the universe.


  • Galaxy is a huge congregation of millions of stars held together by their own gravitational fields. 
  • There are three types of galaxies, as follows: 
    (i) Spiral Galaxies; 
    (ii) Elliptical Galaxies; and 
    (iii) Irregular Galaxies. 

Local Group 

  • A cluster of 23 known galaxies is known as a local group. Milky Way and Andromeda are the two largest known galaxies.


  • Nebulae are distant stellar systems of luminous bodies (made up of gas and dust particles) e.g. the Orion Nebulae.


  • Constellations are clusters of stars, organized into distinct shapes and figures, and named accordingly e.g. Hydra which is the largest constellation.


  • Comets are huge clouds of frozen gases and dust which have their home in the cold outer fringes of the solar system. 
  • A comet's head is formed by the evaporation of solid ice particles when the comet approaches the sun. 
  • Comets have an extremely eccentric orbit but a definite periodicity, such as the Hailey's Comet which is seen every 76 years. The tail of a comet always faces away from the sun. 

Meteors (Shooting Stars) 

  • Meteors are made up of small solid matter which, when coming into atmosphere of the earth, burns out due to friction. While burning, they emit light e.g. Leonid shower is a meteor storm (originated in the constellation Leo).

The World of Stars

  • Stars account for 98 per cent of the matter in a galaxy. A star is formed by gravitational contraction (from vast clouds of galactic gas and dust). 
  • Star-forming matter is richer in hydrogen and helium.