Short Notes - Astronomy

Category : UPSC




NASA defines astronomy "the study of stars, planets and space”.




All existing matters, energy & space as a whole form universe. It contains both normal and dark matters. The normal matters are the visible parts such as sun, star, galaxies which contribute only 5% and the rest 95% are called the dark matter which is generally invisible.




In astronomy & cosmology, space is the vast 3-dimensional region that begins where the earth's atmosphere ends. There are inter steller & intergalactic spaces.


  • Study of Universe is known as Cosmology.
  • Big Bang Theory explains the origin of the universe.
  • Universe comprises galaxies, that are huge concentration of stars.




  • Galaxies are also known as Island Universe.
  • Galaxies may be-

(i) Spiral      

(ii) Elliptical and  

(iii) Irregular

  • The Milky Way is the galaxy that contains our Solar System.
  • Latest known galaxy is the Dwarf Galaxy.




  • Stars account for most of the Galactic Mass.
  • They tend to form groups called Constellations.
  • A star goes through various stages of evolution.


The Life Cycle of a Star


  • A nebula is a cloud of gas (hydrogen)- and dust in space. Nebulae are the birthplaces of stars.
  • A star is a luminous globe of gas producing its own heat and light by nuclear reactions [nuclear fusion).
  • Stars are born from nebulae and consist mostly of hydrogen and helium gas.
  • Red Giant stars is a dying star, i.e. the later stages of the evolution of a star like the Sun, as it runs out of hydrogen fuel at its centre.
  • In few billion years, the Sun will turn into a red giant star, expand and engulf the inner plan ets, possibly even the Earth. Red Giant stars are very cool, faint and small stars, approximately one tenth the mass and diameter of the Sun. Proxima Centauri and Barnard's Star are red dwarfs.
  • Red Dwarf stars are the most common & longest lived stars.
  • They are the smallest of the stars with low temperature.
  • White Dwarf is very small, hot star, the last stage in the life cycle of a star like the Sun.
  • Black Hole is very small, hot star, the last stage in the life cycle of a star like the Sun. The gravitational pull in a black hole is so great that nothing can escape from it, not even light. So, it is invisible.
  • Renowned Indian physicist
  1. Chandrasekhar has given Chanderasekhar limit, which is about the formation of Black Holes.
  • The closest star to the Earth is Sun.
  • The closest star to our solar system is The Proxima Centauri.
  • Light year, Astronomical unit (A.U.). Parsec is the unit of measurement of interstellar distance that is equal to 3.26 light years.











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