[GS] [Geography] Universe, Galaxy, The Big Bang Theory, Star & The Solar System

Table of Contents



  • The universe is commonly defined as the totality of everything that exists, including all physical matter and energy, the planets, stars, galaxies and the contents of intergalactic space.

  • The study of universe is known as Cosmology.

       Cosmology = cosmos (universe) + logos (science)

  • The universe has no limit.


  • A galaxy is a vast system of billions of stars which also contains a large number of gas clouds (mainly of hydrogen gas) and dust, isolated in space form similar systems.

  • There are about 100 million galaxies (1011 galaxies) in the universe, and each galaxy has on an average, 100 billion stars (1011 stars). So, the total number of stars in the universe is 1022

  • Milky Way Galaxy was formed 5 billion years after the Big Bang.

  • Latest known galaxy is the Dwarf Galaxy.

  • According to the modern thought, universe can be classified into two parts namely- (a) Atmosphere and (b) Space.

  • Origin of the universe is explained by the Big Bang Theory, formulated and proposed by the Belgian astronomer and cosmologist Georges Lemaitre.

  • Andromeda is our nearest galaxy.

The Big Bang Theory

  • All the matter in the universe was originally a concentrated lump called Primeral atom.

  • Big Band was an explosion that occurred 15 billion years ago, leading to the formation of galaxy of stars and other heavenly bodies.

  • Since then, all the galaxies have been flying away from one another causing expansion of the universe.


  • Clumps of dust and gas in a nebula come together due to gravity and form stars.

  • Stars are made of hot burning gases.

  • They emit light of their own and are very large and very hot.

  • Light takes about 4.3 years to reach us form the next nearest star Proxima Centauri.

The Solar System

  • The Solar system consists of the sun, the eight planets and their satellites (or moons) and thousands of other smaller heavenly bodies such as asteroids, comets and meteors.

  • The sun is at the center of the solar system and all these bodies are revolving around it.

  • The gravitational pull of the sun keeps all the planets and other objects revolving round it. Thus, the motion of all the members of the solar system is governed mainly by the gravitational force of the sun.

  • Planets revolve around the sun in elliptical orbit.

  • In the solar system the planet nearest to the sun is Mercury and the planet farthest from the sun is Neptune (not Pluto).

  • The size of solar system has been estimated to at about 105U.

  • The solar system is dominated by the sun which accounts for almost 99.9% of the matter in the whole solar system.

  • The sun is also the source of all the energy in the solar system.

  • Mercury, Venus, Earth, Mars are called terrestrial planets and Jupiter Saturn, Uranus and Neptune are called gaseous planets.


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