• Universe is a vast expanse of space.
  • Earth is a part of the Universe.
  • Universe is believed to have come into existence after the Big Bang explosion.
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  • It consists of billions of galaxies, stars, planets, comets, asteroids, meteoroids and natural satellites which are collectively known as celestial bodies.
  • Universe is also known as Cosmos.
  • Hence the study of Universe is known as Cosmology.
Light year: It is the unit used to measure the distance between the celestial bodies.
A light-year is the distance traversed by light in a year at a velocity of \(300,000\)  km per second.
Light year calculation:
Speed of light  ×1  year in seconds
i.e. 3×108×365×24×60×60=9.46×1015m
Big Bang Explosion
  • The explosion took place about 15 billion years ago.
  • All the stars and celestial bodies came into existence by this massive explosion.
  • These celestial bodies together called as Universe.
Hierarchy of Universe
Universe > Galaxy > Solar systems > Planets > Satellites
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  • It is a huge cluster of stars held together by gravitational force.
  • It was formed about 5 billion years after the Big Bang explosion.
Our solar system is a part of the Milky Way galaxy.