### Theory:

Universe
• 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.
• 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. $3×{10}^{8}×365×24×60×60=9.46×{10}^{15}$m
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
Galaxy
• 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.