- 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 year in seconds
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
- 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.