Our Universe : Origin
The big bang theory explains the origin of our universe. According to this theory, 15 billion years ago, cosmic matter was in a compressed state from which expansion started by a primordial explosion. The super-dense ball broke to form galaxies, which again broke to form stars and
finally stars broke to form planets including earth. Since the outer space is limitless, conventional units for measuring distances are not suitable. Hence new units as follows are used:
• Light Year: Distance covered by light in one year in vacuum at a speed of 3x108 m/s. One light year is equal to 9.46 × 1012 kilometers.
• Astronomical Unit: The Mean distance between the Sun and the Earth (1.49 x 108 km). One light year is equal to 60,000 AU.
• Cosmic Year: Sun's period of revolution around the galactic centre (250 million years). Also called as 'galactic year'
• Parsec: Distance at which the mean ra- dius of the Earth's orbit subtends an angle of one second of an arc. It is equal to 3.26 light years.