There are billions of stars in the universe. In addition, there are innumerable other heavenly bodies such as planets, asteroids, noons, etc. These various heavenly bodies in the universe are separated by very large distances. These distances are so large that the ordinary units of length fall extremely small for measuring them.
The extremely large distances between the various heavenly bodies are expressed in terms of two astronomical units of distance:
One light-year is the distance travelled by the light in one year.
Since the speed of light is 3 * 105 Km/s and
One year is equal to 1* 365 * 24 * 60 * 60 seconds
1 light-year = 3 * 105 * 365 * 24 * 60 * 60 km
= 9.46 * 1012 km.
The star closet to us ‘Alpha Centuari’ is 4.3 light-years from the earth, which means that this star is 4.3 * 9.46 * 1012 km away from earth. Light from this star which we see at a particular night would have left its surface 4.3 years ago.