What does it mean when something is 4 or 5 light years away from us?
Light years is a unit of distance used in astronomy.
THe speed of light in Vacuum is a constant. It is about 300,000 kilometers per second.
When you want to tell the distance o to far away star we can say th e distance i n light years.
Light travels 300,000 x 60 x60 x 24 x 365.242 kilometers in one year.
THis diatnce is called a light year. It works out to be 5878000,000,000 miles or 9461,000,000,00 0 kilometers.
This means the distance a light beam would take 4 or 5 years to travel.
In the universe, distances are very large and hence are not easy to express or comprehend in normal measures.
Initially, astronomers used astronomical unit (A.U.), which is the average distance between the Earth and the Sun, which is about 93 million miles or 150 million kilometers. But this may be useful only within Solar system as though at its closest point, Pluto is only 29 A.U. from the Sun and at its most distant, it can be 49 AU from the Sun, the distance of Proxima Centauri, the closest star to us is 271,000 AU.
It was obvious that A.U. will be too short a measure for measuring size and distances of galaxies, As light travels at a constant speed of about 186,000 miles per second, a light year, as a unit of distance that light traveled in one year (approximately
Another unit to measure astronomical distances is parsec, which is the distance at which one A.U. subtends an angle of one arc-second (explained by me in an earlier answer) A parsec is equal to about 3.26 light-years in length.