The Star Formation
The distribution of matter and energy was not even in the early universe. These primary density differences gave rise to differences in gravitational forces and it caused the matter to get drawn together. These formed the bases for expansion of galaxies. A galaxy contains a large number of stars. Galaxies spread over vast distances that are measured in thousands of light-years. The diameters of individual galaxies range from 80,000-150,000 light years. A galaxy starts to form by accumulation of hydrogen gas in the form of a very large cloud called nebula. Eventually, growing nebula develops localized clumps of gas. These clumps continue to grow into even denser gaseous bodies, giving rise to formation of stars. The formation of stars is believed to have taken place some 5-6 billion years ago.
A light year is a measure of distance and not of time. Light travels at a speed of 300.000 km/second. Considering this, the distances the light will travel in one year is taken to be one light year. This equals to 9.461×102 km. The mean distance between the sun and the earth is 149.598.000 km. In terms of light years, it is 8.311 minutes of a year.