How was the Sun born and how will it meet its end?

Here I shall give a brief description of how stars are born, how they live and die. This is called stellar evolution.

The birth....

Everything starts with a huge cloud of cold gas made up mostly of hydrogen and helium. If the cloud is dense enough it will start to collapse under the forces of gravity. As the cloud collapses, the density increases and it becomes hotter and hotter (gravitational potential energy is being released - see the energy Fact File). When the cloud reaches certain density we call it a protostar. This protostar continues to collapse. If the centre of the protostar becomes very dense and hot then nuclear energy is produced by the fusion of hydrogen to helium. We say that a star is born. The energy generated creates sufficient pressure inside the star to stop the gravitational collapse.

.....mid-life crisis......

For most of their lives, stars `burn' hydrogen in their cores, these are main sequence stars. This can last from a few hundred thousand years to tens of billions of years. If the star is more massive it will burn its hydrogen much faster and its life will be shorter. Stars with masses about a few percent of solar mass burn their hydrogen very slowly and can live virtually forever. Stellar evolution is a very slow process, but we are lucky to see stars in the different stages of evolution.

When a lot of the hydrogen in the star's centre is burnt, the star starts to collapse again. If the mass of the star is high enough and the temperature and the density in the core become large enough, nuclear energy is produced by fusion of helium into heavier elements (carbon, nitrogen, oxygen). The outer parts of the star, however, will expand and it will become a red giant.

The Sun will reach this stage in a few billion years and its radius will be larger than Earth's orbit. Life on Earth will have ended sometimes before this happens.

......The End

When all nuclear reactions in the stellar interior have been exhausted, most stars become white dwarfs - small hot stars that slowly cool. Our Sun will finish its life as a white dwarf.

Stars of high mass are liable to explode with great violence forming a supernova, but our Sun may just blow off its outer layers and produce a nebula like this one.