Solar/stellar evolution

Here I will just briefly try to tell you how stars are born, how they live their lives and how they die. And of course how we know that.

Everything starts with a huge cloud of gaseous cold matter. If the cloud is dense enough it will start to colapse. During the colapse the density increases and the cloud becomes hotter and hotter. When the cloud reaches certain density we call it a protostar. This protostar continues to colapse. If the centre of the protostar becomes so dense and hot that it can support thermonuclear burning of hydrogen we say that star is born. Inner pressure stops gravitational collapse.

Stars live most of their life while they burn hydrogen in their cores. This can lasts from a few hundred thousands years to tens of bilions of years. A rule is that if the star is more massive it will burn its hydrogen much faster and its life will be short. 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 evolutionary stages.

When all the hydrogen in the core is burnt, it starts to colapse again. If the mass of the star is high enough and the temperature and the density in the core reach conditions for the burning of helium, the radiation pressure in the outer parts of the star will expand it and the star will become a red giant.

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

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 moderate and high masses are liable to explode with great violence forming nebulae which are so pretty like this one here.