With thanks to http://cse.ssl.berkeley.edu/bmendez/ay10/2000/cycle/ Stellar Life Cycle With thanks to http://cse.ssl.berkeley.edu/bmendez/ay10/2000/cycle/

The interstellar medium The interstellar medium (ISM) is the gas and dust that is prevalent between the stars in many galaxies. It is mostly Hydrogen with trace amounts of other elements. Some regions have more heavy elements (heavier than Helium) than others. Within the medium there are very large clouds that are slightly more dense than average

Protostars When the ISM clouds get dense enough or there are perturbations within the cloud (shock waves and such) regions within can begin to collapse under their own self gravitation. As material is falling in on a region the pressure in the region begins to rise which causes the temperature to rise as well. Gravitational energy is being converted into heat. A protostar forms at the centre of the collapsing region. It is very luminous and is radiating by gravitational heating only. The star is working toward achieving hydrostatic equilibrium. This collapse may take only a few million years

Main sequence stars                                                              Shortly after hydrostatic equilibrium is achieved the star must ignite hydrogen fusion within its core in order to maintain the balance and resist gravity's grasp. If it does not have the mass to do this it will continue to collapse under gravity and cool as it does. Such failed stars are called Brown Dwarfs .Stars like our Sun live for 10 billion years on the main sequence. Stars with higher masses live shorter lives, and stars with less mass live longer lives. The main sequence is where all stars spend the vast majority of their active existences

Red giants When the star has run out of hydrogen fuel to fuse into helium within its core the core will begin to collapse and heat some more. To counter the core's collapse the outer envelope expands causing the temperature to drop at the surface but also increasing surface area and thereby the luminosity of the star. Within the core temperatures will rise to begin fusion of helium into carbon. A shell around the core will rise to such a temperature as to ignite further hydrogen fusion in that region of the star. The helium produced falls onto the core where it can be used as fuel. This time in the life of a Red Giant is very short compared to the main sequence lifetime, only a few million years

Eventually the core of the red giant becomes hot enough to fuse helium into carbon and oxygen. Thus two fusion processes occur at once: helium fusion in the core and hydrogen fusion in a shell around the core. Eventually, the star runs out of helium to fuse in the core. What happens next to the star depends on whether the star has relatively low mass or high mass.

Low mass stars After a helium-burning red giant runs out of helium fuel in its core, the star's core starts to collapse and heat up. This causes the outer layers of the star to expand and cool, similar to the process that occurred after the star ran out of hydrogen fuel and left the main sequence. As the star swells larger and larger, it eventually becomes a red supergiant

Eventually the outer layers of the star will be shed, creating a planetary nebula, with only a white dwarf left behind