Life cycle of a main sequence star like our sun
1 All stars begin their life in a similar ways
In the nebula - (a cloud of gas, mostly hydrogen, and very fine dust), clumps form because they are pulled together by gravity. Some of the clumps become very large and this increases the pressure and temperature. More energy comes from falling particles so that eventually the clump heats up to form a protostar.
2 As the fusion of hydrogen to helium begins, the star emits energy. The thermal activity prevents it contracting and the star begins a state called Main Sequence, which may be stable for many billions of years.
3 When almost all the hydrogen is exhausted the helium core begins to contract and becomes hot enough for the helium atoms to fuse forming carbon. The outer layers expand and cool. The expanded star is a Red Giant.
4 When the helium runs out the outer layers of gas drift away from the core forming a gaseous shell called a Planetary Nebula
5 About 80% of the original star remains as the core which is small and bright. This is a White Dwarf cooling to a red and finally a black dwarf.
The life cycle of a massive star
1 Massive stars evolve in a similar ways to smaller ones. In the nebula - (a cloud of gas, mostly hydrogen, and very fine dust), clumps form, pulled together by gravity. Some of the clumps become very large and so increases the pressure and energy. Particles of dust and gas falling into the clump heats the gas even more so forming a protostar.
2 Massive stars have a mass from 3 to over 50 times that of the sun. They are very much brighter, not only because they are bigger but also because they are very much hotter. Because they are much hotter the light emitted is of shorter wavelength so they look blue. The high pressure and temperature causes hydrogen fusion to be much faster so the starʼs life is relatively short- perhaps a few hundred million years or less.
3 When all of the hydrogen is exhausted the star expands to form a Red Giant with a core of helium surrounded by a shell of cooler gas. Fusion continues in the core forming heavier elements up to iron. At this stage the core will suddenly collapse, in a fraction of a second, and then rebounds causing a massive explosion.
4 The explosion is a Supernova blasting apart much of the star, forming a nebula. For a brief time the supernova may be brighter than the rest of the galaxy. In the moments of a supernova the extreme conditions enables the formation of heavy elements, that is those with an atomic number greater than 26. These heavy elements on earth came from a supernova and the shockwave of the explosion may have been the trigger in the formation of our sun.
5 If the remaining core is small (1 to 3 solar masses) a very dense and relatively small neutron star will remain. If the core is larger than 3 solar masses it will form a black hole. So called because the strength of the gravity is so large that light cannot escape from it.
