Life Cycles of Stars

Lets star out by saying most stars begin there lives in a molecular cloud.  At some point in this cloud when the big "ball" of matter comes together, the matter begins to heat and then it begins to collapse.  As it collapses the cloud becomes more masses because it gains matter from the surrounding accretion disk. 

In the above photo you see the protostar (ball of matter) the bright spot in the middle, and the accretion disk surrounding it.

Ok, now here is the good part. The protostar after it has gained enough mass and is hot enough it will begin nuclear fusion in its core.  The easiest example I can give to explain what nuclear fusion is, is when a star converts hydrogen into helium in its core.

If a star is larger than 200 times the mass of the sun it will not become a star. In simplest terms it blew itself up....lol! Also a star less than 0.1 times the mass of the sun will not become a star because it cannot reach high enough forces to begin nuclear fusion in its core.

So if a star is above .1 solar masses and below 200 solar masses, what happens next? It becomes what we know as a main sequence stars.  Main sequence star burn hydrogen into helium in the core.  Depending on how massive the star is it can take billions years to burn through its hydrogen or it could take millions of years. A great example of this is the H-R diagram pictured below.  The stars that are in the like line that looks like a negative slope are those of the main sequence.

The higher the mass (the hotter the star) of the star the faster the star will go through its life cycle.

After stars go through their main sequence stage, they have what we call a helium flash, the outer layers expand and the core contracts.  The star is now exponentially bigger and it is red.  I can use our sun as an example. In 4.5 billion years our sun will complete its main sequence stage and turn into a red giant. When it does everyone will burn to a crisp! The sun will become so big that it will engulf the planet.  So it you are around then please have an escape plan (also look out for other stars from Andromeda because our galaxies will be merging around that time too.).  Oh and if you can get away please take pictures and tag me on facebook!

So after the red giant phase, then it gets a little tricky. For stars around the same size as our sun they will become a planetary nebula they eventually end up as a white dwarf(I will do a post of deaths of stars). 

For stars much much more massive than our sun, the stars will go supernova and blow up and later become a neutron star or black hole.