Sunday, September 19, 2010

My (Former) Research Explained: Stellar Evolution

All stars primarily burn hydrogen (through fusion) and some other stars burn heavier elements. All the stars that burn hydrogen are related in some way and are called main sequence stars. All the others live off the main sequence, but have their place as well. What is the main sequence? How does a star go from the main sequence to off of it? What happens after that? Keep reading to find out...
The H-R diagram
That is a picture of the H-R diagram, two names put together where one of them I can't pronounce. It graphs a stars temperature vs its luminosity or the amount of light from it. See that bright band in the middle? That is all the stars in the universe (that we know of) that are still burning hydrogen as their main fuel. All the other stars have some other features.

The stars in the lower left don't burn anything and are slowly cooling, they are white dwarfs. The stars in the top right are giant class stars. Stars that have ballooned in size due to years of overeating and lack of exercise. They are now burning helium, carbon, oxygen, iron, etc to sustain life and are pretty large in radius. Those stars were once on the main sequence, but have retired and moved on.
From left: red supergiant, red giant, blue supergiant, red giant, and black hole
Let's look at the sun as an example. Our sun was formed by a swirling gas made of hydrogen and other light elements. Once the mass settles into a large spherical shape it enters the main-sequence where it is now. Our sun fuses hydrogen atoms into helium and it will do so for several more billion years. As the hydrogen is burned the star's becomes more massive (because it is now mostly helium) and the "added" mass causes whatever hydrogen is left to burn faster. 

This adds to the pressure that keeps the star from collapsing and causes the star's outer layers to expand. The star becomes a red giant and has left the main sequence. From here it becomes crazy. Whatever hydrogen is left burns and then the helium burns, so the star collapses some. The core becomes heavier because it is made from other elements like carbon, lithium, and oxygen. This causes the star to swell again and collapse and swell. 

The collapsing and swelling gives the outer layers of the star some momentum and eventually they will just float away. This leaves the star as a white hot core and a cloud of gas, which becomes known as a planetary nebula. The gas cloud can go on to create planets or other stars. The white hot core is left behind and is known as a white dwarf. The core undergoes no other reactions and just slowly cools over the course of billions of years. Eventually it will have no energy left and will just be a black object (not to be confused with a black hole).

What happens if you are a massive star? Well in this case all hell breaks lose. The gravitational force is so huge that atoms just fuse together incoherently. Hydrogen fuses into helium of course, but as this happens the helium fuses into heavier and heavier elements. This causes great instability in the star and eventually the star will collapse in a massive explosion known as a supernova. 

Some material will escape to be ejected back out into the universe. Everything that doesn't get's sucked into the core leaving behind a super-massive object known as a black hole. 

Now I'm leaving out lot's of details because I haven't thought about this stuff in years, but you can get the point. Super-massive stars are pretty interesting and I suggest you look into them further. For now, our next stop will be the OH/IR maser, which is what I spent the summer of 2005 trying to understand and study.

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