In honor of Star’s upcoming birthday, we are going to celebrate the birth, life and death of stars! Plus this is a great departure from biology and a lovely little vacation from the “how your body does this” posts I have been doing lately. In the beginning there was a gravitational collapse of a molecular cloud reaching over 100 light years across. Just kidding – you know I take things slow for you! Around the universe is a bunch of interstellar medium (ISM). ISM is made up of mainly hydrogen and helium. These clouds of gas are happy and just hang out in space until something basically knocks into them (triggered by supernova explosion, clouds may collide, etc) and makes them unstable. Once the cloud becomes unstable it breaks apart into small chunks. This breaking apart releases lots of heat and increases pressure, causing the superhot gas to ball up and form a rotating protostar. Small protostars never get hot enough to start nuclear fusion and turn into brown dwarfs. Brown dwarfs burn cool and dim and die away over hundreds of millions of years. The big protostars will continue to get hotter and hotter and eventually get hot enough (10,000,000K) to start the nuclear fusion.
A beyond basic definition of nuclear fusion: you take two hydrogen atoms and smash them together, at such high temperatures and with such great pressure, that the two nuclei combine and turn into helium. This releases a shit-ton of energy and that is why fusion is the power source of stars.
Once a star has reached the point of using fusion for energy, it has lost most of the energy it gained from collapsing and is said to be in its main sequence. It’s now known as dwarf stars. A dwarf star is a giant ball of gas with a core of helium undergoing fusion and an outer layer of helium. The amount of helium will continuously increase since the star is constantly producing more as a product of fusion. To maintain fusion and size, the star will continue to get hotter and brighter. This is the phase our sun is in – it continues to get brighter and brighter all the time. The amount of time a star stays in this phase depends on how much hydrogen it started with. Huge stars will burn fast and live short lives, while smaller stars will burn slow and live much longer. These small stars (known as red dwarfs) will eventually just get dimmer and dimmer and burn out after hundreds of billions of years. The bigger stars (including our sun) will move on to the post-main sequence.
In about 5 billion years the sun will have used up all of its hydrogen and will start to cool. This cooling will allow the helium to expand. The sun will then be known as a red giant and will be 250 times bigger than it is now. To put that in perspective: it will eat up the earth. Eventually the core will get hot enough to start helium fusion and the sun will shrink up and heat up again. Red supergiants will burn through helium and a bunch of other elements (getting hotter and hotter with each element) until it starts producing iron. In between each element it will grow and shrink. Iron will not produce energy for a star so everything stops there.
Average stars will then shed some layers and turn into small (earth sized) balls called white dwarfs. Eventually they just fade into black dwarfs and disappear. Just like A-list stars in Hollywood, A-list stars in the sky go out with a bang. The iron core will become so big and heavy it will collapse under its own wait and explode as a supernova. The remains will blow away and leave neutron stars and black holes. We’ll talk about neutron stars another time - they are fun.
This was kind of a long and dense post, but I think it was still fun and interesting to know. But now I do have the song, “supernova girl” stuck in my head.
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