What Happens to Our Sun?

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Inspired by the supernova and neutrino post I wanted to put together a little life cycle sketch of our sun.

The sun was formed about 4.6 billion years ago from a hot, boiling molecular cloud of hydrogen.  This molecular cloud expanded to the point of equilibrium between the energy pushing it outward and it’s own gravitational forces pulling it inward.  Eventually the cloud cools past it’s point to maintain an equilibrium and it collapses.  The hottest, densest, part of the cloud eventually became our sun.  Cooler, less dense bits became our earth, planets, asteroids, and all the other debris that makes up the solar system.  The protostar begins as a little ball of gas that becomes heavy enough that it generates enough gravity to start pulling in more and more hydrogen gas.  As more the gasses begin to pummels the stellar core, it begins to heat up.  Once it reaches about 10 million degrees it’s hot enough to fuse the hydrogen into helium and it’s now a star.  The earth gets its start at the same time.  Life will start up about 500 million years later.  Currently, the sun is halfway through what they call it’s “Main sequence”.  In the main sequence the sun goes about it’s day as the solar system’s largest fusion reactor.  At the core of the sun, about 620 million tones of hydrogen are burned to create helium every second.  As a result, about 4 million tonnes of matter are converted into energy.  If that sounds explosive, you’d be correct.  That constant explosion at the heart of the sun is what is keeping the sun from collapsing under the weight of it’s own gravity. Right now the sun is in equilibrium between it’s fusion and it’s gravity.  As the gasses collapse into the core of the sun, this increases the rate of fusion pushing the gasses away from the core.  The further away from the core the less fusion can happen and the gasses begin to fall towards the center again until an equilibrium is reached.

However, at some point in the next four of five billion years, the sun is going to run out of hydrogen to fuse.  When this happens, It’ll turn into a Red Giant.  A red giant star begins when most of the hydrogen fuel in the core of the sun is used up and turned into helium.  Gravity will force the gasses to collapse once again until the hydrogen in the outer layers becomes hot enough to begin fusing.  The energy released in the outer layers will cause the Star will expand to several hundred times it’s current diameter absorbing mercury, venus, and possibly earth.  Because the surface area of the sun is so much larger the temperature at the outer edges will be much cooler causing the sun to look red.  The star will burn this way for about a billion years.  The fusion of  hydrogen will create a thin shell of helium between the core of the star and the hydrogen fuel.  This layer intially isn’t hot enough to fuse, and it can’t contract due to gravity because it’s already at its densest possible state.  As more helium is added gravity will cause the helium shell to increase in temperature.  When the shell is hot enough, the helium will fuse with itself  creating carbon and release a tremendous amount of energy and matter into the solar system.  It’s called a helium flash.  It marks the dying stages of our star.  After a series of helium flashes, the sun will have shed most of it’s outer layers.  Gravity will force what remains of the sun into a dense little ball that will radiate light and heat as a White dwarf.  It doesn’t have the fuel to produce energy but it’ll have enough stored energy to shine as a little candle for trillions of years.


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