Getting Sidetracked

I started this post with what I thought was a rather clever idea (that I may revisit in another post) that involved a bit of research into the final stages of the sun's life. Apparently it wasn't clever enough to keep my attention, because I ran into a few articles about the helium flash and got a little bit distracted.

The sun isn't always going to stay its current friendly size. In five billion years or so, it will begin to run out of hydrogen to fuel nuclear fusion and begin to fuse heavier and heavier elements (at this point it'll also have about double its present-day luminosity). As it does so, it will expand to many times its current size, careening off the main sequence and ballooning into a red giant. Initially it's dramatic but slow--a billion years spent growing to 200 times its current size and thousands of times its current luminosity. Then things get crazy.

The sun's core is now made up entirely of helium, which is added onto by helium "ash" that sinks into the core as a product of the last of its hydrogen fusing in an outer shell. For stars the size of our sun (and up to around twice its size), this core doesn't get hot enough to begin burning helium. As a result, there is no thermal pressure to counteract gravity, and the star begins to collapse in on itself, and the core gets so densely packed that the helium in the very center actually turns into degenerate matter. Degenerate helium heats up, but cannot expand to compensate as a normal gas would. Eventually, it hits a temperature of around $10^8$ K, and the helium in the core suddenly begins to fuse in a runaway chain reaction--as helium ignites it heats the core up even faster, which simply leads to the ignition of even more helium. In a few rather dramatic moments (and this really is on the timescale of seconds), 6% of the core's helium turns into carbon, and the sun's core is producing as much energy as our entire galaxy.

I wanted to do a clever scale-relating analogy like I did in this post, but there is literally no way to scale down something that insane. So here's the Milky Way. Imagine packing it into something around the same size as Jupiter. (Source)

And yet, despite that patently insane fact, that's not even the most astounding part. You would expect that a galaxy of luminosity packed into the center of a solar system would fry every rock, gas, and dwarf planet all the way out to Quaoar--but it doesn't. The thermal pressure given off by the helium flash is finally enough to counteract the crushing gravity of the rest of the overblown star pushing down on it, puffing our beleaguered sun back out and returning the core to its previous nondegenerate state. But doing so requires so much energy that the entire explosion is absorbed into the core that gives rise to it--despite the prodigious amount of energy being released, the process is unobservable from outside the star.

It's over in seconds, but it's really the death knell for the sun. Ten billion years old, it's life is 99.99% over. It will become increasingly unstable over the next few million years, fusing what helium it has left and blowing out mass in the solar wind. Meanwhile, a series of massive pulses will briefly but dramatically increase its size and luminosity. After the last of these, the sun will finally release its outermost shell, which will expand out into a planetary nebula. The core that saw so much dramatic action will be left as a white dwarf.

The downside? We'll all be fried. The upside? Aside from being ridiculously cool, our solar system will be left looking something like this:

Mosaic of the Helix Nebula, composed of Hubble and Kitt Peak National Observatory images. The white dwarf is visible at the very center. A much larger (as in 16,000 x 16,000) version is available here. (Source)

At least we'll be going out in style.