A post on the Developer Forums about degraded chemical weapons being found in Iraq sort of evolved into a discussion of stellar physics while our collective backs were turned. I'm thinking maybe that particular discussion should be moved onto the blogs - it's starting to clutter up developer central.
Loduwijk, the reason why the fusion of helium continues after the star expands is fairly simple - once you've started fusing helium, that fusion produces enough heat to keep itself burning. As a (very) rough analogy, think about starting a fire by raising the temperature of a block of wood. Once the wood actually catches fire, trying to lower the temperature does nothing - the fire keeps itself going, by producing enough energy to set more wood on fire. It's sort of a similar thing.
Generally, stars die because of one of two reasons:
1 - They run out of things to 'burn', and collapse into a white dwarf
2 - They start trying to fuse iron.
Low mass stars do #1, high mass stars do #2. The 'turning point' of mass is about 1.4 solar masses, I believe - it's called the Chandrasekhar limit, after the indian astronomer who originally worked it out. The reason why the limiting factor is mass is also pretty simple - in order to fuse higher and higher elements, you need a higher and higher 'ignition temperature', although it's not strictly speaking temperature that sets it off - the atoms of that element need to have enough kinetic energy that they're completely ionised, and they need enough kinetic energy to overcome their mutual repulsion. Then, you need enough pressure to stick them together. Once the star starts to run out of hydrogen, it starts to collapse, increasing the temperature and kinetic energy of the atoms in its core, which can start the next stage. But if the star isn't massive enough, it can't get a high enough pressure or temperature, so it can't get the fusion going, and it just keeps collapsing.
Iron is a different matter, because it's got the highest nuclear binding energy of all elements. Trying to fuse iron, or any element above iron, takes energy, trying to fission iron, or any element below iron, takes energy. Once a star hits iron, it can go no further - higher elements have to be created in supernovas, or other such situations.
Red giants do have fairly shot lifespans, compared to 'main sequence' stars. I'm not sure about most of the galaxy being red giants - it may be that it's most of the observable galaxy, which wouldn't be surprising, given that red giants are brighter then the main sequence star they came from. Plus, the main factor in the lifespan of a star is actually its mass - bigger stars burn fuel faster, go through the main sequence faster, and then explode. The heavier a star is, the shorter its life is. Some of the blue-white stars, at the upper end of the mass scale, have lifespans that are only in the millions of years. Red giats could outnumber those easily, given how long stars below the Chandrasekhar limit live - generally a few billion years.
Astrophysics was my second preference for university. I got into my first, Software Engineering, but I still knew a lot about astrophysics before then, and probably more now. It helps that I can do physics as an elective. :)
Oh no, hydrogen can maintain a continual fusion reaction. Helium needs a higher temperature to get started, that's all. Once it's started, the pressure can be relaxed a bit.
The problem with fusion reactors is that we're trying to control it. And you still need pressure, even when it keeps itself going. To get self-sustaining nuclear fusion that will contain itself, you need something the size of a star, which is very much unworkable. That's the big problem - we're trying to sustain nuclear fusion with very limited fuel. Plus, the reactions we use aren't aneutronic - there are a fair few neutrons spat out by the reactions, and they're travelling pretty fast. That would almost certainly kill anybody exposed to them for a few seconds, without shielding, and would probably toast the reactor fairly quickly, too. | |
I recall hearing that MIT has a hydrogen fusion project going on where they are fusing hydrogen with magnetically generated preassure and using that to run a nuclear fusion powered electric generator. Obviously, they don't create any excess energy for use since they get out of it less than they put in, and so they just put part of what they generate right back into the superconducting electromagnets, but still this made me think... if we could initiate a helium fusion reaction, and helium can sustain itself in a chain fusion reaction, then that could be used to produce energy.
Of course, I'm thinking only hypothetically. Still, it has my curiosity peaked. Does your book say exactly how many times more energy is required for helium fusion than in hydrogen?