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TriangleMan
2004-Feb-28, 03:38 PM
I read about how in large mass stars that once you get past core helium fusion the star doesn't have much time left. Core carbon fusion takes only centuries, oxygen less than a year. Are there any noticable changes to the star once it gets to the carbon-core stage so that we could predict when it goes nova?

Ut
2004-Feb-28, 06:02 PM
I read somewhere that when the metals are being fused in the core, the star pulsates, and ejects mass. I think it had something to do with there being enough energy released to expand the core, lower the gravitational pressure, and shut down fusion for a short time. I can't, for the life of me, remember where or when I read this, though. It could have been last year, or when I was in grade school. So I wouldn't put a whole lot of faith in it.

TriangleMan
2004-Mar-03, 11:54 AM
I thought that even when a star has a carbon core it still has hydrogen and helium layers above the core. I was just wondering if we would be able to detect when a star has a carbon core so that we have a decent idea of when it'll go nova.

Ut
2004-Mar-03, 04:29 PM
The fusion can take place in the concentric spheres surrounding the core, out to some distance. They play their part in mass expulsion. Core activity, I'd expect, would be more central to such things as the star pulsing. My stellar theory is kind of weak, but this makes sense to me. The fusing of heavier elements in the core is less efficient than of hydrogen, resulting in the need for higher pressures for it to take place. As the pressure increases (i.e., the core tries to collapse), the energy output increases and the core expands, slowing down fusion. These fluctuations should be noticably strong in a star burning heavy elements, like carbon. As the end gets closer, the star, I think, pulsates faster. This is really speculation on my part, though. I don't know this to be what's theoretically expected.

Grand_Lunar
2004-Mar-06, 11:59 PM
Only way I think we might be able to tell is by using a spetroscope (or whatever its high tech equivalent is).
I know that when a star starts producing iron, its had it. Iron nucli cannot fuse. It takes more engery to bind them than they release. They require an input of engery. That's how fission works (but a supernova's not the same!).
Our best bet, I think, is to watch the star Betelgeuse. See what that star is doing. It seems the right kind to want to go "KABLOOIE!"
Oh, wait, in space, no one can hear you sream!
So then, we watch for the star to go _____________________!
:wink:

Normandy6644
2004-Mar-07, 12:50 AM
Only way I think we might be able to tell is by using a spetroscope (or whatever its high tech equivalent is).
I know that when a star starts producing iron, its had it. Iron nucli cannot fuse. It takes more engery to bind them than they release. They require an input of engery. That's how fission works (but a supernova's not the same!).
Our best bet, I think, is to watch the star Betelgeuse. See what that star is doing. It seems the right kind to want to go "KABLOOIE!"
Oh, wait, in space, no one can hear you sream!
So then, we watch for the star to go _____________________!
:wink:

From what I remember, once iron is in the core, it does indeed fuse but for a very short time, since whatever it fuses into (I don't feel like looking it up right now) is too unstable, at which point fission does occur. This will happen until the core's pressure becomes too high due to all the unstable reactions and then it goes boom. :lol:

Kaptain K
2004-Mar-07, 02:40 AM
Iron fusion and fission are both endothermic. If either (or both, for that matter) occurs, it just exacerbates the situation, since these reactions absorb energy instead of releasing it.