Shrike
BWO Team Member
Registered: Jul 2000
Location: Lo-Fidelity All Star
Posts: 2273 |
quote:
Originally posted by Turnsky
errr... no red giants are when stars roughly the size of earth's run out of fissionable materials and start to burn out slowly.
Supernovas happen when a really big star's materials react all at once and explode.
Neutron stars happen when a star explodes and then all the widely distributed material from that explosing starts going in again and forms a rather dense object with a very high gravitational force, black holes happen when a neutron star collapses even further in on itself.
Not quite. 
Red Giant stars are stars that have moved off the main sequence (which is Hydrogen burning) and begin burning Helium (this is where our Sun would stop, after Helium burning, blowing off the outer layers and forming a white dwarf made up of Carbon, Oxygen and a few other elements) and if the star is massive enough, heavier elements such as Carbon, Oxygen, Neon, etc, right up so Silicon. There's some theorized numbers, and as a little FYI, Silicon -> Iron burning for a 1 solar mass core takes something like half a day but I'll leave that.
Also, I'm not sure how many people know, but a star is in equilibrium between inward pressure (gravity) and outward pressure. (radiation) So if you get less radiation, the core of the star collapses until the pressure is high enough for the next reaction to begin. You may have heard of a 'Helium Flash', that is when Helium burning begins, and this causes a re-equilibration.
So our presupernova star is basically like a giant plasma onion, with a core made of Silicon and concentric shells of progressively lighter elements. Now, each fusion reaction has a threshold pressure-temperature regime, and every time a reaction starts, you get more energy to stave off the collapse of the star, but at ever higher P-T. Now, the thing is, the binding energy of Iron is higher than the energy released from the reaction of 2Si - > Fe. So basically you star Iron burning, and the energy keeping the core of the star from collapsing vanishes. Bang, collapse.
Over about 1/4 of a second or so, the core is in free-fall as it compresses to nucleonic density, and since nucleonic matter is nearly incompressible, it acts like a very stiff spring. This is called core rebound, and is why supernova spew all their outer layers (which could be tens of solar masses) into space. I'm not sure about this, but I believe that this collapse converts gravitational energy to EM energy, which helps power the supernova. The core can be left as a neutron star (if it's over the Chandresekhar limit of ~1.4 solar masses) or collapse unstoppably to a black hole if it's over about 2.5 solar masses or so.
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11-14-2001 02:14 AM
