How do supernova's work?

[Leovinus]

I get the idea that a star the size of our Sun will eventually run out of fuel. When that happens, the pressure of the nuclear furnace inside the star can no longer push out to balance the weight of the matter above and the star collapses to a white dwarf.<br /><br />What I don't understand is this: If the star is bigger and the collapse happens, you don't end up with a bigger white dwarf; you end up with a supernova. Why? What causes the explosion? <div class="Discussion_UserSignature"> </div>

 

[Leovinus]

Thanks. That was an excellent explanation. <div class="Discussion_UserSignature"> </div>

 

[Maddad]

Leo<br />While what tigerbitten said was correct, the answer to your question is even simpler. <br />A star our size can support its own weight with degeneracy pressure, so it doesn't go supernova when it dies.<br /><br />There are two types of supernovas, a Type I and a Type II, with some minor variations of both. The Type I involves a dwarf star bleeding off material from a binary companion, usually a giant star. As the dwarf grows more massive it crosses a point where the degeneracy pressure can no longer support its weight, so it collapses. (In the collapse most all of its remaining protons (10⁵⁸ or so) combine with electrons producing neutrons. Each new neutron also releases a neutrino. That flood of neutrinos helps power the explosion.)<br /><br />A Type II supernova starts out massing more than the critical amount. It's degeneracy pressure would not support its weight, but its nuclear reactions in its core will while it's still fusing one element into another on its way to iron. When it finally runs out, then it also suddenly shuts down and collapses.<br /><br />One difference between Type I and II supernovas is that the Type I's always have the same profile because they all have exactly the same mass. This means that their absolute brightness is a constant, which makes them excellent for measuring really distant objects since we can see them from across the universe.<br />

 

[thalion]

Q: Why do Type II supernovae form neutron stars and not massive white dwarfs?<br />--If the mass of the remnant core is greater than 1.4 solar masses, it is too massive to become a white dwarf, and must collapse to either a neutron star or a black hole.<br /><br />Q: What causes the explosion?<br />--The very short answer is this: the degenerate core collapses to neutron star-size very quickly, in less than a fraction of a second. The massive layers of the star over the core collapse down as well, compressing the core to a maximum density (called--imaginatively--"maximum scrunch"), after which the core rebounds, sending a shock wave through the previously-collapsing layers of the star. This shock wave is what blows the star apart, along with some extra energy added by neutrinos during the formation of the neutron star (indeed, 99% of a supernova's energy is released as neutrinos).

 

[Saiph]

Tigerbiten: I don't believe the cause of the shell fusion is due to released neutrons. That probably helps, but the main cause is the shockwave hitting the core, as Thalion points out.<br /><br /><br /><br />The main difference between a massive star and small star dying, is the rate at which the core stops. When a small star's core ceases to fuse, it's really a relatively slow process. Also, the hydrogen envelope of the star (i.e. everything non-core) rests upon the core, or near the core. It's density can help trap the last remaining light for a while, and it slowly dissipates over thousands of years as pure thermal energy. Gravitational contraction also lengthens this process.<br /><br />A massive star however, gets to iron. Iron requires more energy to fuse than the process releases. It's a huge energy sink. As such the fusion shuts down, rapidly, once significiant levels of iron are produced. Since there is no fusion there, and the core is massive (usually several solar masses) the core collapses into a neutron star. The envelope of the star is being supported by radiation, not gas pressure. I.e. light alone is holding it up. <br /><br />Guess what happens when the fusion goes out? The light's go out too! And since there is little gas near the core to hold the light energy and slowly ration it out, you've knocked the foundation out from under 10 solar masses + of material. Even if it wasn't supported by radiation, the rapid shinking of the core from something similar to the sun's radius, to something smaller than earth certainly knocks out the foundation as well.<br /><br />It plummets to the now shrinking core at relativistic speeds. The leading edge hits the core, and at those speeds compacts a lot. Much of the energy is converted straight to thermal. Now you've got incredibly high pressures, and absurdly high temperatures. This allows fusion of Hydrogen (and basically anything else!) causing not only a rebound shockwave, but one boosted by the run-am <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>