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>