Why do stars go nova?

It seems like its violating all sorts of natural laws considering a star’s actually burning up its fuel as it ages and shrinks. So why doesnt it simply wink out of existence like a fire running out of oxygen?

And I dont understand why larger stars collapse into bigger supernovas. As a star dies, shouldnt they all, no matter how big they are, reach a certain smallness that initiate the nova process? Why wouldnt that size be the same for every size of star? After all, if a huge star goes nova at, for example, a tenth of its size, then shouldnt a small star already a tenth of the size of the larger one go nova immediately?

Stars work by fusing hydrogen nuclei to helium. When a star has used up its supply of hydrogen it can no longer maintain its size against the pull of gravity, so it collapses. As it gets smaller it gets hotter. Eventually it gets hot enough for helium to fuse to carbon. So it doesn’t just “wink out”, since it now has all that helium to use up.

A critical factor missing from the otherwise good explanation above is pressure. The heat of fusing hydrogen into helium supplies the pressure to keep the star expanded against the inward pull of gravity. When that pressure weakens because the fuel runs out, the star collapses. If it’s a large enough star, this collapse is violent enough to heat the core hot enough to fuse huge amounts of material into all the elements as heavy as uranium. Oh, and also blow the star apart. That’s a supernova. Smaller stars, like our Sun won’t go supernova. Instead, when the Sun’s hydrogen fuel runs out and it collapses, the collapse won’t be so violent. It will heat the core, but only enough to make it puff up into a red giant. Eventually it will shrink down to a white dwarf as the core temperature is no longer sufficient to fuse heavier elements.

Obligatory plug: Just got done reading Phil Plait’s “Death From he Skies!” Fun reading. Talks about several forms of star deaths including supernovas* in an easy to understand way.

Obligatory Wikipedia link: http://en.wikipedia.org/wiki/Supernova

  • Since I speak English, not Latin, I don’t use the -ae ending.

It’s important to realize that not all of the star’s mass ends up blasted hither thither and yon. A significant portion of it ends up compressed into a really small, really dense remnant in the middle (either a neutron star or a black hole). That means that the portions that end up in the remnant have really low potential energy, which leaves a lot of kinetic energy available for the parts that do get blasted away.

There are several different types of novas, and the mechanisms are different for each.

Stars are basically just a ball of gas subject to the tug-of-war of two forces. Gravity is trying to crush the gas inward, while the energy released by fusion reactions tries to inflate the gas outward. Moreover, the heavier, denser elements build up towards the core of the star, while the lighter elements remain near the surface. Pressure is greater in the center than at the surface.

Initially, the heavier elements are usually unable to fuse with one another because, in general, heavier fusion reactions require greater temperature and pressure to occur spontaneously. However, as the lighter fusion reactions closer to the surface of the star begin to dwindle, fusion begins to lose the tug-of-war against gravity. Gravity causes the star to collapse inward again, but this causes the pressure inside the star to increase. In some cases, the increased pressure may allow those heavier elements that couldn’t fuse before to suddenly “ignite”. This may cause the star to swell (like a red giant), or in some cases the particular reactions may be so energetic that the outer mantle of lighter elements are imparted with enough outward velocity to escape the star entirely. This is one sort of nova.

Other, more exotic forms of novas are possible, such as what happens when a white dwarf collapses into a neutron star, but it’s kind of complicated. I’ll let someone who’s more knowledgeable about fundamental particle physics explain how inverse beta decay works, if you really want to know. The general principle is similar, in that as the star collapses, the pressure caused by gravity finally crosses some threshold needed to make a new kind of reaction occur. The difference, in this case, is that the reactions aren’t nuclear fusion, but rather the electrons in the star can “react” in some sense with the quarks in a proton in order to convert them both into a neutron and (I believe) a very high-energy tau neutrino. In laymen’s terms, this makes at least part of the star go boom.