BDanielMayfield wrote:It’s interesting that a “runaway thermonuclear event” involving hydrogen burning can happen on or near the surface of a white dwarf, while in the cores of stars hydrogen is fused at such a slow pace that stars can stay on the main sequence for millions, billions and in the case of the lightest stars, trillions of years.
How can the same nuclear reaction produce such differing effects?
Bruce
The utter non-mathematician that is me should not even attempt to answer that. Nevertheless, this is how I think of it.
Inside main sequence stars, hydrogen fusion takes place at a slow pace because the stellar core is not strongly compressed and the temperature of the core is not exceedingly high. When a star leaves the main sequence and core hydrogen fusion ceases, the stellar core shrinks and its temperature rises. Although this very greatly affects the star, whose outer layers expand mightily, there is no danger at this stage of the star being destroyed.
When the core has been sufficiently compressed, and the temperature in the core has risen sufficiently, core helium fusion will take place. Core helium fusion will be the end of the line for our own Sun. When the Sun, several billion years into the future, has used up its core helium, its core will be inert. It will shrink and heat, but the mass of the core (and the layers of the Sun weighing down on the core) will not be enough to heat the core to the sort of temperature where other kinds of fusion can take place. Similarly, the mass of the core and the layers of the Sun weighing down on the core will not be nearly enough to compress the core to the point where it can become a black hole. The core of the Sun will have reached the end of the line.
Yes, but for a while the Sun will still keep both helium fusion and hydrogen fusion going in shells around its core. But the way that the helium-fusing and hydrogen-fusing shells of the Sun will interact with each other is going to make the Sun unstable. This instability will lead to worse and worse convulsions in the Sun, until the Sun casts off its outer layer and becomes first a planetary nebula and then a cooling white dwarf. And that
will be the end of the line for the Sun.
Okay. But now imagine that the Sun had been more massive than it is. Imagine that it had been "teetering on the brink" as to whether or not it would be able to get carbon fusion and oxygen fusion going. It
almost got there. But not quite.
Now image that the inert core that
almost got more fusion going in itself has a stellar companion. And imagine that this stellar companion dumps matter on the inert core. And imagine that suddenly a large enough helping of gas is dumped on the core to start a "runaway thermonuclear event". How can this happen?
This is how I understand it. I believe that the core is relatively homogeneous. All of it is quite tightly packed, and all of it is made up of more or less the same elements. And all of it, mind you, is prime "fuel". All it takes to turn this inert core into a super-duper amazing super-bomb is to add a smidgen of mass. Remember that the only thing that prevented this core from getting another round of fusion going while the core was inside a functioning star was just a bit more mass coming into the core. Now the core is naked, but it is still able to get a brand new round of fusion going if only it gets a sufficient helping of new mass. And if this mass is coming, from a companion star, then every part of this previously inert core is ready to start turning itself into another kind of matter and also into pure energy. Remember that there is no "stellar envelope" to absorb the shock of the onset of this fusion. Every part of the core is turning itself into matter and pure energy almost simultaneously. Remember that the core is its own fuel, and remember that the core is so closely packed that every part of it has its own fuel right at hand.
Okay. That's how I understand a Type Ia supernova. I have probably missed some very important aspects of it, and I would be grateful to anyone who can set me straight!
And as for why most novae just involve the outer parts of the white dwarf, and not the whole star, well, that is something that I'd appreciate some information about!
Ann
Naked Eye Nova Centauri 2013
Well, at least put a monocle on it 
