[tejtm]

There are different types of nova. This type happens when one of a close binary pair is a is a compact stellar remnant, it already spent its fuel and is a *dense* inert planet sized mass of neutrons. The other star in the binary pair is "not dead yet" but getting there. Late stage stars swell up as the core loses its grip on the outer layers. In this case the binary pair is close enough that the outer shell the red giant is loosing can be captured by the white dwarf. It is snowing star dust!

This material landing on the white dwarf is the ordinary (light-ish) matter found in the outer layers of stars. The ball of neutrons that the star-snow is falling on, does not care. When the star-snow lands it settles into a (nigh) perfect sphere. So no mountains, moguls nor mole hills on neutron stars everything will be perfectly smooth. This keeps up until the star-snow is coating the entire planet sized ball of neutrons to a depth of maybe six feet at which point the pressure of its own weight causes it to spontaneously undergo fusion.

Rapid fusion of the top layer of mass over an entire planet sized ball does goes brrrr. And again, the ball of neutrons does not care that its entire surface just became an atomic explosion.

This process gets used because we can calculate allot of things.

We can bound about how big the ball of neutrons could be, smaller it would not form and bigger it would be a black hole.

We can bound how much matter could pile up before it has to go boom.

We can calculate what an atomic explosion with X amount of matter should look like.

So these explosions become sign posts, first because we can see them in other galaxies and the compare what is seen/measured with predictions which gives things like how far away it would have to be to seem X bright, or what must be in the way for this part of the spectrum to be wonky.

This particular one is predictable because it is close enough we can have a pretty good estimate to how fast the star-snow is accumulating, and how deep it can get before it pops off, again.

Disclamer: no formal ed in this stuff.

[foxyv]

To add to this, White Dwarfs consist of electron degenerate matter, not pure neutrons. They have not yet reached the point where the electrons and protons fuse to become neutrons. Essentially the atoms have been compressed to the point where the only thing keeping the star from collapsing further is a quirk of quantum mechanics and the Pauli exclusion principle.

https://en.wikipedia.org/wiki/Electron_degeneracy_pressure

In addition, when the White Dwarf accumulates too much mass (1.4 Solar Masses), it will cause a Type Ia supernova where the entire star will explode leaving behind nothing.

https://en.wikipedia.org/wiki/Type_Ia_supernova

https://en.wikipedia.org/wiki/Chandrasekhar_limit

[tejtm]

  Thank you! 

I was hoping if I spewed some vague hand waving someone knowledgeable would fill in with accurate details.

[foxyv]

I liked your comment! It was well thought out and typed. I just wanted to add a couple details I thought were really neat. The idea that stars can be destroyed at all blew my mind when I first learned about it. I always love to talk about it!