Novae vs Type Ia Supernovae

[ajtribick]

As far as I understand it, both novae and type Ia supernovae are caused by the accretion of matter onto white dwarf stars. My question is, why do some accreting white dwarfs only undergo nova events, and others go supernova? What is the difference in the setup?

Thanks.

[Malenfant]

Novae are when white dwarfs in close binary systems collect matter from their companions and then that ignites in the accretion disk around the WD.

IIRC, Type Ia are when so much matter piles up on the WD in this scenario that its mass goes to more than 1.44 solar masses, at which point it should collapse. Except in this case, it doesn't actually collapse, it instead blows up. Not sure off the top of my head why, I'd search but I have a thumping headache right now...

hopefully that's enough to get you started anyway

[Ynjevi]

In the case of novae, only the outer hydrogen layers of the white dwarf start fusion reaction and explode. Type Ia supernovae form when the critical density is reached and fusion reaction starts in the whole star at once.

[ajtribick]

So what determines which of the two outcomes happens?

Presumably you'd need a white dwarf close to the critical limit to have a supernova or you'd end up with enough hydrogen to trigger a "mere" nova before you got to the limit? Would accretion rate have an effect?

[Ynjevi]

Mass, as usual, determines what happens. Supernova Ia progenitors must lie very close to the maximum white dwarf mass, called Chandrasekhar limit, about 1.4 Solar masses. Only a very few white dwarfs are that massive.