Why do novas shine?

  • #1
Brzohn
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It’s my understanding that a nova happens when you have a close binary star, and one component is a white dwarf. The white dwarf sucks hydrogen from its companion, the hydrogen piles up on the surface of the dwarf until it’s hot and dense enough to undergo nuclear fusion. Ka-boom. Right?

Why is this energy visible light? Nuclear reactions produce energy in the form of gamma rays and moving particles. The reaction is on the surface of the star; the energy isn’t emitted in the middle of an atmosphere that it could be Compton scattered down the spectrum, or heat up the atmosphere till it glows. By my reasoning, a nova should emit gamma and cosmic rays, but not visible light. What’s going on?
 
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  • #2
Until just about now I've had the same misconception as you, I think. That the white dwarf is essentially a naked core emitter.
Take a look at this paper:
https://articles.adsabs.harvard.edu/pdf/2012MmSAI..83..779K
There's a brief overview of the process in the introduction.

The key takeaways seem to be:
The nova white dwarf has an atmosphere accreted from its companion. It then emits light from its photosphere, much like a regular star. The photosphere expands significantly during the nova event, on the order of a hundred solar radii, giving rise to the initial luminosity spike. Subsequently, Bremsstrahlung radiation in the ejecta take over as the main contributor to the light curve.
I.e. it doesn't appear to be the case that the fusing surface is ever exposed.
 
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  • #3
By the same argument, the sun should not emit visible light either. Does that point you in the right direction?
 
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