# Astronomy question: Mechanism(s) of novae **URL:** \ **Category:** Factual Questions\ **Created:** [September 13, 2004, 3:19am UTC](https://boards.straightdope.com/t/astronomy-question-mechanism-s-of-novae/264208 "2004-09-13T03:19:01Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [September 13, 2004, 3:19am UTC](https://boards.straightdope.com/t/astronomy-question-mechanism-s-of-novae/264208/1 "2004-09-13T03:19:01Z")
Are all stellar novae due to the initiation of a massive fusion reaction on the surface of a star (? always a white dwarf) which has received infalling matter (? always hydrogen) from a companion? Are other mechanisms known or postulated? (BTW, is the classical mechanism above synonymous with “cataclysmic variable”? I’m pretty sure it is, but would appreciate confirmation.) Thanks! ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [September 13, 2004, 3:36am UTC](https://boards.straightdope.com/t/astronomy-question-mechanism-s-of-novae/264208/2 "2004-09-13T03:36:40Z")
Actually, the most common mechanism for triggering supernovas occurs inside ordinary, if fairly massive, old stars. As the hydrogen fuel is depleted, the interior of the star begins to cool. Eventually, it cools to the point where the internal pressure can no longer counteract gravity, and the star collapses in on itself. If the mass of the star is below a certain critical value, this collapse triggers a new burst of fusion activity, which causes the star to balloon up into a red giant which slowly burns itself out to a white dwarf. This is believed to be the ultimate fate of our own sun. However, if the star is above the critical mass value (I’m not sure what it is, but IIRC it’s more than 3 or 4 times our sun’s mass), the collapse triggers such an intense burst of fusion that it blows the star apart. A supernova. Vastly simplified, but hopefully you get the idea. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [September 13, 2004, 3:39am UTC](https://boards.straightdope.com/t/astronomy-question-mechanism-s-of-novae/264208/3 "2004-09-13T03:39:07Z")
Just so I don’t get more confused, I’ll ask: Are you referring to novae or supernovae? ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [September 13, 2004, 3:54am UTC](https://boards.straightdope.com/t/astronomy-question-mechanism-s-of-novae/264208/4 "2004-09-13T03:54:29Z")
> [@KarlGauss](#): > > Just so I don’t get more confused, I’ll ask: Are you referring to novae or supernovae? Ah, sorry. You’re asking about novae, and I’m talking about supernovae, which are two different beasts. If you’re asking generally about stellar outbursts (which most people call novae, regardless of cause), then [this page](http://www.madsci.org/posts/archives/mar97/854312977.As.r.html) ought to answer your questions about novae, in particular: > [@](#): > > A nova is basically caused by a nuclear explosion on the surface of an unusual type of star. A nova outburst can only occur in a special type of star system. There must be two stars orbiting each other (we call this a binary star system). One of the stars is fairly normal, perhaps like our Sun. The other star, however, is a white dwarf. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [September 14, 2004, 2:01am UTC](https://boards.straightdope.com/t/astronomy-question-mechanism-s-of-novae/264208/5 "2004-09-14T02:01:00Z")
> [@me](#): > > Are other mechanisms known or postulated? > > (BTW, is the classical mechanism above synonymous with “cataclysmic variable”? I’m pretty sure it is, but would appreciate confirmation.) > > Thanks! Any takers for these questions? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 14, 2004, 6:13am UTC](https://boards.straightdope.com/t/astronomy-question-mechanism-s-of-novae/264208/6 "2004-09-14T06:13:41Z")
> [@](#): > > (BTW, is the classical mechanism above synonymous with “cataclysmic variable”? I’m pretty sure it is, but would appreciate confirmation.) I really ought to know this one, but I don’t. The terms “cataclysmic variable” and “recurring nova” seem to be used distinctly, but I’m not sure what the difference is, if anything. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [September 14, 2004, 6:04pm UTC](https://boards.straightdope.com/t/astronomy-question-mechanism-s-of-novae/264208/7 "2004-09-14T18:04:08Z")
I see **Angua** is about, so I’m bumping this up to the top to (hopefully) catch her attention.