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I was reading about the helium flash, the short but sudden onset of helium fusion in certain red giant stars. As I understand, the upper (nondegenerate) layers of the star absorb the energy as they quickly expand, which leads to the stabilization of the core as thermal pressure takes over, leading to the period of helium burning.

I was curious as to whether this could be generalized to the onset of fusion of heavier elements later in a more massive star's life, and I came across a reference to a hypothetical silicon flash in Woosley & Heger (2015). More digging led me to a passage in Fundamental Astronomy (page 250), which states

In stars with masses around 10 $M_\odot$ either carbon or oxygen may be ignited explosively just like helium in relatively low-mass stars; there is a carbon or oxygen flash. This is much more powerful than the helium flash, and may make the star explode as a supernova.

Let's say that a star undergoes a relatively low-power carbon, oxygen, or silicon flash, and enough energy is absorbed by the outer layers such that the star does not undergo a supernova. Would enough energy escape the outer layers of the star so that we be able to detect the flash using optical or neutrino telescopes? Obviously, this is distance-dependent, so I'll say that the hypothetical star is within five parsecs of Earth.

As another way to put the question, how luminous would the flash be?

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    $\begingroup$ I don't have nearly enough time for a thorough answer now but there's some recent modelling in this paper. If you understand Kippenhahn diagrams, Fig. 3 shows degenerate carbon ignition in a series of masses. Sometimes you get one flash, sometimes a series of flashes, and sometimes a steady "carbon flame" that burns towards the centre of the star. As to the question, my suspicion is that it wouldn't be observed for the same reason that the He-flash isn't: it's buried too deep. Then again, a Type Ia supernova is basically a "naked" carbon flash! $\endgroup$
    – Warrick
    Commented Aug 1, 2016 at 6:48

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There is also something called a "giant eruption", like eta Carinae, which is a type of "failed supernova" because it didn't destroy the star. It is not known what causes giant eruptions, but perhaps one thing to think about is a flash of the type you mention. Giant eruptions certainly are visible, eta Car got quite bright when it blew off much of its envelope.

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  • $\begingroup$ The question was: " Would enough energy escape the outer layers of the star so that we be able to detect the flash using optical or neutrino telescopes?" So my answer was, "it is not known what causes giant eruptions, but if they can be caused by the flashes you are talking about, then the answer is yes, they can be observed optically." It depends on the state of the star at the time-- most stars absorb their own flashes so they cannot be observed, but in the question it was pointed out that some stars might actually be destroyed by their own flashes. A giant eruption is between those. $\endgroup$
    – Ken G
    Commented Sep 13, 2016 at 13:54
  • $\begingroup$ I didn't vote on this, but I'm assuming that the downvote came because of the assumption that there is such a connection; the basis for the answer is just speculation. Also, both components of Eta Carinae (as well as many LBVs in general) may be too massive for carbon flashes to occur. $\endgroup$
    – HDE 226868 ♦
    Commented Dec 29, 2016 at 20:41
  • $\begingroup$ The question was not restricted to carbon flashes, it also mentioned silicon flashes, which presumably would happen in stars more massive than the 10 solar mass carbon flash mentioned in the question. But yes, the possibility that nuclear flashes produce LBV eruptions is speculative, but so would be any explanations for LBV eruptions. One might also want to consider something more akin to thermal pulses, which don't happen in the core and don't require degeneracy, but are otherwise similar to "flashes" in that they involve unstable fusion. $\endgroup$
    – Ken G
    Commented Dec 29, 2016 at 21:50

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