With the recent news about the "fainting" of Betelgeuse and the speculation that this might be a precursor to a supernova, I'm wondering if there is any theoretical/observational basis for this interpretation, or whether this is a case of reading too much into a greater-than-average change in an intrinsically variable star. Why would a star become dimmer in the run-up to a supernova?
1 Answer
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The connection between the dimming and a putative supernova relies on the interpretation that the decrease in luminosity may be due to circumstellar material, ejected in the years/decades/centuries immediately preceding a supernova. There are several mechanisms that could lead to this sort of mass loss (see slides 24-25), including
- gravity-wave driven envelope loss (in red supergiants, during neon/oxygen core burning)
- pulsations due to pair-instability in the days to decades prior to the supernova
- turbulent eruptions à la luminous blue variables
These then lead to Type IIn supernovae, with narrow lines arising from interactions with the previously ejected circumstellar material. If the dimming is due to extinction by circumstellar dust from eruptions due to any of these mechanisms, it could mean that a supernova is imminent on a timescale of days to years.
answered Jan 2, 2020 at 16:03
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$\begingroup$ Wouldn't the star become brighter before the supernova as more energetic fusion reactions happen forming bigger atoms and the star expands, then get dimmer as iron forms and fusion stops, and then brighter again from the gravitational collapse right before the supernova? $\endgroup$– CatersCommented Jan 3, 2020 at 19:52
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2$\begingroup$ @Caters, the gravitational collapse at the end of the few days where the nickel-56 core grows to 1.4 solar masses lasts only a few milliseconds. Any brightening from the gravitational collapse is due to neutrinos interacting with the outer envelope, and not due to that envelope contracting in any measurable way, as there are no pressure waves in the star that could travel that fast (and indeed, the first pressure wave that reaches the envelope is the visible part of the supernova event). $\endgroup$ Commented Jan 3, 2020 at 23:49