system
Given the following post, respond as if you're commenting on Reddit:
human
Is it possible to accurately predict the death of a given star?

If you were close enough to a red giant star would it be possible to accurately predict the point at which it would go super nova? How accurate could that prediction be? Would you be able to pinpoint the precise minute, year, decade? And from how far in advance? Also, same question for a smaller star collapsing into a white dwarf.
gpt
No.  The problem is that the changes which will result in the star exploding all occur deep within the core of the star, and it takes of order a million years for structural changes in the core to be visible on the surface.  This not coincidentally about the same amount of time it takes for the thermal energy in the core to make it out to the surface.

Supernovae in massive stars occur because the core has fused up to iron and doesn't have another energy source to fall back on and support the star against gravity.  There are five or six fusion stages the star goes through, roughly H -> He -> C, O -> Ne, Mg, Na -> Si -> Fe -> kaboom.  For a massive star like Betelgeuse, the first step takes 2-3 million years.  Helium to carbon/oxygen, another hundred thousand to million years.  Going up the alpha chain from oxygen takes no more than a few hundred.  After that, you get a silicon core in a few months to years, and an iron core in just a few weeks, if that.  This is because each rung of the nuclear burning ladder releases less and less energy per reaction and requires much higher temperatures to proceed, so the star goes through its fuel faster and faster.

And therein lies the problem.  Once the star moves off the main sequence and becomes a red supergiant (red giants don't go supernova, as a rule), all you know is that it has at least started burning helium in its core.  You have no real idea how far along it is in that process because the energy released by carbon burning and higher simply has no time to make it to the surface and produce visible changes in the structure of the star.  Now, you might have a shot if you were really close to it by detecting neutrino emission, since neutrinos stream freely out of all stars - in fact, solar neutrinos are some of our best physical evidence for the fusion processes occurring in the core of the Sun.  But you'd never see a high enough neutrino flux when viewing the star from afar.  Once the star goes supernova, you would get some slight amount of warning from an enormous surge of neutrinos resulting from the formation of the proto-neutron star at the center of the core, but that's it.  And it only amounts to a couple hours' warning before the supernova shockwave makes it to the surface and finishes blowing up the star.

Similar story for low-mass stars destined to become white dwarves.  Although we can definitely classify stars as being asymptotic giant branch (AGB) stars, which are no longer burning helium in their cores and are well on their way to completely shedding their outer layers, this phase is still very long (AGB stars are still burning hydrogen and helium in shells around the inert core).  The process by which an AGB star ejects its envelope and becomes a white dwarf takes of order 10-100,000 years.  This is blisteringly fast on an astronomical timescale, but we humans really can't say anything until a star is already well into this phase.  This is accentuated by the fact that this phase is so comparatively short, so there just aren't that many stars out there currently in it and thus we don't have a great handle on what they should look like.