How long from star collapse to supernova explosion? According to some reports, Betelgeuse might "be ready to explode into a supernova". It's luminosity has decreased "considerably" in the recent weeks. 
If we consider the "stable period" as that when luminosity is stable, what is the time between a star beginning to dim (i.e. leaving the "stable period") and exploding as a supernova? To put it more crudely, for the case of Betelgeuse, how long might it be until it explodes as supernova? In the end, I want to know whether we are talking about a process that takes millions of years or rather is a "sudden", few weeks/months/years process that I can see in my lifetime.
I get the feeling the answer is somehow hidden in this part of the Wikipedia entry on supernovas, but I am not trained enough to see it. It probably depends on the actual size of the star (but in this particular question, we can take Betelgeuse as reference).
 A: As I wrote in an answer on Astronomy Stack Exchange, the supernova hypothesis (which remains rather unlikely) doesn't imply that the dimming of Betelgeuse is a part of the collapse, or even that the star could collapse today or tomorrow. One explanation of the recent photometry is that the star has expelled matter from its outer layers, forming a shell of circumstellar gas. Light from the star that passes through this gas is absorbed or scattered, leading to extinction, and so it appears to us that the star is getting dimmer, even though the dimming is extrinsic, not intrinsic.
The various processes that could lead to this mass loss happen in the late stages of nuclear fusion, typically while the core is burning neon, oxygen, or silicon. Silicon fusion lasts on the order of a day or so; instabilities during silicon burning would indicate that a supernova should occur on a timescale of days. Neon and oxygen fusion take place on the order of about a year (though some late-stage mass loss events could also occur a few decades prior), and gravity wave pulsations during this phase could lead to the requisite mass loss.
In short, even if the dimming indeed originates from one of these mechanisms (gravity waves, pulsational pair-instability events, etc.), it could simply mean that Betelgeuse will undergo a supernova within a few years or decades. The supernova probably wouldn't happen tomorrow, or next week, or even next month. But it would be sooner than we previously expected.
