How soon?
Last week, next month, next century.
12/21/2012...
“How soon?”
In the year 2525.....
This article does the best job of describing the process to those at least somewhat technically minded that I’ve seen:
http://cass.ucsd.edu/public/tutorial/SN.html
So the answer to “how long” before the supernova is highly dependent on which “shell of fusion” is now being exhausted. Taking these numbers as exactly accurate, if it is exhausting the “helium burning” phase, the Betelgeuse will expand again and be in the Carbon burning phase for another 300 years. On the other hand if it is going into the Oxygen burning phase, the core collapse is imminent. Note that the core collapses independently of the external “surface” and MUCH more rapidly, in fact “catastrophically”. (I’ve seen estimates near 70,000km/sec.) [Of course, it is possible that that this is merely a size oscillation which is not understood or covered by current theory.]
The distance believed to be “safe distance” from a supernova of this type is about 300 light years, and if so, we’ll be OK here, provided the star’s pole is NOT pointed “at us”. It is estimated that there is only about one supernova in the Milky Way Galaxy per century. If Betelgeuse DOES go soon, it will be a remarkably close- Ring Side seats!