Jackie wrote:

"Bob Greschke" wrote in message
...
But Sirius IS big enough, AND it's big enough, AND it will go off someday.
Then we may just be toast.

Sirius "A" IS one of the closest of stars to our own Sun (Sirius is a bit
over 8 light-years away), but it's not large enough to go supernova. If
current theories of stellar evolution hold, when Sirius has exhausted its
nuclear fuel, it will eventually expand to a red giant phase (like our own
Sun will) and eventually cool down to a white dwarf star. The type of
supernova that many giant hot stars may experience is the Type II, which is
the result of the collapse of the great star's core. Our own Sun is neither
hot enough nor large enough to experience such a core collapse, and while
Sirius *is* hotter than our Sun, it's not large enough to go Type II
supernova (core collapse supernovae are reserved for the largest, hottest
and most unstable stars).

However, there is another type of supernova, the type IIa. These take the
scenario of a duo of stars in which both companions revolve close to one
another and one is larger than the other-- the typical scenario here is a
red giant star and a close white dwarf companion. The small white dwarf star
has a huge amount of matter compressed within its small sphere; the red
giant companion is also massive, but its huge outer atmosphere is relatively
sparse and has become massive via its expansion. If the two stars are close
enough to each other, the smaller and more massive of the two can pull some
of the atmosphere off the larger star. At a certain point (the Chandrasekhar
limit, or approx. 1.3-1.4 solar masses) the small star cannot hold any more
mass, and the 'borrowed' mass of its companion beyond that critical point
ignites a thermonuclear reaction.

Insofar as I know, Sirius's white dwarf companion star, the "Pup", orbits
too far away from Sirius to cause the type 11a supernova.

Jackie

I'm sure you meant Type Ia supernovae for binary pairs involving a
white dwarf. http://www.edu-observatory.org/eo/white_dwarfs.html