[[How about Procyon B and Sirius B - the white dwarf companions to theDogtsar &little Dogstar?

Are they likely to result in supernova - esp. given Procyon A is already evolving towards gianthood? ]]

No. There is something called a “type I supernova” which involves matter from a red giant star falling onto a white dwarf, but the two stars have to be very close together — Sirius A and B, and Procyon A and B, are separated by several AUs — they are “visual binaries.” A close binary of the type required would only be visible from Earth as a “spectroscopic” or “eclipsing” binary.

[[If it is 20000 lightyears away, doesnt that mean its likely already blown up?(and we havent seen it yet) Is there any way to measure these things before the light reaches us?]]

Yes, it probably has blown up already, and we’re just not seeing it yet. No, there’s no way to measure it before the light reaches us. As far as present-day physics can tell, there is no easy way to send a signal faster than the speed of light. (Technically you can do it by warping space or with a “wormhole,” but creating or using either takes enormous amounts of power and requires something called “negative energy density matter” which may not exist.)

Incidentally, the name Sanduleak comes from the star catalogue by Nicholas Sanduleak (pron. “Sand-you-lee-ak) of 1,272 hot blue stars in the Large Magellanic Cloud & the stars observed spectral type - pre-Supernova - was B3 Ia or bright supergiant of spectral type B3.

“tacitus writes:

[[I assume no one knows for sure that the type of star SN1987A was before it blew up ]]

Actually, I think they did know the type, though I don’t recall what it was. The star was catalogued as Sandoval -69 degrees, and I forget the index number, in one of the Magellanic Clouds, and I think it was a B-type blue supergiant.”

Yeah - blue supergiant.

The precursor star that exploded as SN 1987 A was catalogued as Sanduleak (or Sk) - 69 202.

(See in ‘End in Fire’ by Paul Murdin,Cambridge Uni. Press, 1990.)

Are they likely to result in supernova - esp. given Procyon A is already evolving towards gianthood?

(BTW. There was, incidentally, an SF novel based on this for Sirius’es “Pup” blowing up - ‘Supernova’ by Roger MacBride Allen & Eric Kotani, Avon books, 1991.)