The red supergiant variable star Betelgeuse, estimated to be 530 light years from Earth (although measurements by different means vary from 495LY to 640LY; 530LY is considered the "best compromise" measurement) is the ninth brightest star in the sky, the ninth largest star known to exist in the universe, and has the third largest apparent size as observed from Earth of any known star, after the Sun and R Doradus, which is almost three times closer to Earth. R Doradus is believed to lie 200 +/- 25 LY from Earth, and has an angular diameter of approximately 0.057 arcseconds, making it roughly 370 times the diameter of the Sun, or about 3.46 AU, where one AU is the average radius of Earth's orbit. Placed where the Sun is, R Doradus would contain all of the inner planets and most of the main asteroid belt. Betelgeuse's angular diameter of just under 0.055 arcseconds makes it almost three times larger, 950 to 1000 times larger than the Sun (8.8 to 9.3AU, or roughly to the orbit of Saturn). It is one of only about a dozen stars whose apparent size is so large it has been imaged telescopically as a visible disk rather than a point.

Why is this important?

Well, you see, Betelgeuse has been shrinking continuously since 1993, at an increasing rate. By June 2009, it had shrunk 15% from its size as measured in 1993.

But wait! There's more. It is rumored, though I have been unable to find any reliable confirmation of the source (which is claimed to be first-hand) that the latest observations from Mauna Kea show that Betelgeuse is now shrinking so fast it is no longer round. (Due to conservation of angular momentum, when a massive star collapses gravitationally, it collapses faster at the poles, becoming increasingly oblate — flattened — as its final collapse accelerates.)

What does this mean?

Well, briefly, what it means — if true — is that Betelgeuse could be within as little as weeks of a Type II (core collapse) supernova. (Astronomers have considered for some time that Betelgeuse has the potential to go supernova any time in the next thousand years or so. "Any time" may just turn out to be rather sooner than expected.)

IF this happens, not to put too fine a point on it, it will almost undoubtedly be among the most dramatic astronomical events ever observed by human eyes. A type II supernova can briefly outshine an entire galaxy ... and this one will be only a little over five hundred LY away. The supernova that created the Crab Nebula, SN 1054, was bright enough to see in daylight for 23 days, and remained visible for 653 days ... and it was 6,300 LY away. Betelgeuse is almost 12 times closer, and can be expected to appear around 140 times brighter by virtue of that alone. And as noted at the beginning of this post, Betelgeuse is the ninth largest star known to exist in the universe.

If the rumor is true, this is going to be one hell of a show, and we'll have a front-row seat. (Relatively speaking.)

(Don't panic, though. It is not believed that a Betelgeuse supernova would present any threat to Earth, and we're not anywhere near Betelgeuse's axis of rotation and therefore in no danger from a gamma-ray burst.)

Clarifications:

When we say Betelgeuse "is within a few weeks of" going supernova, what we really mean is the light front from the speculative supernova is a few weeks from reaching Earth. If the rumored Mauna Kea observations are both true and correct, we can infer that in fact, Betelgeuse already went supernova about 530-540 years ago, and the light from the final stages of collapse leading up to the supernova are just now reaching Earth.

It should also be observed, as Dan Neely points out in comments, that the brightness estimates in the doomers.us article are wildly exaggerated. Supernova SN1054 is estimated to have been around apparent magnitude -6; the 11.9:1 distance ratio alone between Betelgeuse and the Crab Nebula would be expected to put a Betelgeuse supernova in the -11 to -12 range. Supernova SN1006, however, is estimated to have reached apparent magnitude -7.5, despite being a thousand light years further away than SN1054; if SN1054 and SN1006 had the same absolute luminosity, SN1006's apparent brightness would have been only around three quarters that of SN1054, whereas in fact it is believed to have been around four times brighter. This would imply that SN1006's absolute luminosity was around 5.2 times higher than SN1054's. If we assume that the speculative supernova SN2010A is no brighter in absolute luminosity than SN1006, its distance means it could approach apparent magnitude -14, about 2.5 times brighter than the full Moon. The Sun's magnitude -26.7, though, is FAR out of reach.

(As also pointed out by Dan, one also cannot directly compare Betelgeuse to SN1006, as SN1006 was a Type 1a supernova. Nevertheless, it serves as a useful yardstick for this purpose.)

Update (finally, solid data!)

20100601-22:02: However much fun it would have been to watch the light show, it seems finally a reputable source, the Bad Astronomy Blog on discovermagazine.com, has weighed in. And, as most of us expected from the start, the original rumor is just that and nothing more: An unsubstantiated rumor. There's no actual new observations of Betelgeuse that would suggest it's any closer to going supernova than we already believed it was 20 years ago. "No BOOM! today."

I can't say I'm surprised, but I'll admit to being a little disappointed. A relatively nearby supernova would have been a truly memorable event.

Ah, well. Just remember — "There's always a BOOM! tomorrow. Sooner or later ... BOOM!"