Betelgeuse Luminosity in Upheaval

As recently as October, Betelgeuse glowed around magnitude 0.5, considerably brighter than its nearby Aldebaran (0.9). But observations made this month by both amateurs and professionals indicate a steep drop in brightness. On December 28.2 UT, I used Aldebaran (magnitude 0.9) and Bellatrix to estimate the star at a feeble magnitude of 1.5, nearly equal in brightness to Bellatrix. In just two months it's fallen from 10th place to 21st, according to astronomer James Kaler's 26 Brightest Stars list, a remarkable decline — and a historic low.

Betelgeuse is classified as a pulsating red supergiant. It physically expands and contracts as its atmosphere alternately traps and releases heat radiating from its core. When the star is smallest and hottest, it would extend to the orbit of Mars if put in place of the Sun. When largest and coolest it would balloon to span Jupiter's orbit. Although Betelgeuse is 20 times more massive than the Sun, its expanding shell has only 1/10,000 the density of air — it might be better described as a red-hot vacuum.

Betelgeuse is a semi-regular variable star with multiple periods of variation. The primary pulsations repeat every ~425 days, but the star also shows additional changes in brightness with periods of 100-180 days and 5.9 years. Dark patches that resemble monstrous sunspots as well as bright blobs of upwelling gas are behind some of these fluctuations. Betelgeuse is clearly in upheaval and will continue to surprise us before it eventually runs out of fuel, collapses, and explodes as a Type II supernova.

While the supergiant's current behavior is out of the ordinary, it doesn't necessarily mean an eruption is imminent. Astronomers predict a star-shredding blast sometime in the next 100,000 years or so.​

What's Up With Betelgeuse? - Sky & Telescope

If the star does become a supernova, Betelgeuse would likely be as bright as, or even brighter than the moon for weeks or even more.

At 642.5 light-years from Earth, it would be the closest supernova observed and recorded by humans (closer than the Crab Nebula, which is 6,523 light-years from Earth and is the result of a supernova reported to have taken place in A.D.1054). This also means that, if we see Betelgeuse explode tonight, the supernova really took place over 600 years ago, we're only seeing it now.
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Will Bright Star Betelgeuse Finally Explode? A Look at the Dimming Red Giant in Orion's Shoulder | Space

 

Betelgeuse luminosity has not yet reached nadir:

Betelgeuse continues to dim, diminishes to 1.506 magnitude

Betelgeuse continues to dim, diminishes to 1.506 magnitude

 

Hubble spies the culprit behind Betelgeuse star's dimming - CNN