Betelgeuse in Orion is one of the brightest stars in the sky. The 10th brightest if you check the list, but that’s only an average because its light isn’t constant. Instead, Betelgeuse is a variable star with a brightness that varies from about magnitude 0.2 — nearly the equal of Rigel in the knee of Orion — to about 1.3 or similar to its next-door neighbor Bellatrix.
A celestial object’s brightness is measured on the magnitude scale. The smaller the number the brighter the object. A typical bright star has a magnitude of1 1 or first magnitude. Medium-bright stars like the ones in the Big Dipper shine at magnitudes 2 and 3. Under a very dark sky you can see down to sixth magnitude. Stars brighter than first magnitude are numbered backwards from 1, so a zero or negative magnitude star is very bright indeed. Vega, for example, shines at magnitude 0.0 (a little brighter than Betelgeuse), while Sirius, the brightest star in the sky, outdoes them all at magnitude –1.5.
As recently as October, Betelgeuse glowed around magnitude 0.5, considerably brighter than its neighbor Aldebaran (0.9). But observations posted online in the past few days drew attention to the fact that the star had faded to possible historical lows. There was even speculation that the dramatic dimming could be a preamble to a supernova explosion. Whoa — talk about getting ahead of ourselves!
Last night (Dec. 22) I looked up and was surprised to see that Aldebaran looked much brighter than Betelgeuse. Using it and Bellatrix (1.6) as my comparison stars I estimated Betelgeuse at magnitude 1.4. Very dim! Amateur astronomer Piqui Díaz of Buenos Aires called it 1.3 on Dec. 21.
I’m a member of the American Association of Variable Star Observers (AAVSO), so the first thing I did was look back over the record to see if Betelgeuse had ever faded so deeply before. I discovered that several times in the past, most recently in late 2008-2009, some observers had pegged it around magnitude 1.3.
Another source and this analysis report that Betelgeuse is the faintest it’s been for at least the past 50 years. That’s exciting news. Few bright stars show such obvious changes. That means you and I have the opportunity to see it during this historical low and then watch it re-brighten over the coming weeks … or do something else entirely. The timing couldn’t be better. Betelgeuse is already well-placed in the eastern sky around 8:30 p.m. local time and will remain easily visible in the evening for the next four months.
To make your own magnitude estimate, look briefly from one star to the next. Don’t linger. Then write that number down and return every few nights or maybe every week and make another estimate. Over time you’ll see it slowly change right before your eyes.
Betelgeuse is no ordinary star. It’s classified as a pulsating red supergiant with an atmosphere that expands and contracts as it alternately traps and releases heat radiating from the core. Think of it as an enormous gasbag. When smallest and hottest, the star spans some 300 million miles and would extend to the orbit of Mars if put in place of the sun. When largest and coolest it would reach all the way to Jupiter! Although the star is 20 times as massive as the sun that material is spread so thinly that Betelgeuse has 1/10,000 the density of air. Someone once described it as a “red-hot vacuum.”
The pulsations repeat roughly every 420 days, but the star also shows additional changes in brightness with periods of 5-6 years and 100-180 days. Dark patches that resemble monstrous sunspots as well as bright spots that appear to giant gushers of upwelling gas are responsible for some of the fluctuations. As they come and go the star brightens and fades in a flickering sort of way that we can train our eyes to see even from big, overlit cities.
Massive stars like Betelgeuse often end their lives as supernovae. When a supergiant star runs out of fuel in its core there’s nothing left to stave off the crushing force of gravity — no outward pressure from burning to resist. Almost instantly, the star collapses, and when the material strikes the core it creates a shockwave that reverberates across the star, tearing it to shreds in one of the most violent events in the cosmos.
Will we see this happen in our lifetime? Maybe, but don’t count on it. It could happen tonight or in the next 10,000 years. (But I sure hope it’s tonight!)