Feb 21st is the day we’ll know if the puzzling dimming of Betelgeuse is just an alignment of dimming cycles in a highly variable star, or a prelude to a supernova we’ll be able to witness with our bare eyes. If it happens, the dying star will be visible in the daytime and shine as bright as the moon.

From Spaceweather.com :

For months, astronomers have been keeping a wary eye on Betelgeuse, the bright red star in Orion’s shoulder. What’s attracting their attention? All of a sudden, Betelgeuse isn’t bright anymore. Its luminosity has “fallen off a cliff”–a sign that the star could be on the verge of going supernova.

“The most recent measurements put the visual magnitude of Betelgeuse at about +1.66, the dimmest its been in our 25 years of photometry,” says Edward Guinan of Villanova University. 

Above: The horizontal axis is Heliospheric Julian Date (HJD). For reference, Jan. 30, 2020, the date of the most recent measurement, has an HJD of 2458879.

Betelgeuse is a highly evolved red supergiant–the type of star that could collapse and explode at any moment. Indeed, the dimming of Betelgeuse could be explained if the star has suddenly contracted to about 92% of its previous radius. But that’s not the only possibility. Betelgeuse might be dimmed by a giant starspot–or maybe it is shrouded by an outburst of stardust from its own cool outer layers–or something else entirely. No one knows.

Answers might be forthcoming on Feb. 21st. Astronomers have long known that Betelgeuse is a variable star. It pulsates with many periods, as shown in this Fourier analysis of Betelgeuse’s light curve:

Above: A period analysis of 23 years (1995-2018) of Betelgeuse photometry. Credit: Peranso.

“This shows a dominant (probable pulsation) period of P = 430 days,” note Guinan and colleague Richard Wasatonic in a recent Astronomical Telegram. Given this result, “the minimum brightness is expected on 21 (+/-7d) February 2020.”

If Betelegeuse starts to bounce back on Feb. 21st, this whole episode might just be a deeper-than-average pulsation, and perhaps the supernova watch can be called off. However, notes Guinan, “even if the 430-day period is still working, this would indicate a minimum brightness near 0.9 mag–much brighter than the current value near 1.6 mag. So something very unusual is going on.”

Stay tuned for updates as Feb. 21st approaches.

From Astronomy Picture of the Day:

Explanation: Why is Betelgeuse fading? No one knows. Betelgeuse, one of the brightest and most recognized stars in the night sky, is only half as bright as it used to be only five months ago. Such variability is likely just normal behavior for this famously variable supergiant, but the recent dimming has rekindled discussion on how long it may be before Betelgeuse does go supernova. Known for its red color, Betelgeuse is one of the few stars to be resolved by modern telescopes, although only barely. The featured artist’s illustration imagines how Betelgeuse might look up close. Betelgeuse is thought to have a complex and tumultuous surface that frequently throws impressive flares. Were it to replace the Sun (not recommended), its surface would extend out near the orbit of Jupiter, while gas plumes would bubble out past Neptune. Since Betelgeuse is about 700 light years away, its eventual supernova will not endanger life on Earth even though its brightness may rival that of a full Moon.