20 August 2020

Sky & Telescope: “The Fall and Rise of Betelgeuse”

After an unprecedented decline to a record-breaking minimum Betelgeuse has finally turned the corner. Astronomer Edward Guinan of Villanova University reports in Astronomer’s Telegram #13512 that the star bottomed out  with a mean minimum magnitude of 1.614 +/- 0.008 from February 7-13. More recent observations acquired Feb. 18-22 at the school’s Wasatonic Observatory show the star brightening from 1.585 to 1.522, a clear sign of a turnaround.

But the story of this supergiant is hardly over. Is the dip in its light caused primarily by its throbbing atmosphere or are other factors at play? In a related Telegram, a team of astronomers at the University of Minnesota report that Betelgeuse has remained “steadfast” in infrared light for a very long time. They performed infrared photometry of the star on Feb. 21, and after examining the star’s spectral energy distribution — a plot of energy output versus the frequency and wavelength of the emitted light  — they saw virtually no change in the star’s total radiation output compared to observations made 50 years ago!

Bob King

One of the more discussed astronomy news of late last year was the sudden dimming of the star Betelgeuse. Changes in brightness are not uncommon as Betelgeuse is a variable star, but this time astronomers registered its faintest value in 50+ years. In ancient times people would have probably interpreted this as a bad omen – I’m sure the chaos of 2020 is just a coincidence…

At some point during winter I have also looked up at the sky and indeed Betelgeuse looked significantly fainter than usual. Many were anxiously waiting for signs of a supernova explosion, but unfortunately that is very unlikely to happen yet. The star has brightened since, but scientists are still debating the causes of this exceptional event. Some are arguing that unusually large star spots formed on the surface of Betelgeuse, others that the star ejected a large cloud of gas in our general direction.

Betelgeuse light curve
Betelgeuse light curve for the past 1,000 days

There may be a new twist to the story, as observations from the STEREO mission hint that Betelgeuse is dimming again, much sooner than expected according to its regular cycles:

Surprisingly, instead of continuing to increase or level off in brightness, Betelgeuse has decreased by ~0.5 mag from mid-May to mid-July. Spanning the STEREO observations, the star dimmed at a rate of 5 mmag/day. The decrease in brightness over 26 days between our first and third observation is comparable to the photometric behavior in late October 2018, and slower than the faster decline, between October 22 and November 10 2019, that marked the beginning of the historic Great Fainting event in February 2020. The next ‘minimum’ according to the 420-430 d periodicity is predicted to occur in early April 2021. If this current decline continues, another dimming event may occur much earlier. However, Betelgeuse has (not-well understood) complex light variations and it is difficult to make predictions. It will be important to continue to follow Betelgeuse closely through 2020/21.

Edward Guinan

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