With its recent uptick in brightness, Betelgeuse appears to be returning to normal. Astronomers urge us to keep watch.
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.
Guinan points out that the present resurrection is occurring approximately 424+/-4 days after the star's last minimum in mid-December 2018 when it dimmed to magnitude 0.9. If you've been following Betelgeuse you'll know that this is just one of its several pulsation periods. The oscillations are caused by turbulence below the surface layers of the star that make it ring like a bell. They may also shift the position of or modify a large bright spot seen in photographs of the star, affecting a change in its brightness.
Observations submitted to the American Association of Variable Star Observers (AAVSO) by Thomas Calderwood show the beginnings of a brightening trend from magnitude 1.589 on Feb. 17 to 1.556 on the 19th. These and other observations this month confirm the Villanova data.
"Betelgeuse has definitely stopped dimming and has started to slowly brighten," writes Guinan, adding that "observations of all kinds continue to be needed to understand the nature of this unprecedented dimming episode and what this surprising star will do next."
What does it look like with the naked eye? On Feb. 25.21 UT, I compared Betelgeuse to Bellatrix (magnitude 1.6) and Aldebaran (0.9) and found it equal to Bellatrix, an increase of 0.1 magnitude from several previous estimates I'd made over the past two weeks. Yes, we have a pulse!
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!
In other words, if you could tune your eyes to see Betelgeuse in the infrared the drama of the past few months wouldn't even register. That makes the current "fainting" unrelated to a significant change in the star's energy output. The U of M team suggests that the fading at visual wavelengths is "due mostly to local surface phenomena" such as changes in the amount and opacity of foreground dust belched out by the star along Earth's line of sight through the star's complex dust shell and atmosphere. Surface temperature fluctuations may also contribute to the fading.
My favorite line in the telegram comes near the end: "Thus, while Betelgeuse may explode tomorrow or any time in the next few 1e5 yr (100,000 years), the unprecedented current visual faintness is unlikely to be a harbinger of its impending core collapse." I guess that means you can cancel that supernova party you had planned.
The group plans to continue observations and include them along with historical data in an upcoming paper. If watching Betelgeuse fade got you excited, stick around. Its return to normal may offer another taste of the unexpected.