Betelgeuse, a red supergiant star located around 650 light-years away in the constellation of Orion, underwent a historic dimming between December 2019 and March 2020. Many observations across the star’s spectrum were obtained prior to, during, and subsequent to this dimming event. These observations reveal that a substantial surface mass ejection (SME) occurred on Betelgeuse and moved out through the extended atmosphere of the star.
Betelgeuse, the second brightest star in the constellation of Orion, is a red supergiant located 650 light-years away from Earth.
Otherwise known as Alpha Orionis or HD 39801, the star has a diameter of 1,400 times that of the Sun.
It is also one of the most luminous stars known, emitting more light than 100,000 Suns.
With an age of only 8 million years, Betelgeuse is already nearing the end of its life and is soon doomed to explode as a supernova. When it does, the supernova should be seen easily from Earth, even in broad daylight.
Betelgeuse periodically changes in brightness, which was first noted in the 1830s by the British astronomer John Herschel.
It experienced an unexpected dimming during December 2019 and the first quarter of 2020, reaching an historic minimum on February 7-13.
“Betelgeuse continues doing some very unusual things right now; the interior is sort of bouncing,” said Dr. Andrea Dupree, an astronomer at the Harvard & Smithsonian’s Center for Astrophysics.
“The new observations yield clues as to how red stars lose mass late in their lives as their nuclear fusion furnaces burn out, before exploding as supernovae. The amount of mass loss significantly affects their fate.”
“However, Betelgeuse’s surprisingly petulant behavior is not evidence the star is about to blow up anytime soon. So the mass loss event is not necessarily the signal of an imminent explosion.”
In their study, Dr. Dupree and colleagues analyzed new spectroscopic and imaging data from the NASA/ESA Hubble Space Telescope, the STELLA robotic observatory, the Fred L. Whipple Observatory’s Tillinghast Reflector Echelle Spectrograph (TRES), NASA’s STEREO-A spacecraft, and the American Association of Variable Star Observers (AAVSO).
“We’ve never before seen a huge mass ejection of the surface of a star,” Dr. Dupree said.
“We are left with something going on that we don’t completely understand.”
“It’s a totally new phenomenon that we can observe directly and resolve surface details with Hubble. We’re watching stellar evolution in real time.”
Betelgeuse’s SME was possibly caused by a convective plume, more than a million km across, bubbling up from deep inside the star.
It produced shocks and pulsations that blasted off the chunk of the photosphere leaving the star with a large cool surface area under the dust cloud that was produced by the cooling piece of photosphere. The star is now struggling to recover from this injury.
Weighing roughly several times as much as our Moon, the fractured piece of photosphere sped off into space and cooled to form a dust cloud that blocked light from the star as seen by Earth observers.
The supergiant’s 400-day pulsation rate is now gone, perhaps at least temporarily.
“The star’s interior convection cells, which drive the regular pulsation may be sloshing around like an imbalanced washing machine tub,” Dr. Dupree said.
“TRES and Hubble spectra imply that the outer layers may be back to normal, but the surface is still bouncing like a plate of gelatin dessert as the photosphere rebuilds itself.”
The team’s work will be published in the Astrophysical Journal.
Andrea K. Dupree et al. 2022. The Great Dimming of Betelgeuse: a Surface Mass Ejection (SME) and its Consequences. ApJ, in press; arXiv: 2208.01676
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