From October 2019 to February 2020, Betelgeuse (the bright orange star at Orion’s right shoulder, not Tim Burton’s magical necroprankster) was seen to dim dramatically, even more so than it typically does. It was something that wasn’t just observed with telescopes but also it was quite obvious to the naked eye from most locations. This led many observers to wonder if that was in fact an indication that the red supergiant was about to explode in a supernova—which, based on its size and type, it one day will.* When that does happen it will be the closest such event to Earth in modern history and, although at its distance there would be no direct physical danger to us, it would put on a very dramatic show in the sky for at least a few months. So even while most astronomers were nearly 100% certain that Betelgeuse’s explosive demise wasn’t actually upon us, it became one of those “what-if” moments when far-fetched speculation was coupled with a bit of astronomical fantasizing.
But what did cause the obvious dimming of Betelgeuse, during which time it was just a third of its usual brightness? Recent analysis of data acquired with the Hubble Space Telescope points to an outburst of material from the enormous star that cooled, darkened, and blocked some of the light from it from our point of view here on Earth, about 643 light-years away.
From a news release from NASA on August 13, 2020:
Observations by NASA’s Hubble Space Telescope are showing that the unexpected dimming of the supergiant star Betelgeuse was most likely caused by an immense amount of hot material ejected into space, forming a dust cloud that blocked starlight coming from Betelgeuse’s surface.
Hubble researchers suggest that the dust cloud formed when superhot plasma unleashed from an upwelling of a large convection cell on the star’s surface passed through the hot atmosphere to the colder outer layers, where it cooled and formed dust grains. The resulting dust cloud blocked light from about a quarter of the star’s surface, beginning in late 2019. By April 2020, the star returned to normal brightness.
This sudden dimming mystified astronomers, who scrambled to develop several theories for the abrupt change. One idea was that a huge, cool, dark “star spot” covered a wide patch of the visible surface. But the Hubble observations, led by Andrea Dupree, associate director of the Center for Astrophysics | Harvard & Smithsonian (CfA), Cambridge, Massachusetts, suggest a dust cloud covering a portion of the star.
Several months of Hubble’s ultraviolet-light spectroscopic observations of Betelgeuse, beginning in January 2019, yield a timeline leading up to the darkening. These observations provide important new clues to the mechanism behind the dimming.
Hubble captured signs of dense, heated material moving through the star’s atmosphere in September, October, and November 2019. Then, in December, several ground-based telescopes observed the star decreasing in brightness in its southern hemisphere.
“With Hubble, we see the material as it left the star’s visible surface and moved out through the atmosphere, before the dust formed that caused the star to appear to dim,” Dupree said. “We could see the effect of a dense, hot region in the southeast part of the star moving outward.
“This material was two to four times more luminous than the star’s normal brightness,” she continued. “And then, about a month later, the south part of Betelgeuse dimmed conspicuously as the star grew fainter. We think it is possible that a dark cloud resulted from the outflow that Hubble detected. Only Hubble gives us this evidence that led up to the dimming.”
“With Hubble, we see the material as it left the star’s visible surface and moved out through the atmosphere, before the dust formed that caused the star to appear to dim. We could see the effect of a dense, hot region in the southeast part of the star moving outward.”
— Andrea Dupree, associate director of the Center for Astrophysics | Harvard & Smithsonian (CfA)
Dupree estimates that about two times the normal amount of material from the southern hemisphere was lost over the three months of the outburst. Betelgeuse, like all stars, is losing mass all the time, in this case at a rate 30 million times higher than the Sun.
Dupree hopes to use STEREO for more follow-up observations to monitor Betelgeuse’s brightness. Her plan is to observe Betelgeuse again next year with STEREO when the star has expanded outward again in its cycle to see if it unleashes another petulant outburst.
*Yes, even when the day comes that Betelgeuse goes supernova—be it tomorrow or in 10,000 years from now—it will have “already actually happened” about 643 years prior in real time. But since everything in astronomy is based on current observation, and that is irrevocably tied to the velocity that light and information travels through space, we don’t have to keep saying it every time we mention an event occurring (unless transmission time happens to be the topic of the discussion.) Light takes considerable time to travel to us. We know that. There’s nothing we can do to change it so it’s moot.