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Mystery of the dimming star solved

This illustration plots changes in the brightness of the red supergiant star Betelgeuse. (NASA, ESA, and E. Wheatley (STScI))

Earth's sun is a predictable star, and that's a good thing: if the sun fluctuated in its brightness significantly from year-to-year, the solar system would be a much less hospitable place for life. But while most stars in the universe are as regular as our sun, a small percentage aren't — and when a star suddenly dims or brightens, it usually signifies something odd is happening with it, or even that it could be on the verge of exploding.

So when astronomers noticed in 2019 that Betelgeuse had dimmed, some speculated that the massive star was going to expand into a supernova so large it would be visible from Earth even during the daytime hours. Given that Betelgeuse is the tenth-brightest star in the night sky, citizens of Earth paid attention. Novas or supernovas that are visible with the naked eye are rare, and when they do happen, they tend to be generation-defining events: the last time a nearby star went supernova, in 1604, it was so bright that it was visible during the daytime

Betelgeuse's mysterious behavior made headlines — and then it got more mysterious in February 2020. Then, reports surfaced that Betelgeuse was regaining some of its waning brightness. Before long, the so-called "Great Dimming" had captured the public's imagination. Scientists and amateur astronomers alike obsessed over Betelgeuse's odd behavior, trying to derive the meaning; one independent scientist even set up a Twitter bot, "BetelBot," which issued regular updates on Betelgeuse's varying brightness.  

But now, thanks to a group of scientists using the Hubble Space Telescope, we now know the cause of Betelgeuse's Great Dimming: A coronal mass ejection (CME), or a phenomenon in which a star's corona (or crown) erupts with a massive cloud of highly magnetized and energetic plasma.

"It has very large convective cells on its surface, which means there is hot material moving upward from within, similar to chocolate sauce boiling in a pot," explained study author Dr. Andrea Dupree, associate director of the Harvard & Smithsonian Center for Astrophysics, in an email to Salon.

The paper itself was posted to the preprint database arXiv and accepted for publication by The Astrophysical Journal. 

"It appears that in 2019, [Betelgeuse's] outward expansion seemed to last an exceptionally long time and coincided with the presence of an exceptionally large convective cell," Dupree said. The dimming that observers noted at that time was caused by "an ejection of a substantial part of the star's surface followed by the presence of a cooler spot, presumably due to gas expanding to fill the void."

"Stars live like these live for millions of years, but the end comes relatively quickly," Murphy wrote to Salon.

Since Betelgeuse is a massive star (it is 1000 times larger than our sun), roughly a year passed before people began to note the consequences of this event. Yet astronomers "could see material moving out through the star's atmosphere (in the southern part of the star) using the Hubble Space Telescope," Dupree explained. "And then that southern part became very dim, as if a dark cloud were covering it. So we believe the dimming is attributed to the material that was ejected and cooled, as well as the cool spot from the gas that expanded into the void left by the ejected material."

Dr. Avi Loeb, an astronomer at Harvard University, told Salon by email that part of the reason the discovery about Betelgeuse is so significant is that it is an unusual star — specifically, a red supergiant (which have the largest radii of all known stars). Moreover, Betelgeuse is so large that "if it were at the center of our Solar System, its envelope would engulf the asteroid belt and the orbits of Mercury, Venus, Earth, and Mars."

Speaking of red supergiants, Loeb said that "understanding their properties and evolution is important for understanding their fate when they consume their nuclear fuel and eventually explode. They all go on to burn heavier elements and undergo core-collapse resulting in a supernova."

Loeb also offered some illumination on the star's period of re-brightening in February 2020.

"By 22 February 2020, Betelgeuse started to brighten again," Loeb explained. "Infrared observations found no significant change in brightness over the last 50 years, suggesting that the dimming was due to a change in extinction by large dust grains. Data from the Hubble Space Telescope in 2022 suggested that occluding dust was created by a surface mass ejection and caused the dimming."

"It has very large convective cells on its surface, which means there is hot material moving upward from within, similar to chocolate sauce boiling in a pot," explained study author Dr. Andrea Dupree.

Dr. Phil Massey, an astronomer at the Lowell Observatory in Flagstaff, Arizona, told Salon by email that he did not believe most astronomers seriously believed that Betelgeuse was about to explode as a supernova. Some believed that the Great Dimming had been caused by a giant starspot somewhere on Betelgeuse's surface — a starspot being the interstellar equivalent to our sun's sunspots, blemishes that appear periodically on the surface of stars. Others theorized that Betelgeuse was undergoing a dust formation event — in which a star will "lose mass in an episodic way" that results in a cloud of dust in its vicinity.

Massey noted that he and Dr. Emily Levesque of the University of Washington, with whom he has been studying red supergiants since 2003, had found that the star's temperature had been essentially unchanged over many years. That was an important clue as to what was going on with the second-brightest star in the Orion constellation.

"To us, that completely ruled out the 'star spot' explanation and meant that there had been some sort of large mass ejection, leading to the formation of dust." (They later wrote a paper on the subject for the journal Astrophysical Journal Letters.)

Regarding Dupree's observations, Massey said that they vindicated the earlier research he had done with Levesque: "Their results confirm what we've long suspected — that red supergiants 'burp' out large amount of mass from time to time, and that this mass-loss is more episodic in nature than constant."

Dr. Alex Murphy, an astronomer at the University of Edinburgh, also praised Dupree's work by adding that it allows people to better understand a development which — technologically speaking — they would have been unable to appreciate only a few decades ago.

"Stars live like these live for millions of years, but the end comes relatively quickly," Murphy wrote to Salon. "So we're really lucky to have one so close to us and in this phase now, when mankind has 'just' (astronomically speaking) developed the technology to be able to see what's happening. We have pretty good understanding of what's happening, but there's nothing like seeing it first hand."