The red supergiant star Betelgeuse (center) may have a companion star orbiting it, as shown in this illustration. Credit: Lucy Reading-Ikkanda/Simons Foundation
The bright red supergiant star Betelgeuse has long been a familiar sight to stargazers, winking with a reddish glow from the shoulder of Orion the Hunter. But it is also an increasingly popular target among professional astronomers thanks to its scientific peculiarities, ranging from pulsations to mysterious dimming events.
Now, astronomers think they’ve found a key clue to Betelgeuse’s strange behavior: two independent studies published on the site arXiv The preprint server claims that Betelgeuse is actually part of a binary system, with a previously unknown companion star.
This second proposed star is much smaller than Betelgeuse and so far remains invisible; both teams inferred its presence based on the way its gravity gently pulls Betelgeuse back and forth across the sky. But if astronomers can confirm the star with observations, it could explain Betelgeuse’s longer pulsation, which has baffled astronomers for decades.
Playing for the long haul
Betelgeuse’s strange behavior comes in the form of variations in brightness. The star pulses with brightness not with a single regular beat, but with a series of overlapping rhythms with periods as short as a few hundred days to as long as thousands of days. One of these is directly related to how long Betelgeuse has left before it inevitably explodes. Most of the others are overtones or harmonics related to that supernova time bomb. This has been a source of disagreement among researchers, but most agree that a 416-day cycle is the star’s so-called fundamental mode, the one that most directly portends its demise, and shorter modes are its nuances.
But the longest cycle, which completes every 2,170 days, remained an enigma all its own. There are some researchers who believe it is the fundamental mode of the star – and if true, it would indicate that Betelgeuse is twice as large as generally thought, and destined to go supernova at any time. But most researchers disagree, speculating that this longer cycle is something stranger and more subtle.
Furthermore, for a few months in late 2019 and 2020, Betelgeuse suddenly dimmed completely off-cycle, surprising astronomers and leading them to speculate that the star must have been swept away by a giant cloud of dust. Additionally, it has spurred a new wave of investigations into the familiar star, including the latest development, which came independently in two different studies currently on the planet. arXiv preprint server, which Betelgeuse probably has a companion.
A companion star was first proposed more than a century ago as a possible explanation for Betelgeuse’s periodic dimming and brightening. But this hypothesis fell out of favor as astronomers learned more about the life cycles of massive stars. It has become clear that Betelgeuse has reached a stage in life where it physically pulsates on its own, expanding and contracting in a cycle. These pulsations release clouds of dust and gas from the star’s surface into a shell of material surrounding the star.
However, these intrinsic pulsations generally occur over periods of hundreds of days or less, so they do not explain the much slower pulsations found in Betelgeuse and other similar stars. These long secondary periods (LSPs) typically require thousands of days to complete a brightening and dimming cycle.
Low-mass companion stars or large planets could explain some of these LSPs, as they transit or occult their central star and block some of its light. But finding them, especially around super-bright variable stars with extended atmospheres like Betelgeuse, is a difficult challenge, so their existence is mostly more conjecture than observed fact.
Searching in history
Key to both studies was the comparison between Betelgeuse’s LSP and astrometric and radial velocity data. Both reveal how Betelgeuse moves slightly across the sky, perfectly as if pulled by an invisible companion.
A team led by Jared Goldberg of the Flatiron Institute’s Center for Computational Astrophysics in New York used astrometric data from the European Space Agency’s Gaia space telescope and painstakingly tackled every other possibility for Betelgeuse’s LSP, poking holes in each theory until there was only one left: a low-mass companion star orbiting every 2,170 days. Their paper was published on arXiv on August 17 and has been accepted for publication in The astrophysics diary.
Morgan MacLeod of the Harvard-Smithsonian Center for Astrophysicals in Cambridge, Massachusetts, led a second research team, which used radial velocity data from a century of observations, as well as astrometric data – measurements of Betelgeuse’s position – to locate the tiny movements of the star. on the sky. They published their discovery on arXiv just a month later, on September 17.
Both teams are very much in agreement on the invisible companion facts. It can’t be much larger than the mass of the Sun, making it just one twentieth of the mass of Betelgeuse. And it surrounds Betelgeuse at a distance just less than Saturn’s average distance from the Sun – the rough equivalent of a width of Betelgeuse from the surface of the star itself.
But Betelgeuse provided another twist to the story.
“Before our work,” says Goldberg Astronomy“One of the main theories about the behavior of the LSP was that of a companion with a dusty tail obscuring the star. But from the radial velocity, it must be the opposite: the dust blocks the star when the companion is not in sight! So not a ‘eclipse but an anti-eclipse. Or rather, the dust eclipses the star, but the dust is where there is no companion.”
MacLeod’s team came to the same conclusion. Instead of blocking the light from Betelgeuse and causing a dip in brightness, the companion is apparently clearing away dust like a snowblower, causing Betelgeuse’s light to shine a little brighter during the eclipse.
This could tie in perfectly with the dust puff explanation for the Great Darkness of 2020, the Goldberg team proposes in their paper. The dimming occurred near the LSP minimum, when the theorized companion would have been behind Betelgeuse. This would place its L3 Lagrange point, an orbital quirk of gravitational stability, directly between Betelgeuse and Earth-based observatories, possibly funneling the dust cloud along our line of sight and obscuring the star’s light.
A shot in the dark
The next obvious step is also the most challenging: observing your partner. According to Goldberg this “may not be possible even with current tools. Partly it depends on how lucky we are and the properties of the partner. Whether the companion is accumulating material from Betelgeuse and how hot the companion is would produce different signals, which could both tell the team about the companion and make it harder or easier to observe.
“But,” he continues, “it’s easier now that we know when to look: if we’re right, the companion will pass by the Betelgeuse branch around December 6 of this year; at this point they will be separated as much as possible [from our point of view]and that configuration… gives us the best chance of distinguishing the companion from the star.”
He calls their next remarks, which have already been approved, “a little risky. Detecting something about 100,000 times fainter than the object it is next to is difficult!”
But Betelgeuse has always been full of surprises. Perhaps this time the improbability will work in our favor.
