Seven hundred light-years from Earth, a brilliantly bright red supergiant lights up the night sky. This is Betelgeuse, a star that’s fascinated humans for millennia. Archaeologists think ancient people used the star’s progression across the night sky to time the due dates of expected babies. Now, new evidence suggests that Betelgeuse has something like a baby, too. Circling the red supergiant seems to be a tiny companion star that astronomers have termed “a Betel-buddy.”
Betelgeuse (which, pronounced, sounds more like “Baetel-juice”) got its common name in the thirteenth century, when scholars misread an Arabic phrase that they thought meant “hand of the giant.” That was appropriate, given the star’s location in the constellation Orion, which represents a towering hunter, so the scholars translated the name into Latin and kept using it. In keeping with the theme, researchers have named the companion star Siwarha, the Arabic word for “bracelet.”
Until now, certain aspects of Betelgeuse’s behavior haven’t made a lot of sense, but adding Siwarha into the equation “solves a lot of problems,” says Andrea Dupree, Ph.D. ’68, a senior astrophysicist at the Harvard-Smithsonian Center for Astrophysics (CfA) who was involved in Siwarha’s discovery.
One such problem is the fact that Betelgeuse shows changes in brightness and velocity that come at two distinct intervals: one lasting about 400 days and the other lasting about 2,000 days. (Betelgeuse also made the news in 2020 when it mysteriously dimmed much more than it usually does during either of these periods—an anomaly that occurred when the star spit out a massive amount of hot plasma, like a huge sneeze.) The 400-day period is caused by expansions and contractions in the star’s outer layers. But the reason behind the independent 2,000-day period was, until recently, a mystery.
“There were lots of great ideas,” Dupree says, to explain the phenomenon, but all the theories lacked evidence. To fill that void, she and other researchers at the CfA decided to look back at decades of data, collected by many different groups, that document how Betelgeuse has varied in brightness, velocity, and position. Researchers at the Flatiron Institute in New York independently pursued a similar goal. When the two groups put their findings together, they came to the same conclusion, Dupree says, that “a companion was the easiest explanation of this long period.”
The idea caught on, and last summer a group led by scientists from the NASA Ames Research Center published images they took of Betelgeuse with the hope of catching a glimpse of Siwarha. The observations are suggestive, but their statistical power is below what most scientists consider definitive, Dupree says. The difficulty in observing Siwarha directly comes from the fact that it’s a million times dimmer than the star it’s orbiting. It’s like trying to see a tiny Christmas tree light in front of a flood lamp.
To circumvent this problem, Dupree and her colleagues decided to look for the effects of Siwarha rather than the star itself. They started by examining the atmosphere of Betelgeuse, searching for evidence of a disturbance that might indicate the presence of a companion star. Their efforts were rewarded: the way elements in Betelgeuse’s atmosphere absorbed and reflected light changed over time in subtle ways that indicated that a large object was passing through the atmosphere. Dupree compares the pattern they observed to the wake left by a speedboat as it cruises across a lake. Their results will soon be published in The Astrophysical Journal.
Although this indirect evidence still isn’t definitive, a companion star “certainly seems like the simplest and most elegant explanation,” says University of Washington professor of astronomy Emily Levesque, a former Harvard astronomy department predoctoral fellow who was not involved in the work. Next year, when Siwarha emerges from behind Betelgeuse, Dupree and many other scientists will attempt to take pictures of the companion star directly using a variety of high-powered telescopes. If they’re successful, Levesque says, that evidence will be “pretty hard to argue with.”
Siwarha likely causes Betelgeuse to dim at 2,000-day intervals by pushing dense material between the star and Earth, Dupree says. Siwarha’s existence could also address other lingering questions, such as why Betelgeuse’s rotation hasn’t slowed, the way most stars’ rotations do, as the supergiant has aged and expanded. Betelgeuse may be stealing Siwarha’s angular momentum, keeping the supergiant spinning the way a smaller star would.
For now, Dupree is satisfied with having found a likely answer to the long-standing question of why Betelgeuse dims every 2,000 days. “The fact that we figured it out,” she says, “is really very rewarding.”
