Betelgeuse, also known as Alpha Orionis or Alpha Ori, is the second-closest red supergiant to Earth. From November 2019 to March 2020, this star experienced a historic dimming of its visible brightness. Usually having an apparent magnitude between 0.1 and 1, its visual brightness decreased to 1.6 magnitudes around 7-13 February 2020 — an event referred to as Betelgeuse’s Great Dimming. New research shows that the observed dimming is probably caused by an unseen companion star orbiting Betelgeuse. Named Alpha Ori B, or the Betelbuddy, this companion acts like a snowplow as it orbits Betelgeuse, pushing light-blocking dust out of the way and temporarily making Betelgeuse seem brighter.
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Graphical depiction of Betelgeuse and the Betelbuddy. Image credit: Lucy Reading-Ikkanda / Simons Foundation.
Betelgeuse, the second brightest star in the constellation Orion, is an 8-million-year-old red supergiant located approximately 724 light-years away from Earth.
With a radius around 1,400 times larger than the Sun’s, Betelgeuse is one of the biggest stars known.
It is also one of the most luminous stars known, emitting more light than 100,000 Suns.
The star is nearing the end of its life span, and when it explodes, the event will be bright enough to see during the day for weeks.
Astronomers can predict when Betelgeuse will explode by effectively ‘checking its pulse.’
It’s a variable star, meaning it gets brighter and dimmer, pulsing like a heartbeat.
In Betelgeuse’s case, there are two heartbeats: one that pulses on a timescale a little longer than a year, and one that pulses on a timescale of about six years.
One of these heartbeats is Betelgeuse’s fundamental mode, a pattern of brightening and dimming that’s intrinsic to the star itself.
If the star’s fundamental mode is its long-scale heartbeat, then Betelgeuse could be ready to blow sooner than expected.
However, if its fundamental mode is its short-scale heartbeat, as several studies suggest, then its longer heartbeat is a phenomenon called a long secondary period.
In that case, this longer brightening and dimming would be caused by something external to the star.
Scientists still don’t know for sure what causes long secondary periods, but one leading theory is that they arise when a star has a companion that circles it and barrels through the cosmic dust that is produced and expelled by the star.
The displaced dust alters how much starlight reaches Earth, changing the star’s apparent brightness.
Flatiron Institute astrophysicist Jared Goldberg and his colleagues explored whether other processes may have caused the long secondary period, such as the churning of the star’s interior or periodic changes in the star’s powerful magnetic field.
After combining data from direct observations of Betelgeuse with advanced computer models that simulate the star’s activity, the researchers concluded that the Betelbuddy is by far the most likely explanation.
“We ruled out every intrinsic source of variability that we could think of as to why the brightening and dimming was happening in this way,” Dr. Goldberg said.
“The only hypothesis that seemed to fit is that Betelgeuse has a companion.”
The authors have yet to determine exactly what the Betelbuddy is, but they assume it’s a star of up to twice the Sun’s mass.
“It is difficult to say what the companion actually is beyond providing mass and orbital constraints,” said Dr. Meridith Joyce, an astronomer at the University of Wyoming.
“A sunlike star is the most probable type of companion, but that is by no means conclusive.”
Next, the team will try to snap images of the Betelbuddy with telescopes, as there will be a potential window of visibility around December 6, 2024.
“We need to confirm that Betelbuddy actually exists, since our result is based on inference, not on direct detection,” said Dr. László Molnár, an astronomer at Konkoly Observatory.
“So we’re working on observation proposals now.”
The findings will appear in the Astrophysical Journal.
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Jared A. Goldberg et al. 2024. A Buddy for Betelgeuse: Binarity as the Origin of the Long Secondary Period in α Orionis. ApJ, in press; arXiv: 2408.09089
