Scientists have come up with a new theory to explain why the 10th brightest star in the sky, Betelgeuse, has been dimming and generally acting oddly in recent years.
The new study suggests the red supergiant star has a companion – lovingly called its “Betelbuddy”.
Between November 2019 and March 2020, Betelgeuse visibly dimmed.
Some suggested the star might be about to supernova – one of the most violent events in the universe, occurring at the death of the largest stars. Others thought maybe Betelgeuse had just expelled a bit of dust and gas – a cosmic “fart,” which obscured its light temporarily.
Things got weirder when just a couple of years later the star suddenly got 50% brighter.
The new study, accepted for publication in the Astrophysical Journal, suggests that Betelgeuse’s recent strange behaviour might be because the star isn’t alone.
“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,” says lead author Jared Goldberg, a research fellow at the Flatiron Institute’s Center for Computational Astrophysics based in New York. “The only hypothesis that seemed to fit is that Betelgeuse has a companion.”
The companion star has the official designation Alpha Ori B. But everyone prefers Betelbuddy.
Computer simulations run by Goldberg’s team suggest that the companion star – if it exists – acts like a snowplough. As it orbits Betelgeuse, the Betelbuddy pushes dust out of the way and temporarily makes Betelgeuse seem brighter.
Betelgeuse is a massive star, 100,000 times brighter and more than 400 million times the volume of our Sun.
It is nearing the end of its life and is expected to go supernova soon. The resulting explosion is likely to be bright enough to see during the day for weeks.
Astronomers are trying to predict when Betelgeuse will go boom. One way they do this is by checking the star’s “pulse” – the periodic brightening and dimming of the star.
Betelgeuse has 2 pulses. One pulse is about 6 years, while the second is little more than a year.
One of these pulses is the so-called “fundamental mode” – a property of the star itself. If the fundamental mode is the longer-scale pulse, then Betelgeuse could go supernova sooner than expected. Previous studies suggest that the fundamental mode of Betelgeuse is the shorter heartbeat.
The other pulse must be due to external factors such as the presence of a Betelbuddy.
“Nothing else added up,” Goldberg says. “Basically, if there’s no Betelbuddy, then that means there’s something way weirder going on – something impossible to explain with current physics.”
The researchers aren’t sure yet what the Betelbuddy is. But their best guess is that it’s a star about twice the mass of the Sun.
“It is difficult to say what the companion actually is beyond providing mass and orbital constraints,” says co-author Meridith Joyce from the University of Wyoming, US. “A sunlike star is the most probable type of companion, but that is by no means conclusive.”
“A more exotic hypothesis I personally like, though the opinions of my co-authors may differ, is that the companion is a neutron star – the core of a star that has already gone supernova,” she says. “However, in that case, we would expect to see evidence of this with X-ray observations, and we haven’t. I think we should look again.”
The next step is to try and observe the Betelbuddy using a telescope. The team says there is a potential window for this around December 6.
“We need to confirm that Betelbuddy actually exists, since our result is based on inference, not on direct detection,” says co-author László Molnár of Konkoly Observatory at the HUN-REN Research Centre for Astronomy and Earth Sciences in Hungary. “So we’re working on observation proposals now.”