In the past year, Betelgeuse has seen two episodes of dimming. Usually it's one of the ten brightest stars in the sky, and astrophysicists and astronomers have been busy trying to understand what was happening to the red supergiant. Various researches have found some possible answers: enormous star spots, dust accumulations, convulsions in front of the supernova.
Now, a new study introduces another wrinkle in our understanding of Betelgeuse. The authors say Betelgeuse is both smaller and closer than previously thought.
The new study is entitled "Standing on the Shoulders of Giants: New Mass and Distance Estimates for Betelgeuse Using Combined Evolutionary, Asteroseismic and Hydrodynamic Simulations with MESA". The main author is Dr. Meridith Joyce, a postdoctoral fellow in stellar physics and galactic archeology at the Australian National University. The paper is published in the Astrophysical Journal.
Our fascination with Betelgeuse is not new. As the tenth brightest star in the sky, we can't help but notice it. Betelgeuse is a variable star and the brightest star in the sky in the infrared. It has a prominent position in the Orion constellation, and its red hue has made it a fascinating object since ancient times.
The familiar constellation of Orion. Orion's belt is clearly visible, as is Betelgeuse (red star in the upper left corner) and Rigel (light blue star in the lower right corner). Photo credit: NASA Astronomy Picture of the Day Collection NASA.
As a variable star, Betelgeuse's brightness changes sometimes. But from the end of last year, the star showed a new behavior. It got darker at first and weakened to only about 37% of its normal brightness in early February 2020. Then it brightened up again by the end of May 2020. Then another episode of dimming began.
In this new article, the authors wanted to "… conduct a rigorous study of the nearby red supergiant Betelgeuse, using the synthesis of new observational data and three different modeling techniques." They performed hydrodynamic and seismic models to learn more about the physics, which drives the pulsations of Betelgeuse, and to get a clearer idea of what stage of their life Betelgeuse is in.
As a result, they were able to get an accurate prediction for the star's radius. They also received more accurate mass, distance, and periodicity measurements.
In a press release, Dr. Joyce: "It's usually one of the brightest stars in the sky, but we've seen two drops in brightness at Betelgeuse since late 2019. This has led to speculation that it might explode soon." However, our study offers a different explanation. "
“We know that the first dimming event included a cloud of dust. We found that the second minor event is likely due to the star's pulsations, ”explained Dr. Joyce.
According to these results, Betelgeuse is far from exploding as a supernova. The star is still burning helium in its core, and an explosion is hundreds of centuries away.
"Right now, helium is burning at its core, which means it is nowhere near exploding," said Dr. Joyce. "We could look at about 100,000 years before an explosion happens."
Red supergiants deplete the helium in their nuclei in a million or two million years. After that, they move on to carbon fusion. Next, they fuse increasingly heavy elements until an iron core is built up. Then they inevitably collapse and explode as supernovae.
Shortly before the final gravitational collapse of a red supergiant, its core resembles an onion. The iron core is surrounded by layers of silicon and sulfur, oxygen, neon, carbon mixed with some oxygen, helium and finally hydrogen. Outside the core, the composition is mainly hydrogen and helium. (Not to scale.) (Photo credit: Work modified by ESO, Digitalized Sky Survey. CC BY 4.0)
Accurate size and spacing measurements for Betelgeuse were difficult to obtain. Over the past few decades, different estimates have ranged from just 330 light years to 650 light years away. A recent study from 2017 found 724 light years (+ 111 / -156).
Knowing the distance to a star is one of the keys to understanding it. Without knowing the distance, we cannot know the luminosity or physical size of a star. But it's hard to measure.
Astronomers rely on trigonometric parallax to measure the distance to a star. To do this, they measure the apparent movement of a star. Apparent motion is how a star appears to move in relation to stars even further away. It is based on the earth's orbit around the sun, which changes our observation location. The position of the target star changes, while the more distant stars don't seem to move because of the great distance. The apparent movement is known as star parallax and is measured in arc seconds. The further away a star is, the smaller its parallax is in arc seconds.
How star parallax works. Photo credit: Las Cumbres Observatory.
One of the problems with parallax measurements is that the further away something, the less accurate the measurement can be. For example, the star closest to Earth is Proxima Centauri and has a parallax of just 0.772 arc seconds. This is the largest parallax observed for a star. The measurements can be particularly inaccurate if you are observing from the ground, where the earth's atmosphere can disturb.
In this study, the team used data from an instrument on the Coriolis satellite. They also used asteroseismic data to narrow the radius of Betelgeuse. They used three separate models to interpret this data and achieved a distance measurement at a distance of 548 (+ 88 / -49) light years. Your measurement means that Betelgeuse is about 25% smaller and 25% closer than previously thought.
This picture from the study shows the distance measurements to Betelgeuse from various efforts. Hipparcos was an ESA satellite dedicated to precision astrometry. VLA is the Very Large Array Radio Observatory. The new measurement included asteroseismology data, which is represented by the gray bar. Photo credit: Joyce et al., 2020.
"The actual physical size of Betelgeuse has been a mystery – previous studies suggested it could be larger than Jupiter's orbit," said co-author Dr. László Molnár from the Konkoly Observatory in Budapest. "Our results say Betelgeuse is only two-thirds of that, with a radius 750 times that of the Sun."
“Once we got the physical size of the star, we could determine its distance from Earth. Our results show that we are only 530 light years away – 25 percent closer than previously thought, ”said Molnár.
Despite some uncertainties about Betelgeuse's behavior, distance, and size, its ultimate fate is clear. It will eventually explode as a supernova.
Supernovae explosions are spectacular cosmic light shows as long as they are not too close. This is a composite image of SN 1987A from Hubble, Chandra, and ALMA. Photo credit: By ALMA (ESO / NAOJ / NRAO) / A. Angelich. Visible light image: NASA / ESA's Hubble Space Telescope. X-ray image: NASA's Chandra X-ray Observatory – http://www.eso.org/public/images/eso1401a/, CC BY 4.0, https://commons.wikimedia.org/w/index.php? curid = 30512379
Astronomers suspect that the ancient supernova played a role throughout life on Earth, including causing some partial extinctions. A supernova explosion around 360 million years ago during the Devonian period could have triggered an extinction that wiped out up to 75% of all species on earth.
Another supernova just 2.6 million years ago may have wiped out large marine animals, and another may have caused people to walk upright, though that result is far from clear. What is clear, however, is that a supernova that explodes too close to Earth is no good. A 2017 study showed that the supernova killing zone is within 50 light years from Earth.
But when Betelgeuse is blowing, the earth is safe. 530 light years away, the energy of the explosion will be widely dispersed before any of it reaches this planet. So instead of a catastrophe, it would be a fantastic cosmic light show and a cornucopia of scientific knowledge.
If there are still people there, then.