At the end of 2019, the star Betelgeuse faded by about 60%. While it’s impossible to say for sure exactly what caused it, new research suggests a fellow traveler may have played a role. While swinging near the giant star, the intruder may have lifted a tidal bulge, causing Betelgeuse’s surface to darken. While this scenario could not explain the full amount of dimming observed, it may have triggered other effects on the star that compounded the problem, the researchers propose in a new paper.
Betelgeuse is one of the most recognizable stars in the sky. You can see it as Orion’s bright red shoulder and it’s usually the 10th brightest star in the sky. If you were to place the red supergiant in our solar systemit would swallow up everything inside rocky planets and stretch from the sun to the asteroid belt between March and Jupiter.
Betelgeuse is almost ready to die. It’s huge because it stopped fusing hydrogen in its core a long time ago and switched to fusing helium. Around this core is a shell of burning hydrogen. With the intensity of the fusion reactions in and around the core, the energies push the outer layers of the atmosphere outward, forcing the star to expand.
Related: Scientists are still stuck on Betelgeuse antics a year after a bizarre dimming episode
Red supergiants like Betelgeuse are among the largest stars in the universe by volume. They are also incredibly shiny. With their large surface area, they can pump in huge amounts of light despite their relative coolness.
So naturally, astronomers were very surprised at the end of 2019, when Betelgeuse started dimming for no apparent reason. The dimming continued into early 2020, and at its lowest point, Betelgeuse’s absolute brightness dropped by about 60%. Just as randomly, the dimming stopped in February 2020 and the star started to shine again, and it has now reached its normal intensity levels.
Astronomers have records of Betelgeuse dating back half a century, and in those records they couldn’t find any precedent for the 2019 event. So whatever the cause of the “Great Dimming,” as we called her, she must have been really extraordinary.
Whatever the cause of the dimming, it must also stem from a situation external to the star itself, rather than being due to a fundamental change in the internal operations of Betelgeuse. This is because changes in fusion reactions do not stop and start in just a few months. There is simply too much mass in the nucleus and the energies released by fusion reactions are simply too high to support these kinds of rapid changes.
Astronomers have proposed many possibilities, including stellar explosions or giant clumps of dust in orbit. One possibility is that the shape of Betelgeuse’s outer atmosphere has changed, causing a change in brightness. The luminosity of a star’s atmosphere depends crucially on how far this outermost layer is from the nuclear core (and all surrounding shells) at the center. This is because stars are not solid bodies but rather giant balls of gas. The stars stand with the weight of their own gravitybut this force is counterbalanced by the (literally) explosive energies released in their cores.
The surface of a star is therefore always balanced between these two forces. The location of this equilibrium point determines the temperature of the star, and its temperature determines its luminosity.
Astronomers can see the effects when stars spin too quickly. When they do, the rotational force inflates their equators relative to their poles. This pushes the star’s equator away from the core, reducing temperatures and, therefore, brightness. This type of “gravity dimming” causes some stars to appear brighter at their poles than around their midpoints.
An unruly neighbor
Betelgeuse doesn’t spin fast enough for that to be a problem, but things other than rotation can lift bulges on the side of a star. If a random visitor, like a small black holeswung too close to the star, it could raise surface tides in exactly the same way as the moon raises the tides on Earth.
With the tidal bulge in place, the equator would fade, along with the star’s overall appearance. Once the visitor leaves, however, Betelgeuse could return to normal, with all parts of its atmosphere in the right places, and resume its usual abundant radiation output.
A team of astronomers has studied this scenario, and their work is published in the journal Royal Astronomical Society Monthly Notices. After investigating a few options for the mass and velocity of an unknown, hidden visitor briefly hovering in the Betelgeuse system, astronomers concluded that this scenario could not explain the full 60% dimming.
However, the visitor’s passage may have caused other effects, such as a strong stellar outburst. Combined with the darkening of gravity caused by tidal effects, a large amount of ejected material could have briefly obscured our view of Betelgeuse, the total effect explaining the large dimming.
Astronomers may never fully understand what happened to Betelgeuse in late 2019. After all, it only happened once in all of our records of the famous star. Further observations, both of Betelgeuse and other similar red supergiants, may reveal an answer. While the combination of gravity darkening from a tidal bulge lifted by a close encounter with a black hole and the ensuing eruption of enveloping material may seem far-fetched, with limited evidence, we can only cook up the best stories possible.
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