At the end of 2019, the star Betelgeuse dimmed by 60%. While it’s impossible to say for sure exactly what caused it, new research suggests a wandering partner may have played a role. By bobbing close to the giant star, the interloper may have kicked up a tidal bulge, causing Betelgeuse’s surface to darken. While this scenario can’t explain the full amount of dimming observed, it may have triggered other effects in the star that made the problem worse, the researchers propose in a new paper.
Betelgeuse is one of the most easily recognizable stars in the sky. You can see it as the bright red shoulder of Orion and it is usually the 10th brightest star in the sky. If I were to place the red supergiant in our Solar systemI would swallow everything inside rocky planets and extend from the sun to the asteroid belt between Mars Y 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. Surrounding that 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.
Red supergiants like Betelgeuse are among the largest stars in the universe by volume. They are also incredibly bright. With their large surface area, they can pump out enormous amounts of light despite being relatively cold.
So, naturally, astronomers were quite surprised at the end of 2019, when Betelgeuse began to dim for no apparent reason. The dimming continued into early 2020, and at its nadir, Betelgeuse’s absolute brightness dropped by about 60%. Just like randomly, the dimming stopped in February 2020 and the star began to brighten again, and has now reached its normal intensity levels.
Astronomers have records of Betelgeuse going back half a century, and in those records they were unable to find any precedent for the 2019 event. So whatever caused the “Great Dimming,” as it was called, must have been truly extraordinary.
Whatever caused the dimming must also have come from a situation outside the star itself, rather than due to some fundamental change in Betelgeuse’s internal operations. This is because the changes in fusion reactions do not stop and start within 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 such rapid changes.
Astronomers have proposed many possibilities, including starbursts or gigantic orbiting dust clumps. One possibility is that the shape of Betelgeuse’s outer atmosphere has changed, causing a change in brightness. The brightness of a star’s atmosphere crucially depends on how far the outermost shell of the nuclear core (and any surrounding shells) is in the center. That’s because stars are not solid bodies, but giant balls of gas. The stars are held together by the weight of their own gravitybut that force is offset by the (literally) explosive energies released in their cores.
So the surface of a star is always balanced between these two forces. Where that equilibrium point lies determines the temperature of the star, and its temperature determines its brightness.
Astronomers can see the effects of this when stars spin too fast. When they do, the rotational force bulges out of their equators relative to their poles. That makes the star’s equator farther from the core, which lowers temperatures and, in turn, brightness. This type of “gravity dimming” makes some stars appear brighter at their poles than around their centers.
a rebellious neighbor
Betelgeuse isn’t spinning fast enough for this to be a source of the problem, but things other than rotation can cause bulges on the side of a star. If a random visitor, such as a small black holerotated too close to the star, it could generate surface tides in exactly the same way as the Moon rises the tides in Land.
With the tidal bulge in place, the equator would be dimmed, along with the general appearance of the star. Once the visitor was gone, however, Betelgeuse could return to normal, with all parts of its atmosphere in the right places, and resume its usual copious emission of radiation.
A team of astronomers investigated this scenario and their work is published in the journal Monthly Notices of the Royal Astronomical Society. After investigating a few options for the mass and velocity of an unknown, hidden visitor bobbing briefly in the Betelgeuse system, the astronomers concluded that this scenario could not explain the overall 60% dimming.
However, the visitor’s passage could have caused other effects, such as a strong starburst. Combined with gravity dimming caused by tidal effects, a large amount of ejecta could have briefly obscured our view of Betelgeuse, with the total effect explaining the Great Dimming.
Astronomers may never fully understand what happened to Betelgeuse at the end of 2019. After all, it happened only 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 obscuring from a tidal bulge caused by a close encounter with a black hole and the ensuing eruption of enclosing material may seem overblown, with limited evidence, we can only make up the best possible stories.