Betelgeuse's mysterious 'Great Dimming' may have been triggered by a wandering black hole

Jan 27, 2020
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By Paul Sutter, 8 hours ago

"Gravity darkening" might explain the strange episode.

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These images, taken with the SPHERE instrument on the European Southern Observatory’s Very Large Telescope, show the surface of the red supergiant star Betelgeuse during its unprecedented dimming, which happened in late 2019 and early 2020. The image on the far left, taken in January 2019, shows the star at its normal brightness, while the remaining images, from December 2019, January 2020 and March 2020, were all taken when the star’s brightness had noticeably dropped, especially in its southern region. The brightness returned to normal in April 2020. (Image credit: ESO/M. Montargès et al.)

In late 2019, the star Betelgeuse dimmed by about 60%. While it's impossible to say with certainty exactly what caused it, new research suggests that a wandering companion may have played a role. By swinging close to the giant star, the interloper may have raised a tidal bulge, causing the surface of Betelgeuse to dim. While this scenario can't explain the full amount of dimming observed, it may have triggered other effects on the star that made the problem worse, 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 is usually the 10th brightest star in the sky. If you were to place the red supergiant in our solar system, it would engulf all of the inner rocky planets and stretch from the sun to the asteroid belt between Mars and Jupiter.

Betelgeuse is almost ready to die. It's immense because it stopped fusing hydrogen in its core long ago and switched to fusing helium. Surrounding that core is a shell of burning hydrogen. With the intensity of 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 sheer amount of surface area, they can pump out enormous quantities of light despite being relatively cool.

So naturally, astronomers were very surprised in late 2019, when Betelgeuse began to dim for no apparent reason. The dimming continued throughout early 2020, and at its lowest point, the absolute brightness of Betelgeuse dropped by about 60%. Just as randomly, the dimming halted in February 2020 and the star began to brighten again, and it has now reached its normal levels of intensity.

Astronomers have records of Betelgeuse going back half a century, and in those records, they could find no precedent for the 2019 event. So whatever caused the "Great Dimming," as it came to be called, must have been truly extraordinary.

Gravity darkening
Whatever caused the dimming also must have come from a situation outside the star itself, rather than being due to some fundamental change in Betelgeuse's internal operations. That's because changes to the fusion reactions don't stop and start in only a few months. There's simply too much mass in the core, and the energies released by fusion reactions are simply too high, to support those kinds of quick changes.

Astronomers have proposed many possibilities, including stellar outbursts or giants clumps of orbiting dust. One possibility is that the shape of Betelgeuse's outer atmosphere changed, causing a shift in brightness. The brightness of a star's atmosphere depends crucially on how far that outermost layer is from the nuclear core (and any surrounding shells) in the center. That's because stars aren't solid bodies but rather giant balls of gas. The stars hold themselves together with the weight of their own gravity, but that force is counterbalanced by the (literally) explosive energies released in their cores.

So a star's surface is always balanced between these two forces. Where that balance point sits determines the star's temperature, and its temperature determines its brightness.

Astronomers can see the effects of this when stars rotate too quickly. When they do, the rotational force bulges out their equators relative to their poles. That makes the equator of the star sit farther away from the core, which reduces the temperatures and, in turn, the brightness. This kind of "gravity darkening" makes some stars appear brighter at their poles than around their middles.

An unruly neighbor
Betelgeuse isn't rotating fast enough for this to be a source of the problem, but things other than rotation can raise bulges on the side of a star. If a random visitor, like a small black hole, swung too close to the star, it could raise tides on the surface exactly the same way the moon raises tides on Earth.

With the tidal bulge in place, the equator would dim, along with the overall appearance of the star. Once the visitor left, however, Betelgeuse could return to normal, with all parts of its atmosphere in the right places, and resume its usual copious radiation output.

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 briefly swinging into the Betelgeuse system, the astronomers concluded that this scenario couldn't account for the full 60% dimming.

However, the passage of the visitor may have caused other effects, like a strong stellar outburst. Combined with the gravity darkening caused by tidal effects, a large amount of ejected material 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 in late 2019. After all, it happened only once in all of our records of the famous star. Further observations, of both Betelgeuse and other red supergiants like it, may reveal an answer. While the combination of gravity darkening from a tidal bulge raised by a close encounter with a black hole and the ensuing eruption of shrouding material may seem far-fetched, with limited evidence, we can only concoct the best stories possible.

See: https://www.space.com/betelgeuse-great-dimming-passing-star-explained

All the matter on the star feels that same outward push at the equator, so the star bulges outwards at the equator until it becomes an oblate spheroid — think a beach ball that's being slightly stepped on. Points on the star's poles are much closer to the center of the star than points on the equator. Since the poles are closer to the center of the star, they feel a much higher surface gravity than the equator does. Gravity pulls matter towards the center of the star, so a higher gravity area is a higher pressure area. When you exert pressure on a gas, you bring up it's temperature. Areas at the pole of a fast-spinning star are at a higher temperature, and shine brighter, than the rest of the star. This is known as gravity brightening. Its converse, gravity darkening, happens at the equator, where gravity, pressure, and temperature, are relatively low.

That's how a star, spinning fast enough, can glow at its poles and get dark around its middle.

See: https://gizmodo.com/the-mysterious-phenomenon-of-gravity-darkening-1638441719

Due to the speed of its rotation, a star becomes oblate (and hence gravity darkening occurs) because the centrifugal force resulting from rotation creates additional outward pressure on the star. This means that equatorial regions of a star will have a greater centrifugal force when compared to the pole. The centrifugal force pushes mass away from the axis of rotation, and results in less overall pressure on the gas in the equatorial regions of the star. This will cause the gas in this region to become less dense, and cooler and, therefore, darker.
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