Why was Betelgeuse toned down so dramatically?
In October 2019 it began to weaken. It is a variable star, which becomes brighter and weaker in a cycle of about 420 days, so it is not unusual. But this time it started to fade … and then it didn’t stop. Normally varies by about 50% in brightness, decreased by a factor of Three In early February 2020, it was noticeably dimmer than other stars in Orion at first glance. It was not until mid-February that it hit rock bottom and reopened.
Of course, this generated a lot of speculation in the public that it could explode, although I don’t think any professional astronomer has taken it seriously; stars like Betelgeuse explode because of what’s going on in their core, and the upper atmosphere is far, far away. Changes in the nucleus take a long time to reach the surface.
So what was going on? One idea, supported by observations, was that Betelgeuse ejected a large cloud of dust, small grains of silicates and carbon, which are opaque in visible light and dimmed the star from our point of view (full disclosure: this was published by my colleague and friend Emily Levesque and a co-author).
However, another possibility is that Betelgeuse had an unusually large episode of star points: darker, cooler regions on the visible surface caused by magnetic activity within the star. The Sun receives these, and we call them sunspots: Basically, the entangled magnetic fields within the growing volumes of hot plasma prevent them from sinking into the Sun as they cool, so they stay on the surface. Since they are colder, they are darker, and the Sun darkens a bit in visible light.
Red supergiants like Betelgeuse can get huge star patches, much larger than the Sun itself, and indeed can cover large areas of its surface. Could a particularly large outbreak of stellar acne be the cause of the star’s dimming?
A team of astronomers investigated it. They observed Betelgeuse in submillimeter light, a wavelength far from human sight and bordering on radio wavelengths. The advantage of these wavelengths is that any dust Betelgeuse has thrown into space is likely to be transparent in that part of the spectrum, so it would not affect observations. If the dust were to blame, they wouldn’t expect to see any change in the starlight.
They did, however: at wavelengths less than mm, Betelgeuse was attenuated by about 20% at the same time as it was attenuated in visible light. That seems to show that dust is therefore not the reason the star faded.
The light below mm comes from the star itself, so if the light dims, that means the star’s surface did so intrinsically. They postulate that gigantic stars formed, dimming their light.
Therefore, it is expected that the star also cools, since the spots are cooler. Levesque’s earlier work investigated that by looking at temperature-sensitive molecules of titanium dioxide in Betelgeuse’s upper atmosphere. They found that the temperature dropped at most 50 – 100 ° Celsius. A cooler star emits less light, but a fall from this range was not enough to explain the attenuation on its own.
However, the new study shows that this still works for stains. A series of colder 200 ° C star spots covering half the surface would explain both the temperature change and attenuation in visible light and sub-mm.
This also fits in with extremely high-resolution Betelgeuse images taken in December 2019, which showed that the southern half of the surface was dimmer than the other half. It is possible that the ejection of a dust cloud blocked only the southern part of the star, but it is also possible that it was also covered with huge star points. Red supergiant models show that the spots can be this big.
Of course, “large” is well below its size as an adjective. Betelgeuse is approximately 900 times The diameter of the Sun, more than a billion kilometers in diameter, makes these stars much larger than our entire star. Only the spots themselves would extend from the Sun to Jupiter! They are absurdly huge.
So is this the last word on Betelgeuse’s strange behavior? Almost certainly not.
For one thing, Betelgeuse started to light up again just in time, and reached its normal brightness in June 2020. That implies that the spots grew and dissipated fairly quickly, in just a few months. Simulations of red supergiant stars show that their upper layers can light up and dim on time scales of a few months, making it possible for the putative points to strengthen and decrease while observing brightness and dimming. But it is rare that they had dissipated just in time with the normal cycle of star brightness. That bothers me.
Also, we don’t really know what can cause changes in the sub-mm light of a red supergiant. The upper atmospheres of these beasts are complex, with pulsations moving through them, convection that moves things, and other problems.
Then it is possible Those dots are behind all of this, but it’s not a lock, and I don’t think dust can be ruled out.
Clearly, there’s much more to discover about this bloated ruddy supernova in formation. Right now, it’s too close to the Sun to observe, but I bet in late summer when it’s highest in the morning, astronomers will start observing it again.
What will this ridiculously large star do next? We will find out soon.
My thanks to Emily Levesque for answering some questions I had about the upper atmosphere of Betelgeuse. Any persistent error is mine; This field of study is … complex. To say the least.
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