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This article comes from Weixin Official Accounts: SF Chinese (ID: kexuejiaodian), author: SF

Betelgeuse may be one of the most eccentric stars in the sky. It had dimmed sharply in previous years, and had recently brightened strangely. Why is it behaving so strangely? Could it explode into a supernova soon? If that happens, the star will become as bright as the full moon. So, can we really expect this astronomical spectacle to happen?

Wen| Chen Qiang

Betelgeuse is one of the most prominent stars in the winter night sky, a sparkling red bright spot on Orion's left shoulder about 650 light-years from Earth. Betelgeuse is one of the largest stars visible to the naked eye. If it were placed at the center of the solar system, its surface would extend beyond the asteroid belt and devour everything in the orbit of Mercury, Venus, Earth and Mars.

Betelgeuse is a red supergiant, which means it has burned all the hydrogen fuel in its core and expanded hundreds of times. Now, inside its core, it's fusing helium into carbon and oxygen. In general, red supergiant phases are short, lasting only hundreds of thousands to millions of years in total, before the star dies in a supernova explosion. Betelgeuse is in its twilight years.

Normally Betelgeuse is the tenth brightest star in the night sky. However, since early April this year, the star has climbed to No. 7 on the list and is currently nearly 50 percent brighter than usual. This is a bit exciting. Could it explode into a supernova soon?

What happens if Betelgeuse explodes? After the helium is consumed, the red supergiant will sustain itself by burning carbon and oxygen to form neon and magnesium, which will then be burned to silicon. Eventually, the core of the star will be filled with iron. Nuclear fusion of iron, however, absorbs energy, not releases it. At this point, the star's core will no longer be able to draw energy from further fusion reactions, causing the star to collapse under its own gravity and blow itself up into pieces. At this point, the star becomes a supernova.

If Betelgeuse is indeed about to explode, it would be the closest supernova explosion to occur in more than 400 years. When it becomes a supernova, the first warning signs will be silent: a mass of ghostly neutrinos released during the star's collapse that will suddenly sweep across the earth, illuminating detectors around the globe.

Energy-rich photons then emerge from the densely expanding cloud of stellar debris. When the photons reach Earth, we will see Betelgeuse become very bright, even reaching the brightness of the full moon, and all concentrated in a single point of light. Bright enough that you can see it during the day. At night, Betelgeuse's light casts sharp shadows on the ground.

This astronomical spectacle will last for a few weeks. And fortunately, although Betelgeuse is relatively close to Earth, it doesn't do us any harm by becoming a supernova. Astronomers believe that only supernova explosions within about 160 light-years of Earth are likely to harm us.

So it's Betelgeuse's "great darkening"? Astronomers, however, poured cold water on this possibility. They believe that Betelgeuse will not become a supernova in the near future, and the reason for its recent brightening is related to the "great dimming" event that occurred in late 2019.

Astronomers have long known that Betelgeuse is a variable star, meaning that it brightens and darkens periodically. In fact, the indigenous inhabitants of Australia noticed this phenomenon more than a thousand years ago. Over the last 100 years, astronomers have observed Betelgeuse brighten about every 400 days, then drop to about half its peak brightness, and then brighten again.

Between late 2019 and early 2020, the star became dimmer than ever, about 2.5 times less bright than its usual dimmest. This event is known as the "Great Dark." Astronomers think this is because the star spews out a lot of material, creating a huge cloud of dust that then obscures the star's own light.

When the dust cloud clears, Betelgeuse returns to its usual brightness. But since the Great Dimming, the star hasn't been quite as it once was. Its period of brightness variation has become shorter than before, about 200 days, and most importantly, the star has recently become brighter than before.

Astronomers believe that Betelgeuse's ejection of material from the interior caused the outer and inner parts of the star to move in opposite directions to each other, and this change may have caused the star's brightness variation cycle to shorten. Also, there should be a lot of plasma turbulence on the star's surface, which could explain why the star has become brighter than before.

That said, Betelgeuse's surface is now in an unstable state, and astronomers believe that Betelgeuse will return to its normal state within the next five to 10 years.

In fact, every few centuries Betelgeuse spews out large amounts of matter, followed by a "great darkening." But none of this means that the star is about to explode.

When exactly will Betelgeuse erupt? Betelgeuse is relatively close to us, and with the help of astronomical telescopes, astronomers can observe what is happening in the outer layers of Betelgeuse. Although it is impossible to observe directly what is happening inside the star's core, astronomers speculate from the observed data that Betelgeuse is still in the early stages of fusing helium into carbon and oxygen.

Astronomers can also analyze Betelgeuse by studying other red supergiants. For example, a red supergiant called VY Canis Majoris, about 3900 light-years from Earth, is thought to be closer to its death than Betelgeuse. But unlike Betelgeuse, which brightens, this star has been dimming for the past 100 years. VY Canis Majoris was visible to the naked eye more than a hundred years ago, but it has released so much material that we can only see it in infrared now. As the star approaches a supernova explosion, we expect to see this mass ejection. VY Canis Majoris has lost about 60% of its original mass, while Betelgeuse still has 95% of its original mass.

In addition, Betelgeuse was described as a yellow star in ancient times according to historical records, and it was not until more than 2000 years ago that people began to describe it as red. This indicates that Betelgeuse is still in the early stages of red supergiant.

Currently, astronomers mostly believe Betelgeuse is expected to erupt sometime in the next 10,000 to 100,000 years. This means that it is almost impossible for anyone alive today to see Betelgeuse explode.

Betelgeuse will not be visible for the next few months because it is too close to the Sun. Astronomers will have to wait until late summer to continue observing it. If Betelgeuse is still as bright as it is now, or brighter, in September, then we should start thinking about what happened to it.

References:

https://www.space.com/is-betelgeuse-going-supernova

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