At the end of 2019, the red supergiant Betelgeuse, 642 light-years away, mysteriously began to dim, losing up to 60% of its usual brightness. Something really striking for a star that is among the ten brightest that can be seen from Earth and that led astronomers to believe that its explosion as a supernova was imminent. With its 900 million km in diameter, the size of Betelgeuse is such that if it were to take the place of the Sun in the Solar System, its outer edge would lie in the asteroid belt, between the orbits of Marte and Jupiter, so the inner planets of our system, Mercury, Venusthe Tierra y Martethey would be ‘inside’ it.
Betelgeuse, moreover, is a star that is about to die. And when it does it will be exploding as a supernova of Type IIsomething that happens when the core of a star is no longer capable of generating enough energy to ‘sustain’ its enormous mass, which collapses due to its own gravity until the star, unable to resist the immense pressure, explodes releasing a immense amount of energy.
So when the star began to dim for no apparent reason at the end of 2019, astronomers immediately thought the end was imminent. Betelgeuse’s brightness continued to dim until early 2020, but in February of that year, when its luminosity was only 60% normal, the dimming stopped, and the star gradually began to recover. Today, its brightness is back to its normal levels of intensity.
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 occurred in late 2019 and early 2020. The image on the far left, Taken in January 2019, it 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 decreased markedly, especially in its southern region. . Brightness returned to normal in April 2020
Betelgeuse, then, was not about to explode as many thought. And while its end is inevitably near, it may be years, decades, or even a millennium before it occurs. It is impossible to predict the exact moment when that explosion will take place. It will arrive soon, but we don’t know when.
What was it then that caused the dimming of Betelgeuse? Scientists have been trying to find out for years and different explanations have been proposed, from clouds of dust and gas to fragments of shattered planets ‘covering’ part of the star, although none of them has managed to satisfy everyone. What everyone agrees on is that, whatever the cause, it must have been something extraordinary. Something, moreover, that must have come from outside the star, since it is impossible for the nuclear reactions and processes inside it to stop and start again after a few months.
A black hole?
There is, however, another possibility. When a star spins too fast, the rotational force causes it to bulge at its equator, so the distance from the surface to the core is greater there than it is at the poles. And that causes the temperature, and therefore the brightness of the star, to decrease in what is known as ‘gravity dimming’. But Betelgeuse isn’t spinning fast enough for this to happen, although there are ‘other things’ that can cause bulges on the sides of a star. Things like a passing black hole.
That is precisely the idea investigated by a team of astronomers from the Bowdoin College, a private university in Maine, United States. If a black hole had gotten close enough to Betelgeuse in 2019, it would have generated powerful tides on its surface (like those the Moon generates on Earth’s oceans) with an effect similar to spinning too fast: an equatorial bulge with a corresponding decrease in brightness. Afterwards, and once the black hole moved away on its way, everything would return to normal.
In a study published in ‘Monthly Notices of the Royal Astronomical Society’, researchers study precisely this possibility. And his conclusions are that, by itself, the passage of a black hole could not have generated a 60% darkening in Betelgeuse, although other effects could have, such as a strong stellar burst, with the sudden ejection into space of a large amount of star material. The combination of all this material with the aforementioned darkening due to gravity, according to the study, would have been enough to explain the ‘great darkening’.
However, as the astrophysicist writes Paul Sutter on Space.com, “Astronomers may never fully understand what happened to Betelgeuse in late 2019. After all, something like this happened only once in all of our records of the famous star.” In the future, it is possible that new observations, both of Betelgeuse and other similar red supergiants, may find an answer. According to Sutter, the black hole solution may seem far-fetched, but given the limited evidence available, “we can only make up the best possible stories.”