Just a fainting spell? Or is Betelgeuse about to blow?

Is Betelgeuse about to blow?

Probably not, but astronomers are having fun thinking about it.

Over the last three months, the star, which marks the armpit of Orion the hunter, has mysteriously dimmed to less than half its normal brightness, markedly altering one of the great sights of the winter sky.

At the beginning of January the star was fainter than ever before observed, according to Edward Guinan of Villanova University, who has been compiling data on Betelgeuse. In its “fainting” spell, Guinan said, the star has dropped from seventh to 21st on the list of brightest stars in the sky. But even so dimmed, Betelgeuse is still too bright to be easily observed and measured by large professional telescopes — at least not without damaging sensors that were designed to wring every faint photon from the blackness of space.

Rebecca Oppenheimer, an astrophysicist at the American Museum of Natural History in New York, said that she had managed to observe Betelgeuse with the 200-inch telescope at Palomar Observatory in California last week but that it had left an afterimage that put their detector out of service for a day. The next time around, she said in an email, they plan to cover part of the giant mirror, to cut down on the amount of light it receives.

Last week Neil deGrasse Tyson, director of the Hayden Planetarium in New York, circulated a request for amateur astronomers to observe and monitor Betelgeuse’s brightness.

All this has raised the issue of Betelgeuse’s mortality and its cosmic endgame.

That the star will eventually blow up, nobody denies. Betelgeuse — sometimes pronounced “beetle-juice,” and also known as Alpha Orionis — is at least 10 times and maybe 20 times as massive as the sun. If it were placed in our solar system, its fiery gases would engulf everything out to Jupiter’s orbit.

The star is a so-called red supergiant in the last violent stages of its evolution. It has already spent millions of years burning primordial hydrogen and transforming it into the next lightest element, helium. That helium is burning into more massive elements. Once the core of the star becomes solid iron, sometime within the next 100,000 years, the star will collapse and then rebound in a supernova explosion, probably leaving behind a black hole.

That will be quite a show. Betelgeuse is only 700 light years from Earth, far enough to not kill us when it goes but close enough to impress; the supernova would be as bright as a full moon in our sky.

The star’s current diminution probably does not mark The End, astronomers say. Aging stars are notoriously cranky and moody, coughing out bursts of gas and dust that obscure themselves, or sputtering inside as their cores evolve and change.

Even normal stars are subject to periodic fluctuations in brightness. Betelgeuse endures such cycles of ups and downs, and the most likely explanation for the current episode is that two cycles bottomed out at the same time.

“My money all along has been that Betelgeuse is going through a somewhat extreme but otherwise normal quasi-periodic change in brightness,” said J. Craig Wheeler, a supernova expert at the University of Texas at Austin.

Tyson agreed that coinciding cycles was the most sensible explanation for Betelgeuse’s dramatic drop, although if that’s the case, he said, it was surprising that nobody predicted it.