When Betelgeuse explodes, it would rival the brightness of the full moon and possibly be visible even by daylight.

The most visually striking constellation of the entire year, Orion, dominates the mid-winter skies, and the most striking star in Orion is the brilliant orange-red Betelgeuse, which marks the upper left or eastern shoulder of Orion.

This winter, astronomers are keeping their eyes and instruments fixed on that star because since October, Betelgeuse has been undergoing a dramatic decrease in brightness that might be the greatest on record for this fascinating object.

Betelgeuse (the name is garbled from the original Arabic and not pronounced quite like the movie about the ghoul starring Michael Keaton) is a red giant star. This means it is near the end of its life. Its fuel is almost spent, its atmosphere is bloated and bleeding off into space and someday soon, it will fail completely. When that happens, nothing will be able to stop the collapse of its core and the explosive rebound process known as a supernova.

In the meantime, the turbulent processes that engulf the surface of this star make it one of the most easily visible variable stars. A variable star is one that changes in brightness. Many stars do this regularly, though too subtly to be casually noticeable.

Betelgeuse varies regularly over a period of about six years, but there are shorter periods, sometimes irregular, that overlay this cycle. A superposition of cycles seem to be what astronomers are observing right now, as Betelgeuse has dimmed significantly since October.

This decrease has been quite dramatic, taking astronomers by surprise. Usually the 10th brightest star in the sky, Betelgeuse has dropped all the way to be currently the 21st brightest. Because its brightness and position in the sky, it is an ideal target for a backyard observer to track these changes.

Skilled variable star observers can estimate the brightness (known as the apparent magnitude) of stars. Magnitude is given on a scale where smaller numbers indicate brighter stars and larger numbers (confusingly) mean dimmer stars. Rigel, for instance, the bright right or eastern foot of Orion, has a magnitude of about 0.1. Aldebaran, the reddish eye of Taurus the Bull, found by drawing a line through Orion’s belt to the northeast, is dimmer and closer to 1.0 magnitude. Betelgeuse, at times, can appear as bright as Rigel but has decreased steadily, until now, it’s dimmer than Aldebaran.

You don’t have to memorize magnitudes or star positions to be able to watch Betelgeuse and gauge changes in its brightness. The easiest thing to do is compare its brightness with the other stars in Orion.

At its brightest, Betelgeuse can rival Rigel in magnitude. Now, Betelgeuse is nearly as dim if not dimmer than Bellatrix, the left or eastern shoulder of the constellation. If it continues to decrease, it could be comparable in brightness to the stars of Orion’s belt.

Comparing Betelgeuse to the other stars of the constellation from night to night is one way to judge its changes. When you compare brightness between Betelgeuse and another star, experts recommend looking quickly between the two stars because if you let your eye linger on either too long, your brain tends to exaggerate their brightness.

Another trick that works for the nearsighted is to remove your glasses and judge brightness by the size of the fuzzy splotches the stars become.

Does this sudden dimming mean Betelgeuse is heading toward a supernova explosion? Most astronomers think not, though it’s impossible to know for sure. Our models of stellar evolution indicate Betelgeuse could go supernova any time from tomorrow night to a million years from now.

If it does explode though, what could we expect? There hasn’t been a visible supernova in our corner of the galaxy for centuries, and never one as close as Betelgeuse in recorded history.

When Betelgeuse explodes, it would rival the brightness of the full moon and possibly be visible even by daylight. At a distance of more than 600 light years though, it doesn’t pose a threat. We would be safe from the majority of harmful radiation emitted. The chance to study a supernova at such a distance, however, would be unprecedented.

That very distance — approximately 600 light years to Betelgeuse — reminds us that what we’re watching is in a sense a flickering of ghost-light. The dimming of Betelgeuse we see now actually is light that left that star over 600 years ago, so whatever processes or changes causing it actually happened over six centuries ago.

And if it does flare into brightness again or even — though unlikely — explode entirely, then we’ll be watching the end of a star that’s already 600 years dead.

Steve Case is director of Olivet Nazarene University’s Strickler Planetarium and an assistant professor in the department of chemistry and geosciences.

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