Betelgeuse is the closest red supergiant to Earth, an evolved star that is expected to go supernova sooner rather than later in cosmic terms. Over the last 12 months, it has really been messing with astronomers. The star experienced a dramatic dimming that concluded in February, and after a few months of slowly getting brighter, it has now inexplicably dimmed again.
The star is currently too close to the Sun in the sky to be observed by Earth, but fortunately, NASA’s Solar Terrestrial Relations Observatory spacecraft (STEREO-A) is located a nice 69 degrees behind our planet in its orbit; a vantage point that allows for observations of the stars without our pesky Sun in the way.
As reported in the Astronomer’s Telegram, researchers report that the star has been dimming again since May. In a series of three observations on June 24, July 13, and July 20, the star has gotten increasingly dimmer. Enough to make them wonder what might be going on.
“The decrease in brightness over 26 days between our first and third observation is comparable to the photometric behavior in late October 2018, and slower than the faster decline, between October 22 and November 10, 2019, that marked the beginning of the historic Great Fainting event in February 2020,” the group led by Andrea Dupree from the Center for Astrophysics/Harvard & Smithsonian wrote.
Betelgeuse is a variable star, which means its brightness changes on a cycle that in this case lasts about 420-430 days. The Great Fainting was in line with that cycle of dimming but its magnitude was shocking. It dimmed so much that Betelgeuse recorded its dimmest magnitude in 125 years, kicking it out of the top 20 brightest stars in the sky. A possible explanation for the drop in luminosity is a large stellar spot that appeared at the same time, but researchers have now shown evidence of a large stellar eruption from Betelgeuse. As that material expanded and cooled down into space, it blocked some of the light from the star, making it appear dimmer than it already was.
Given the length of Betelgeuse's cycle, it's expected to have a minimum brightness in April 2021, but if the current dimming continues this will happen much earlier.
This latest saga is another mystery to add to the ongoing questions we have about this incredible star. Once again the brightest star in Orion has all eyes firmly on it.