Last winter Betelgeuse hit bottom. Although the star had been known for decades to vary in brightness, it reached a historic low in mid-February when it tumbled to magnitude 1.6, on par with its neighbor Bellatrix. Many of us watched the red supergiant star with great excitement, some even wondering if its behavior presaged a supernova explosion. Astronomers sought to explain its unprecedented dimming as possibly due to giant starspots darkening the star’s surface or alternatively, light-absorbing dust clouds belched out by the monster star.
By March Betelgeuse had turned the corner and began to return to its former brilliance. Before it departed the evening sky in May it outshone nearby Aldebaran in Taurus. What happened?
Thanks to new Hubble Space Telescope observations a team of researchers now suggest that dust was the culprit. A large convective cell made of super-hot stellar gas called plasma welled up from Betelgeuse’s surface. A good way to picture this is to imagine rising air bubbles in a pot of boiling water. The plasma bubble ascended through the hot atmosphere and when it reached the colder, outer layers it cooled and formed dust. The resulting dust cloud blocked light from about a quarter of the star’s surface, beginning in late 2019. By April the cloud had thinned or dissipated, and Betelgeuse returned to its normal brilliance.
“With Hubble, we see the material as it left the star’s visible surface and moved out through the atmosphere before the dust formed that caused the star appear to dim,” said lead researcher Andrea Dupree, associate director of The Center for Astrophysics (Harvard & Smithsonian). “We could see the effect of a dense, hot region in the southeast part of the star moving outward.”
Astronomers kept track of the ejected material which was initially 2 to 4 times brighter than the star’s normal brightness. Then a month later the southern hemisphere of Betelgeuse dimmed as the bright cloud cooled and darkened with dust. Specifically, astronomers looked at the element magnesium in the ejected gases and watched it travel from the surface to the outer atmosphere until it chilled to form dust.
Betelgeuse expands and contracts rhythmically, its surface rising and falling during each pulsation cycle. When the convective bubble erupted, observations show that the star was expanding at the same time. The team suspects that the pulsation may have given the hot gases an extra kick, hurrying them through the atmosphere and encouraging quick condensation.
Every star is a time machine. Betelgeuse is about 650 light years away, so the dimming happened around the year 1370, not long after bubonic plague or Black Death (1346-1353) raged across Europe killing 50 million people. Vaccines were non-existent back then and medical care primitive. Let’s hope science will soon get the current viral plague under control. One wonders what the world will be like 650 years from now. Will Betelgeuse still be around or will it have gone supernova and left a blank spot in Orion’s shoulder?
If you’re getting up to see Orion at dawn, beam in on Betelgeuse and compare it to Bellatrix and Aldebaran. Guess what? The star is dimming again! This is very unusual since it’s normal bright-dim-bright cycle takes 420 days, and it’s only been a couple months since the last brightness peak in late May. Currently equal to Aldebaran, it’s anyone’s guess exactly what will happen next.
The mystery continues.