Between September last year and this February, red supergiant Betelgeuse experienced an unprecedented dimming period that led the supergiant to drop out of the Top 20 brightest stars in the night sky.
Betelgeuse's activity led to speculation it was hinting at its impending supernova. However, as a variable star, Betelgeuse experiences changes in its brightness every 420 days, and this current dimming fits with this just being an extra-strong cycle. Why it reached such an exceptionally low luminosity, dropping to around 40 percent of its usual brightness, is still curious. Now researchers have a new theory, blaming it on a release of dust from its outer layers; basically, a sneeze.
A new paper, accepted in the Astrophysical Journal Letters, points out that the temperature of the star has changed marginally during this period and it is, in fact, warmer than it should be if its dimming was caused by the star's cooling surface. Instead, they think the star got rid of some of its material from its outer layers, which is blocking the light.
“We see this all the time in red supergiants, and it’s a normal part of their life cycle,” lead author Professor Emily Levesque from the University of Washington said in a statement. “Red supergiants will occasionally shed material from their surfaces, which will condense around the star as dust. As it cools and dissipates, the dust grains will absorb some of the light heading toward us and block our view.”
Levesque and her co-author Philip Massey were part of the team who had previously estimated the temperature of the surface of the star in 2004. They took observations on February 14 2020 to estimated how much Betelgeuse had cooled down. It turned out, not very much at all. The average surface temperature of the star is 3,325 °C ( 6,017 °F). That’s only between 50-100°C (90-180°F) cooler than the previously estimated temperature.
“A comparison with our 2004 spectrum showed immediately that the temperature hadn’t changed significantly,” said Massey. “We knew the answer had to be dust.”
Clouds of dust have been observed around other stars as well and the team is confident that new observations of Betelgeuse will reveal them here as well. However, this is only one reason to continue looking at this star. Betelgeuse is the nearest red supergiant to Earth and it is a perfect laboratory to work out how these celestial bodies evolve.
“Red supergiants are very dynamic stars,” said Levesque. “The more we can learn about their normal behavior — temperature fluctuations, dust, convection cells — the better we can understand them and recognize when something truly unique, like a supernova, might happen.”