The final round of observations of the star Betelgeuse has begun, and the blackout that some scientists have predicted as a precursor to a supernova has come to an end. Now there is evidence of a clarification.
This means the star is not coming to a premature end as some had hoped; but still a little puzzled as to why the star has dimmed so much.
Betelgeuse, located in the constellation Orion about 700 light-years from Earth, is a very old star of this type. It spent years of its main sequence – with an active core fused with hydrogen atoms – like a hot, glowing, bluish-white star, about 10-25 times the mass of the Sun. These stars live wildly and have a relatively short lifespan.
Betelgeuse is now about 8–8.5 million years old, and its main sequence days ended about 1 million years ago (compare this to a sun-like star only halfway through the main sequence, 4.6 billion years old).
It cooled down and turned into a red supergiant about 40,000 years ago. By now, Betelgeuse has run out of hydrogen in its core and is converting helium to carbon and oxygen.
The nucleus has also contracted, which brings more hydrogen into the area immediately around the nucleus, forming a hydrogen shell; this hydrogen shell fuses with helium, which is dumped into the core to fuel the helium fusion.
Before it explodes, the star is expected to dim quickly before dramatically illuminating the sky here on Earth.
While Betelgeuse does have variable brightness, its recent darkening was much deeper than previously observed – between September 2019 and January 2020, the luminosity decreased by 25 percent.
Now, instead of the fireworks that we all hoped to see, Betelgeuse is returning to normal brightness levels, in sync with the star's cycle of variability.
'Photometry over the last ~ 2 weeks shows that Betelgeuse has stopped its large delta-V decline of ~ 1.0 mA compared to September 2019,' wrote Astronomers Telegram.
'Based on these and additional observations, Betelgeuse definitely stopped fading and began to gradually lighten. Thus, this episode of 'fainting' is over, but additional photometry is needed to determine the lightening phase. '
(ESO / P. Kervella / M. Montargès et al., Acknowledgment: Eric Pantin)
The big question remains: what caused the blackout? The possibilities are being explored by astronomers. One of them is cooling on the star's surface. Something really strange must be happening in Betelgeuse for it to fade like this, but it's not impossible.
Another is a gigantic cloud of dust ejected from the star towards us. Stars on the branch of the red giant create and eject vast amounts of material long before they go supernova, and infrared images show Betelgeuse surrounded by jets of dust. You can see them in the image above; a black disk blocks a star, an area the size of Jupiter's orbit.
Both of these explanations would be consistent with the dimming asymmetry seen in the December 2019 images.
Sources: Photo: (ESO / Digitized Sky Survey 2. Acknowledgment: Davide De Martin)
