Betelgeuse could have remained a common star among the others. On December 8th 2019, though, an unusual decreasing in brightness surprised worldwide astronomers. Such a deep minimum of intensity had not been measured for 25 years. Ordinary classified as the tenth brightest star of our sky, Alpha Orionis has currently dropped to the nineteenth rank! Would not this peculiar attitude be a premonitory sign of an upcoming supernova event? Let us recap the story of a well-known star.

Sky-map of the Orion Constellation. Source: Stellarium.

The star of the stars?

Even before the December 2019’s dimming event, Betelgeuse has been considered as one of the most studied stars of our sky. For more than a century, astronomers have literally been fascinated about it since a lot of comparisons can be drawn to the rest of the stellar population in our Galaxy.

Astronomer John Herschel was for example the first to witness the variable behavior of this star in 1836. These alternating phases of changing in brightness were later verified during the XXth century, and Betelgeuse became part of the semi-regular variable stars category: a type of star displaying noticeable periodic intensity changes whose cycle and amplitude may vary in time. A property we will explain later in the article.

This red super-giant was not originally famous for its variable properties, which are very well-known in stellar astronomy, but for its angular diameter.  Although Betelgeuse is distant of more than 400 light-years, its radius is about 1000 times larger than the Sun’s, which levels it up to the largest star visible in the starry sky! A superlative that was kept until 1997 when a larger angular diameter value was measured on R Doradus star.

Image of Betelgeuse taken by ALMA. The star is here compared to the size of the Solar System. Credit:
ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella

A star having a high value of angular diameter is very interesting as long as we want to observe details on its surface. Whereas the majority of the stars can be considered as infinitely small dots which are impossible to resolve with our telescopes, Betelgeuse has an angular diameter large enough to be noticeable with optical instruments!

Moreover, Betelgeuse was the first star (other than the Sun) to get an experimental estimation of its linear diameter, carried out with the recently-invented interferometric method. To do so, physicists Albert Abraham Michelson et Francis Gladheim Pease used the telescope at Mount Wilson observatory in 1921 and ended up with a value of angular diameter of 0.047”.

By combining light from several telescopes to increase angular resolution on the sky, this instrumental method is now being used by the most advanced astronomical observatories of the world, such as the VLTI (Chile) and CHARA (USA)..

The popularity of Alpha Orionis does not rely only on this pioneer study. Betelgeuse was one more time on the fore in 1995 when the Hubble Space Telescope unveil the very first direct image of the surface of a star. 

View of the surface of Betelgeuse as captured by the Hubble Space Telescope (1995). Credit : A. Dupree (CfA), R. Gilliland (STScI), FOC, HST, NASA

In the recent years, a handful of images using more developed instruments and techniques were captured. In 2017, the Atacama Large Millimeter/Submillimeter Array (ALMA) provided what is still the sharpest image of Betelgeuse ever.

A strange dimming behavior

The way Betelgeuse is going fainter does not comply either with the observations that have been gathered for 25 years nor with the most recent theoretical models. Until now, astronomers had noticed these brightness variations between magnitudes 0.0 and +1.3 could come from two coupled phenomena:

  • The first one would have a ~400-day-cycle and would be caused by pulsation of the star’s atmosphere 
  • The second, whose period is much longer – a bit more than 5 years – would be the result of the motion of huge convective cells on the surface of Betelgeuse. The latter theory is likely to be tested within few months, in respond to the great interest worldwide observatories have showed towards the recent changing state of Betelgeuse.
Evolution of brightness of Beltegeuse as of February, 10th 2020. Source: @Betelbot

Is Betelgeuse about to explode?

The cataclysmic phenomenon of Supernovae remains without a doubt a pure fascination for mankind because of its rarity and of the spectacular show it ignites. Indeed, the most massive stars of our Galaxies like Betelgeuse should end their life releasing part of their matter in the form of luminous energy. A light such intensively bright the star could be visible in a daytime sky for several weeks. A historic example of a supernova Humans were able to witness was the 1054 supernova, after which the Crab Nebula is born ; the first object of French Astronomer Charles Messier ‘s catalog (M1).

Infrared image of Betelgeuse photographed by an adaptive optics system at the VLT. It shows giant structure of circumstellar gaz around the photosphere (2009). Credit:
ESO and P. Kervella

Nevertheless, astronomers are today not entirely convinced a simple and progressive dimming in brightness would lead to an immediate explosion of Betelgeuse. Observing light variations coming from the surface only does not necessarily contain all the information of the change that could occur inside the stellar core. Betelgeuse will end up very likely in a supernova. Even if it is currently going fainter, astronomers still struggle to identify this change as an time indicator that could inform exactly when does this explosion take place.

For sure, no hypothesis is entirely off the point, and professional astronomers as well as amateurs should be ready at anytime. Betelgeuse did not unveil all its secret yet, and could surprise us as it always did. So, observe this red super-giant as it is still possible! 

Photographing Betelgeuse with the STELLINA smart telescope

Betelgeuse is so bright in the sky that you can observe it in many ways: with the naked eye, using a pair of binoculars or a telescope… In order to give STELLINA‘s users the possibility of monitoring the variation in brightness of the red supergiant, the Vaonis team added the star in its software update of February 2020 (MAJ Stellinapp 1.17) to its catalog of objects. Users are now invited to follow the evolution of the star over the weeks or months, in the hope of witnessing an exceptional supernova.

Betelgeuse STELLINA
Betelgeuse captured by the STELLINA smart telescope in February 2020