A simulated view of Betelgeuse [Michele Diodati]

All About Betelgeuse. The Most Conspicuous Drop in Brightness From a Century Now

In the last four months, the red supergiant in Orion has lost about one magnitude, reaching the same brightness as Bellatrix. Despite this remarkable drop in brightness, there is no indication of an imminent supernova explosion

Michele Diodati
Feb 14 · 5 min read

Betelgeuse is one of the most easily recognizable stars in the sky due to its vivid orange-red color and the position it occupies in Orion, less than ten degrees north-east of the , which, with its three aligned stars, represents the center of the constellation.

Known and admired since ancient times, it is one of the most observed and stars of all time. The photoelectric measurements of its brightness began almost a century ago. With their superior reliability compared to purely visual observations, they have made it possible to reconstruct with high precision the cycles of variation in brightness that have occurred over the years. Betelgeuse is, in fact, a semi-regular variable with complex oscillations due to the various overlaps of two different periods, one of the approximate duration of 5,9 years and the other of 425 days.

At maximum brightness, the red supergiant in Orion reaches a visual magnitude of around +0.3. It is then the 6th or 7th brightest star in the sky. At its minimum, however, the visual magnitude can drop to +1.3, which brings it beyond the 20th place in the list of the brightest stars visible to the naked eye.

Thanks to the knowledge gained from the vast archive of available photometric measurements, Betelgeuse’s periodic changes in brightness are certainly not a surprise for astronomers. But when such variations surpass, and not by little, the extremes reported in the historical series, then even the experts of variable stars begin to wonder if something new is happening. The theme is particularly topical, given the obvious changes in brightness observed in recent months.

It all started in October 2019, when the light from Betelgeuse began to fade. By December 7, the apparent visual magnitude to +1.12, the lowest in the past 25 years of photometric measurements. On December 19, : the visual magnitude is at +1.29. We must go back to the period between the end of 1926 and the beginning of 1927 to find a similar value, around +1.25. But the loss of brightness does not stop. On January 17 and 18, 2020, two new minima , +1.49 and +1.50. Betelgeuse is now markedly less bright than Aldebaran (V = +0.9).

A photo taken on December 22, 2019. Betelgeuse appears distinctly less bright than Aldebaran and only a little brighter than Bellatrix. Rigel plays in a different category []

From mid-January onwards, the speed with which the star loses light begins to decrease, but the decrease does not stop. At the end of the month, there is a new negative record, which is unprecedented since instrumental measurements of stellar brightness exist. The visual magnitude of Betelgeuse on January 30 was +1.614, practically equal to that of the blue Bellatrix (V = +1.62), which is usually the third brightest star in Orion, after Rigel and, well, Betelgeuse.

Compared to September 2019, the red supergiant has lost about one magnitude, which means that it now appears to us 2.5 times less bright than five months ago. The drop from its maximum is even higher (around 1.4 magnitudes). From the observational data, it is also getting colder. Compared to September, the effective temperature decreased by almost 100 degrees, from 3650 K to 3565 K.

If you were to take the current data as the effect of an isotropic change, which affects the entire radiant surface of the star in the same way, then it would mean that Betelgeuse’s radius has shrunk by 8% in recent months. But Guinan and Wasatonic, the two astronomers of Villanova University, to whom the observations described above are due, warn that this unusual decrease in brightness could depend on local phenomena, such as the formation of opaque dust in the extended atmosphere of Betelgeuse or the appearance of locally colder regions on the stellar photosphere, produced by the powerful convective motions that shake the outer layers of the supergiant.

Two photos in comparison that show the evident decrease in brightness of Betelgeuse. In the left half of the picture, Betelgeuse imaged in February 2016. In the right half, Betelgeuse imaged on December 31, 2019. As you can see, the background stars look the same. Only Betelgeuse has faded []

Many think, indeed hope, that this sudden loss of brightness is the prelude to the Betelgeuse supernova explosion. It is a massive and highly evolved star, which we know has reached the final stages of its existence. One day it will explode like a , becoming for some time at least as bright as the full moon, so bright that it can be admired even in broad daylight. However, that fateful moment is presumably still quite far away. The most reliable forecasts indicate in about 100,000 years the time that still has to elapse before a supernova explosion destroys Betelgeuse.

The current drop in brightness, undoubtedly unusual, could be due then, as explained by the aforementioned Guinan and Wasatonic, to the simple overlapping of the respective minima of the two periods of the star. If this really is the cause, we will understand it in March. The 425-day period will reach its minimum around the end of February. Therefore, in the following days, the brightness of Betelgeuse could slowly begin to rise again.

While waiting to verify the goodness of this prediction, it is worth knowing better this immense and fascinating star, trying to reconstruct what we know about it and to circumscribe what we do not know yet, through a necessarily incomplete summary of the countless scientific researches dedicated to the study of Betelgeuse.

This is the first part of a five-part story. Read the other four parts here:

3. All About Betelgeuse. How Big Is It?

4. All About Betelgeuse. A Boiling and Asymmetrical Star

5. All About Betelgeuse. Live Fast Die Young

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Michele Diodati

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Science writer with a lifelong passion for astronomy and comparisons between different scales of magnitude.

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