What is going on? Betelgeuse is a variable star, meaning that its brightness changes. While Betelgeuse is mentioned many times in ancient sources, the first likely person to comment on its variability was John Herschel in 1836. Betelgeuse has varied throughout history, but no one noticed it before Herschel. So, it’s not unusual for Betelgeuse to fade. What is strange is how much Betelgeuse has recently faded. Late last year and early this year, Betelgeuse was fainter than in any modern records. Likely, if Betelgeuse had ever faded this much in antiquity, there would have been some record of it.
What does this unusually faint brightness of Betelgeuse mean? That isn’t clear. Most astronomers think that Betelgeuse eventually will become a supernova, a titanic explosion that will disrupt most of the star. However, they also doubt that its recent activity necessarily is a harbinger that this will happen soon. Indeed, a recent report indicates that Betelgeuse is brightening once again. Betelgeuse reached minimum brightness in early February and began to increase in brightness in mid-February. If this trend continues, Betelgeuse will soon return to “normal” brightness.
A star’s brightness depends upon its size and temperature, so as the star pulsates, its brightness changes.
What causes Betelgeuse to vary, and what caused this unusual recent activity? While the data are clear enough, the interpretation is not. Many stars vary because they pulsate–they periodically swell and shrink. A good example of pulsating stars are Cepheid variables, which are used as important tools in finding stellar distances. As a pulsating star changes size, its surface area changes, and its temperature also changes, if only slightly. A star’s brightness depends upon its size and temperature, so as the star pulsates, its brightness changes. What causes the pulsations? The answer to that question is a bit complicated. There is a region inside a pulsating star that is responsible for this. There is a slight displacement from an equilibrium position. As the material in this region reacts to the displacement to reestablish equilibrium, it overshoots equilibrium, resulting in an imbalance in the opposite direction, and the cycle repeats. There are many examples of this sort of behavior elsewhere. For instance, overshooting equilibrium after displacement is what happens when a spring oscillates or a pendulum swings back and forth.
Many pulsating stars have regular periods of variability, but some do not. Betelgeuse is in the latter category, classified as a semi-regular variable. There appear to be two, and possibly three, different periods of oscillation going on simultaneously in Betelgeuse, with the shortest period being at least a few hundred days. When two or more periods of oscillation take place simultaneously, the oscillations occasionally are out of phase, greatly reducing the amount of oscillation during that time. But there are times when the oscillations are in phase, resulting in larger than average swings in oscillation. This is a well-understood phenomenon in physics called a beat frequency. This effect may be playing a role in the recent unusual activity of Betelgeuse.
But this doesn’t seem to explain all the recent activity in Betelgeuse. Early this year, astronomers at the European Southern Observatory (ESO) in Chile used an instrument called SPHERE (Spectro-Polarimetric High-Contrast Exoplanet REsearch) on the Very Large Telescope (VLT) to obtain images of Betelgeuse. SPHERE on the VLT has the capability of producing the highest resolution images possible of astronomical bodies. Stars are very far away, rendering their light as mere pinpoints. Still, Betelgeuse is one of the largest appearing stars (a dime viewed from 50 miles is the apparent size of Betelgeuse). That is due to Betelgeuse’s large size (if it were to replace the sun, its surface would extend to the orbit of Jupiter) and its relative proximity (only 700 light-years). Fortunately, these astronomers happened to have taken an image of Betelgeuse a year before, long before the recent fading. A comparison of the two images is striking. In the earlier photo, Betelgeuse looks round, but in the following picture, it looks asymmetrical, as if its shape has changed. However, close examination reveals that the asymmetry of the second image largely is due to a vast dark region on or around Betelgeuse.
Chaotic processes acting in Betelgeuse could have recently produced a large amount of dust and then expelled it.
What is that dark region? The answer to that question isn’t clear either. Many stars are known to have spots similar to those on the sun, but much larger than sunspots. Some stars vary in brightness as huge spots develop and disperse or areas with large spots rotate onto the side of the star we see and then rotate off. Thus, we could be seeing a region of huge spots on Betelgeuse. But there is another, more likely, explanation. This dark region could be a large cloud of dust that Betelgeuse recently ejected. Astronomers think that the conditions in the atmospheres of red giants are conducive for the formation of dust grains and are the primary site of dust grain formation. Chaotic processes acting in Betelgeuse could have recently produced a large amount of dust and then expelled it. We know that Betelgeuse has done this sort of thing in the past because astronomers have detected what appears to be concentric shells of dust around Betelgeuse. We may just be witnessing the latest episode of this ejection.
Whatever the cause of Betelgeuse’s recent hiccup, it will make for interesting watching over the next few months. Unfortunately, in May, Betelgeuse will disappear behind the sun for a couple of months. Amateur and professional astronomers are sure to keep watching until then when Betelgeuse emerges once again from behind the sun to see what unfolds. While we know quite a bit about the stars, it is humbling to recognize that there is much that we don’t know. That is a good lesson for us. Many times, the pronouncements of scientists are claimed to have proven the Bible wrong. However, just how well-founded are those pronouncements? We often find that later those pronouncements end up being proved wrong. It’s best to place our trust in God’s Word than in the uncertain conclusions of men.