Skip to main content

Betelgeuse is going dim because it’s covered in star spots, research finds

Betelgeuse is normally one of the brightest stars in the night sky, but late last year something strange began happening to it: The star was becoming much less bright than it used to be. In December, astronomers announced that Betelgeuse was dimming dramatically, leading some to think that it could soon explode in an epic supernova.

Although it’s typical for red giant stars like Betelgeuse to vary in brightness over time, in this case, the star’s output dropped so dramatically that scientists figured something unusual must be going on.

Further study of the star in February this year showed that it had dimmed by almost two thirds, going down to just 36% of its usual brightness. That difference was enough that it could be observed with the naked eye, even from 500 light-years away. At the time, researchers theorized that the variations could either be due to a dust cloud being ejected from the star and blocking its light, or that the irregular surface of the star could be creating the brightness variations.

Red Supergiant Betelgeuse
An artist’s impression of the Red Supergiant Betelgeuse. Its surface is covered by large star spots, which reduce its brightness. During their pulsations, such stars regularly release gas into their surroundings, which condenses into dust. MPIA graphics department

New research suggests that this second theory is more likely, and that Betelgeuse’s unusual behavior is due to temperature variations in the lowest layer of the star’s atmosphere caused by star spots. Star spots are similar to sunspots, dark patches that temporarily appear on the surface of the sun due to magnetic fields and give rise to solar flares. These star spots cover between 50% and 70% of Betelgeuse’s entire surface.

“Towards the end of their lives, stars become red giants,” lead author Thavisha Dharmawardena of the Max Planck Institute for Astronomy explained in a statement. “As their fuel supply runs out, the processes change by which the stars release energy. As a result, they bloat, become unstable and pulsate with periods of hundreds or even thousands of days, which we see as a fluctuation in brightness.”

Elderly Betelgeuse has bloated to a size so large — 1,000 times the size of our sun, while only being 20 times the mass — that gravitational forces can easily pull off its outer layers. That leaves behind areas of cool dust and gas, with areas of varying temperatures across the surface that reduce the brightness of the star overall.

One open question is whether Betelgeuse will continue to dim or will rebound in brightness. We know that sunspots on our sun follow an 11-year cycle, but scientists aren’t yet sure if the same is true of star spots on giant stars.

“Observations in the coming years will tell us whether the sharp decrease in Betelgeuse’s brightness is related to a spot cycle,” Dharmawardena said. “In any case, Betelgeuse will remain an exciting object for future studies.”

The findings are published in the Astrophysical Journal Letters.

Editors' Recommendations

Georgina Torbet
Georgina is the Digital Trends space writer, covering human space exploration, planetary science, and cosmology. She…
Astronomers find remnants of planets around 10 billion-year-old stars
Artist’s impression of the old white dwarfs WDJ2147-4035 and WDJ1922+0233 surrounded by orbiting planetary debris, which will accrete onto the stars and pollute their atmospheres. WDJ2147-4035 is extremely red and dim, while WDJ1922+0233 is unusually blue.

Far away in the depths of the Milky Way lie two small, dim stars that are in the final stage of their life. At over 10 billion years old, white dwarfs WDJ2147-4035 and WDJ1922+0233 are among the oldest stars in our galaxy, and recently, astronomers discovered something special orbiting around them: the remains of planets, making this one of the oldest known rocky planetary systems.

Astronomers used data from GAIA, the Dark Energy Survey, and the X-Shooter instrument at the European Southern Observatory to peer at this system. They identified debris from orbiting planetesimals, which are globs of dust and rock which are created during planetary formation. The researchers used spectroscopy to look at the light coming from the two white dwarf stars and break it down into different wavelengths, which can show what materials the stars and the surrounding matter are made of.

Read more
Betelgeuse blew its top, leaving an interior jiggling like jelly
Illustration of changes in the brightness of the red supergiant star Betelgeuse.

One of the brightest stars in the sky, the nearby red supergiant Betelgeuse, has been the source of fascination in the last few years as it dimmed and then re-brightened dramatically. Now, new data from the Hubble Space Telescope shows that the star blew off a huge chunk of its mass in 2019, creating a cloud of dust that obscured its light and caused the dimming effect.

The explosion of such a big chunk of matter is a rare event called a surface mass ejection, similar to the coronal mass ejection events seen in our sun and other stars but much, much bigger. The surface mass ejection blew off an almost unimaginable 400 billion times as much mass as a standard coronal mass ejection, creating huge changes to the star's structure and behavior.

Read more
James Webb Space Telescope may have spotted its first supernova
The potential supernova spotted by the James Webb Space Telescope.

The James Webb firsts keep coming, with the new space telescope having recently spotted what could be the most distant galaxy ever observed. Now, it may have spotted its first supernova.

The potential supernova spotted by the James Webb Space Telescope. pace Telescope Science Institute
As reported by Inverse, researchers using Webb believe they have observed a supernova using the NIRCam instrument. They compared the Webb data to data collected using Hubble and found a bright object which could be a star that has just gone supernova.
A supernova occurs when a massive star runs out of fuel and comes to the end of its life. As the star collapses, it throws off much of its material in an enormous explosion which gives out large amounts of light. This light is so bright it can be spotted from great distances away. Webb spotted one such bright flash in the galaxy SDSS.J141930.11+5251593. The telescope made two observations of the galaxy five days apart, and in the second observation, the flash was less bright, suggesting it is dimming over time.
"We would need more time series data to make a determination, but the data we do have does match that of a supernova, so it's a very good candidate," lead author Mike Engesser of the Space Telescope Science Institute said to Inverse.
This finding is rather surprising, even with Webb's extreme sensitivity. Because supernovae are transient events, meaning they don't last for long, you have to get lucky to observe one when it happens. Although technically, the supernova happened billions of years ago, we are only just seeing it now because it takes time for the light to travel to us from the distant galaxy.
Webb wasn't designed to detect supernovae, but researchers are making the most of the data collected so far and are finding surprising uses for it. The advantage of looking at this kind of target with Webb is that it will be able to observe the area around the supernova to see its effects and the aftermath of such a large explosion.
Understanding more about supernovae is important not only for understanding the life cycles of stars, but also for measuring the expansion of the universe. A class of supernovae called Type 1a are used as "mile markers" for measuring distances because they have consistent levels of brightness and can be seen from great distances.

Read more