Introduction
Betelgeuse, the well-known red supergiant star in the constellation Orion, has puzzled astronomers recently with its unusual brightening and dimming behavior. Betelgeuse is typically the 10th brightest star in the night sky, but in late 2019 and early 2020, it dimmed dramatically, dropping out of the top 20 brightest stars. Now, Betelgeuse appears to be brightening again, returning to its normal brightness. What could be causing these strange brightness changes in this massive star?
Betelgeuse’s Normal Brightness
Betelgeuse is classified as a red supergiant, an evolved massive star that has used up its core hydrogen fuel and expanded into a huge, bright, and cool star. With a radius over 1000 times that of our Sun, if Betelgeuse were placed in our solar system, its surface would extend beyond the orbit of Jupiter. Betelgeuse emits a reddish light, as its peak radiation is in the red and infrared.
Normally, Betelgeuse ranks as the 9th to 10th brightest star in Earth’s night sky, with an apparent magnitude of around 0.5. This puts it just below stars like Rigel and Deneb in the brightness rankings. Since Betelgeuse is a variable star whose brightness changes slightly from time to time, its exact magnitude ranking can shift. But most of the time, it remains visible as a distinctly red dot to the naked eye in Orion.
Betelgeuse’s Unusual Dimming Event
Starting in late 2019, astronomers noticed an unprecedented dimming in Betelgeuse’s brightness. By February 2020, the star had faded to about 36% of its normal luminosity, dropping in the brightness rankings to 21st place. This was by far the lowest luminosity ever observed for this star, and the dimming meant it was no longer distinctly visible to the naked eye.
Several hypotheses emerged to explain this unusual “Great Dimming” of Betelgeuse:
| Explanation | Description |
| Dust clouds | Clouds of dust passed between Earth and the star, blocking some visible light. |
| Star spots | Cool surface regions formed on the star, making it appear darker. |
| Pulsations | Betelgeuse expanded and contracted due to stellar pulsations. |
| Pre-supernova | Betelgeuse was collapsing prior to a supernova explosion. |
Of these, dust clouds and pulsations were considered the most likely scenarios. Astronomers continued observing Betelgeuse closely over the following year, waiting to see if it would brighten again.
Betelgeuse’s Return to Normal Brightness
In 2021, Betelgeuse did indeed begin brightening again. As of September 2022, visual observations and photometric measurements indicate the star has returned to close to its normal luminosity. It now appears as a distinctly red star visible to the naked eye again in Orion.
The most plausible explanation for Betelgeuse’s dimming and brightening behavior seems to be stellar pulsations – periodic expansions and contractions in the star’s outer layers. As a red supergiant, Betelgeuse is nearing the end of its evolution and experiences complex, chaotic pulsations in size, brightness, and temperature. Its radius periodically grows and shrinks by about 20%, altering its brightness.
The Great Dimming event was likely caused by an unusually strong pulsation that expanded the star’s radius far more than normal. This cooled the surface temperature and blocked visible light emissions. As the pulsation reversed, Betelgeuse then contracted again to its typical size, warming up and brightening. While dramatic, this dimming was not necessarily a sign Betelgeuse was about to explode in a supernova.
Betelgeuse’s Future Evolution
What does the future hold for Betelgeuse? This red supergiant is destined to end its life in a Type II supernova explosion, sometime in the (astronomically) near future. When this happens, Betelgeuse will briefly outshine the entire night sky and be visible in daylight for weeks or months. Fortunately, at a probable 700 light-years distant, the star is too far from Earth to pose risks to our planet or life.
Prior to exploding, Betelgeuse may exhibit more pulsation-driven dimming events. Each of its 420-430 day pulsation cycles likely causes some brightness variations. It’s rare for the surface temperature and visibility to decrease as much as was observed in the Great Dimming – but not impossible. Astronomers are keeping a close watch for any signs Betelgeuse may be preparing to go supernova.
Whenever it happens, Betelgeuse’s end as a supernova will be a once-in-a-lifetime astronomical spectacle. It will also leave behind a spectacular supernova remnant nebula visible for thousands of years as its ejected material expands outward through space. Until then, this semi-regular variable star will continue to varying between about magnitudes +0.2 to +0.8, holding its place as one of the 15 brightest stars visible in our sky.
Conclusion
Betelgeuse’s recent dimming and re-brightening has fascinatingly illustrated the complex late-stage evolution of a massive star. While the exact mechanisms driving its pulsations and brightness changes remain uncertain, ongoing observations are revealing insights into stellar physics – as well as reminding us of this giant star’s eventual fate. For now, Betelgeuse shines again as a prominent red supergiant – but its days as one of the brightest stars in Earth’s sky are numbered.
