Why does Betelgeuse keep fading? The mystery of Orion's giant star

Betelguese, one of the brightest stars in the night sky, has been putting on quite a show lately — but not in the way you might expect. The famous red supergiant in the constellation Orion has been growing noticeably fainter in recent months. Of the 21 brightest stars in the night sky, Betelgeuse is perhaps the most enigmatic. Its changing brightness poses a puzzle: how do the star's pulsations, dust eruptions, and a possible companion contribute to its fading?A giant star with a puzzling nameEven the translation of Betelgeuse's name is contentious. For a long time, it was thought to be a corruption of three Arabic words, usually translated as "The Armpit of the Giant." However, other experts in star nomenclature now dispute that interpretation, proposing instead "The Hand of al-Jauzā," with al-Jauzā referring to Orion.Although classified as a red supergiant, Betelgeuse usually appears more orange than red— golden orange or deep topaz. It is one of the most famous stars in its class. Henry M. Neely (1877-1963), for many years a popular lecturer at New York's Hayden Planetarium, described its restless behavior in his book A Primer for Stargazers:"(Betelgeuse) expands and contracts spasmodically, like an old man with his strength almost entirely spent, panting in the asthmatic decrepitude of old age." The rhythms of a changing starBetelgeuse naturally expands and contracts like a beating heart. As an aging red supergiant, it has a regular pulsation cycle of roughly 430 days: when it expands, its outer layers cool and it dims; when it contracts, they heat up and it brightens. Among first-magnitude stars — the brightest category of stars in the night sky — it is the only marked variable. Its scale is equally remarkable: located 650 light-years away, Betelgeuse is among the largest and most luminous stars of its class, with a luminosity of about 14,000 suns at maximum and 7,600 suns at minimum. Only a few such objects have been identified in our galaxy.British astronomer Sir John Herschel (1792-1871) first noticed Betelgeuse's variability beginning in 1836. Astronomers describe apparent brightness using magnitude: lower numbers mean brighter stars. In April 2023, Betelgeuse reached magnitude 0.0 — as bright as Vega, the fifth brightest star. At minimum brightness, such as in 1927 and 1941, its magnitude may fall to +1.2, making it appear about three times fainter.Three threads run through this mystery: Betelgeuse's roughly 430-day pulsation, its much longer recurring dimming cycle, and the exceptional Great Dimming of 2019—2020. Keeping them, separate helps explain why one cause need not account for every change in brightness.Betelgeuse shines in the constellation Orion. (Image credit: Franco Tognarini via Getty Images)A longer cycle and a possible companionAlongside its roughly 430-day pulsation, Betelgeuse shows a mysterious 2,100-day (5.75-year) dimming cycle. Known as a "Long Secondary Period," this longer cycle puzzled astronomers for a century. Some suggested that one or more stars might orbit Betelgeuse; others found no evidence for such companions or actively refuted their existence.At long last, in January 2026, researchers using the European Southern Observatory's Very Large Telescope (ESO's VLT) located in on Cerro Paranal in the Atacama Desert of northern Chile and NASA's Hubble Space Telescope, published the clearest images and direct evidence of a faint companion star orbiting the red supergiant. Formally named Alpha Ori B, Jared Goldberg, an astrophysicist researching the phenomenon, nicknamed the companion star "Betelbuddy," though in deference to the new definition of Betelgeuse's name meaning "the hand of al-Jawzā," the name Siwarha was ultimately chosen for the probable companion star, which means her bracelet in Arabic. In this explanation, Siwarha orbits deep within Betelgeuse's outer chromosphere, a layer of its atmosphere, circling about every six years. As it moves through gas and dust ejected by the supergiant, its gravity creates a dense wake of gas, with ripples and shockwaves that disrupt stellar winds and mass loss. When the concentrated dust trail passes between Earth and Betelgeuse, it can make the main star appear decidedly dimmer. Compare Betelgeuse with AldebaranBetelgeuse recently experienced another sharp drop in brightness. Since this past August, it has faded from magnitude +0.5 to +0.95 and now appears just a trifle dimmer than the similarly hued orange star Aldebaran in Taurus the Bull. This decline may be another example of the dimming described above, although the brightness change alone does not establish its cause.Aldebaran can be used as a comparison to Betelgeuse, since they both happen to be in the general vicinity of each other. If you want to make such a determination yourself, keep in mind that Aldebaran currently rises in the east-northeast before 7 p.m. local standard time, followed about 90 minutes later by Betelgeuse, which will emerge from beyond the eastern horizon. By 11:30 p.m., both stars will be well placed for viewing in the east-southeast part of the sky.Betelgeuse marks Orion's right shoulder. To locate Aldebaran, look for the three bright, evenly spaced stars that form Orion's belt. Draw an imaginary line through the three belt stars and extend it upwards and to the right. Follow that line for about two fists' width at arm's length (roughly 22 degrees) until you hit a sideways, V-shaped cluster of stars known as the Hyades, which forms the face of Taurus. Aldebaran sits right at one end of that V-shape, glowing with a distinct orange-red color as the "angry eye" of the bull charging toward Orion. View them late tonight; which star appears brighter?The red supergiant Betelgeuse shines in the constellation Orion, left of center, while the reddish star Aldebaran glows in Taurus, near the Pleiades star cluster, in this view from Alberta, Canada. The image also captures Mars and the red "Blood Moon" during the total lunar eclipse of Nov. 8, 2022. (Image credit: Alan Dyer/Stocktrek Images via Getty Images)The Great Dimming of 2019—2020The changes visible now invite comparison with a far more dramatic episode: The Great Dimming Event of 2019—2020.Between October 2019 and February 2020, Betelgeuse experienced an unprecedented, dramatic plunge in brightness — losing over two-thirds of its luster, falling from magnitude +0.5 to +1.7; the rank of a second magnitude star. It then returned to a more normal brightness, reaching an unusually bright peak of 0.0 by the spring of 2023. While the sudden dimming caused internet and media speculation that Betelgeuse was about to die as a spectacular supernova, infrared structural and chemical models showed no significant change in the star's luminosity; Betelgeuse was still firmly in its helium-burning phase and is expected to live for another 100,000 to perhaps many hundreds of thousands of years before it explodes.So, what caused The Great Dimming? The mystery was ultimately solved by compiling data from the Hubble Space Telescope and ground observatories. Apparently, Betelgeuse "sneezed." A massive bubble of superhot plasma rose from a giant convection cell inside the star and blasted into space. As this ejected gas moved millions of miles away from the star, it cooled rapidly and condensed into solid silicon monoxide dust. This giant stardust cloud acted like a literal screen, blocking a substantial portion of the star's light from our viewpoint. This event was exacerbated because the massive eruption perfectly aligned with the lowest point of the star's 430-day pulsation cycle as well as the orbital interference of the companion star Siwarha. From the standpoint of producing the absolute maximum possible dimming of Betelgeuse, this cosmic trifecta was the perfect storm!Untangling the mysteryBetelgeuse has long been known as a variable star, and recent breakthroughs are helping astronomers untangle its erratic behavior. Internal pulsations and massive dust eruptions contribute to its changing brightness, while gravitational interactions with the newly reported, probable companion explain the longer dimming cycle. Together, these mechanisms paint a richer picture of why Orion's giant star fades and brightens.Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope, The Old Farmer's Almanac and other publications.