Hidden Companion May Be Shaping Betelgeuse’s Atmosphere
Radio observations show the red supergiant's outer atmosphere has mostly returned to normal, and new evidence points to a nearby star stirring its gas.
Key points
- Radio measurements indicate that the outer atmosphere of Betelgeuse has mostly settled back after the Great Dimming.
- Researchers report evidence that a close companion star, Siwarha, may be disturbing the giant star's atmosphere.
- The companion is too faint to see directly at radio wavelengths, so scientists looked for indirect signs in the star's shape.
- The findings were published in The Astronomical Journal, with more observations needed to confirm the link.
A team of astronomers reports new evidence that a small companion star may be physically disturbing Betelgeuse, the red supergiant in the constellation Orion. According to ScienceDaily, the measurements show that the star’s enormous outer atmosphere has mostly recovered from its unusual fading episode. The research was published in The Astronomical Journal.
Betelgeuse became the focus of global interest in late 2019 and early 2020. Its brightness fell to a level not seen in over a century, an event known as the Great Dimming. The cause of that drop puzzled scientists for years.
Atmosphere Returns Toward Normal
The study used data from the U.S. National Science Foundation’s Karl G. Jansky Very Large Array and the Atacama Large Millimeter/submillimeter Array, known as ALMA. Together, the two telescopes made observations between 2021 and 2026 at seven radio frequencies, from 22 GHz to 485 GHz. Each frequency reveals a different depth in the star’s atmosphere.
The team was led by Lynn D. Matthews of MIT Haystack Observatory. Scientists from the Center for Astrophysics | Harvard & Smithsonian, the University of Manchester and the European Southern Observatory also took part. By combining the data, they mapped the outer layers from just above the visible surface out to several times the star’s radius.
An earlier study led by Matthews had found unusually cool gas in the atmosphere before the Great Dimming. Researchers then suggested that a shock wave from deep convection may have pushed gas outward. That disturbance may have helped create the material that later formed dust and blocked part of the star’s light.
The new data suggest the atmosphere has since warmed. At a point roughly 2.7 times the star’s radius out, the gas registers a brightness temperature near 3,500 Kelvin, close to the star’s own surface temperature.
A Cold Layer Confirmed
The researchers also confirmed a cool region that scientists had long suspected. It sits between the visible surface, called the photosphere, and the higher chromosphere. Using ALMA at its top frequencies, the team looked closer to the surface than earlier radio work could reach.
They measured temperatures falling to about 2,200 Kelvin in this layer. That is more than 1,000 degrees below the photosphere. The region matches the molecular layer known as the MOLsphere, which had mostly been identified through infrared observations.
A Companion Star Passing Through the Atmosphere
Attention now turns to Siwarha, a smaller star thought to orbit very close to Betelgeuse. Astronomers first proposed it in 2024 to explain a cycle in which the star’s brightness changes roughly every 2,100 days. Later direct imaging and spectroscopic work has added support for its existence.
The names carry a link to the pair. Betelgeuse comes from an Arabic phrase meaning the hand of al-Jawza. Siwarha is said to mean her bracelet. Its orbit is estimated at only about twice the giant star’s radius, so it passes through the extended atmosphere.
Matthews said the companion orbits so close that it moves through the star’s outer gas. Based on their calculations, the team judged Siwarha to be between tens and hundreds of times too dim for the radio data to pick up directly. They therefore searched for indirect signs, looking at the star’s shape instead.
They found that Betelgeuse does not always look round at radio wavelengths. Its atmosphere sometimes appears stretched into an oval. The team measured this departure from circularity over time and found it tended to be more elongated when Siwarha was expected to be farthest from the star in the sky. When the companion was expected to pass in front of or behind the star, the atmosphere looked rounder.
Further Work Needed
The authors note that more observations are needed before the link can be confirmed. Data covering more of the companion’s orbit, especially at radio frequencies that have seen little study, could show how strongly Siwarha affects the star. The planned Next Generation Very Large Array may later allow more detailed studies of nearby stars.
Such work could also show how aging red supergiants lose material near the end of their lives. Understanding these interactions may help astronomers explain how giant stars like Betelgeuse evolve.
Frequently asked questions
What is the Great Dimming of Betelgeuse?
The Great Dimming was a sharp drop in Betelgeuseu0026#039;s brightness in late 2019 and early 2020. According to ScienceDaily, it pushed the star to its faintest point in over a century.
What is Siwarha?
Siwarha is a smaller star believed to orbit very close to Betelgeuse. Astronomers first proposed it in 2024 to explain a brightness cycle of about 2,100 days.
Has Betelgeuseu0026#039;s atmosphere recovered?
New radio observations show that the staru0026#039;s vast outer atmosphere has largely returned to normal. The gas has warmed to temperatures more typical of the star.
Sources
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