Betelgeuse, the red star marking Orion's shoulder, appears to have a companion that orbits so close it travels through the giant's own atmosphere. New radio observations show Betelgeuse's outer layers stretching into an oval when the smaller star, named Siwarha, is at its widest point from the giant, and relaxing toward a circle when it passes in front of or behind it.

The study, published Nov. 3 in The Astronomical Journal, combines data from the NSF Karl G. Jansky Very Large Array in New Mexico and the ALMA observatory in Chile. "This companion is orbiting so close to the star that it actually passes through its extended atmosphere," said lead author Lynn Matthews of MIT Haystack Observatory, according to the ScienceDaily release.

A Star Inside Another Star's Atmosphere

Betelgeuse is a red supergiant roughly 700 times wider than the Sun. It sits about 650 to 700 light-years from Earth and is one of the brightest stars in the night sky.

Siwarha, whose name means "her bracelet" in Arabic, is thought to orbit at only about twice Betelgeuse's radius, according to the National Radio Astronomy Observatory. That places it deep within the supergiant's extended, tenuous outer atmosphere.

Astronomers first proposed the companion in 2024 to explain a roughly 2,100-day brightness cycle, separate from Betelgeuse's shorter, familiar pulsations. Direct imaging and spectroscopy have since supported its existence. Earlier this year, researchers using Hubble reported a dense wake trailing the companion, and imaging with ESO's Very Large Telescope captured the faint star near its predicted position, Science News reported.

That long cycle is the key clue. Betelgeuse has long been known to brighten and fade over roughly 425 days, but a second, slower pattern of about 5.9 years lines up with the companion's likely orbital period, Universe Today reported. Assuming the two stars formed together, Siwarha may be roughly 2.6 to 3.1 times the mass of the Sun.

Radio Waves That Reveal a Hidden Orbit

Siwarha is likely tens to hundreds of times too faint to see directly at radio wavelengths — so the team, which includes Andrea Dupree of the Center for Astrophysics, Harvard and Smithsonian, looked for its fingerprints instead.

From 2021 to 2026, they observed Betelgeuse at seven frequencies, from 22 to 485 gigahertz. Different frequencies probe different depths of the star's atmosphere, letting the team build a layered picture.

The pattern was consistent. When Siwarha was expected to be at its greatest apparent distance from Betelgeuse, the radio atmosphere looked more elongated. When the companion lined up in front of or behind the giant, the atmosphere looked rounder. That is what you would expect if a star moving through the gas is stirring and stretching it.

"It was the combination of the incredible data from the NSF VLA and ALMA that gave us a convincing result here," Matthews said. The highest ALMA frequencies also revealed a cool layer near 2,200 Kelvin between the surface and the star's hotter chromosphere, consistent with a molecule-rich zone known as the MOLsphere.

Strong Evidence, Not Final Proof

The authors describe the link between the atmosphere's shape and the companion's orbit as compelling, not proven. The companion was not detected directly in these data, and its presence is inferred from its effect on the giant.

There are also loose ends. The oval shape tracks Siwarha across frequencies, but the orientation of the best-fitting oval does not agree perfectly between them. The team says more observations are needed across a larger part of the orbit, especially at radio frequencies that have been sampled only sparsely.

The Great Dimming and the Coming Supernova

Betelgeuse became headline news in late 2019, when it dimmed by about 60% in an event called the Great Dimming. NASA says observations showed the star had blown off a large chunk of its surface, which then formed a dust cloud that blocked part of its light. Matthews' earlier work suggested a shock wave driven by convection may have helped launch that material.

The new paper does not blame Siwarha for the Great Dimming. But a companion churning through the atmosphere could influence how the star sheds gas over time. The data show the atmosphere has since warmed to about 3,500 Kelvin at roughly 2.7 stellar radii, suggesting it has recovered from the 2019 episode.

Confirming the companion also matters for Betelgeuse's future. The star is expected to explode as a supernova, but NASA says that is not likely for about another 100,000 years. A close companion can change how a dying star loses mass, which affects what kind of explosion it eventually produces. Some researchers think Siwarha may spiral into Betelgeuse and be swallowed in roughly 10,000 years.

There is no danger to Earth — even when Betelgeuse explodes, it is far enough away that the supernova would be a spectacular sight, briefly visible in daylight, rather than a threat.

Watching for the Next Pass

The next major opportunity comes in late 2027, when Siwarha is expected to reach the other side of Betelgeuse. Astronomers already plan new imaging and radio observations to test whether the atmosphere stretches again on schedule.

For skywatchers, Betelgeuse is easy to find on winter evenings in the Northern Hemisphere, glowing orange-red at the upper left of Orion. The newest research suggests that when you look at it, you are seeing not one star but two locked in a very close embrace.

What Readers Want to Know

What did astronomers find about Betelgeuse? Radio observations suggest its companion star, Siwarha, orbits inside Betelgeuse's extended atmosphere and changes the giant's shape.

How close is the companion? About twice Betelgeuse's radius, deep inside the supergiant's outer atmosphere.

Has Siwarha been confirmed? Earlier imaging and spectroscopy support it. The new radio study is strong evidence but not final proof.

Did the companion cause the Great Dimming? The new study does not say so. The 2019 dimming has been linked to a surface mass ejection that formed dust.

When will Betelgeuse explode? NASA says a supernova is not likely for about another 100,000 years.

Is Betelgeuse dangerous to Earth? No. It is too far away to harm Earth, even when it explodes.