After more than a century of speculation, astronomers have finally captured the first direct images of a companion star orbiting the red supergiant Betelgeuse, solving a long-standing mystery behind the star’s erratic behavior. The discovery, made with a specialized instrument on the Gemini North telescope in Hawai’i, provides a definitive answer to why the massive star’s brightness fluctuates on a predictable six-year cycle. This breakthrough not only deepens our understanding of one of the most famous stars in the night sky but also offers new insights into the evolution and activity of massive stars as they approach the end of their lives.
The newly confirmed star, nicknamed Siwarha, is a Sun-like star in a surprisingly tight orbit, circling Betelgeuse from within its vast, extended atmosphere. Its presence helps explain the periodic dimming and velocity shifts that astronomers have tracked for decades, suggesting the companion’s gravitational influence stirs up dust and gas in the supergiant’s outer layers. Led by a team at NASA’s Ames Research Center, the finding challenges existing models of binary star systems due to the extreme difference in mass between the two objects and opens a new chapter in the study of Betelgeuse, a star destined for a spectacular supernova explosion.
A Century-Old Mystery Solved
Betelgeuse has long been an object of fascination and study. Located in the shoulder of the constellation Orion, the red supergiant is one of the brightest and most recognizable stars visible from Earth. Despite being only about 10 million years old, it is already in the final stages of its stellar life. Its immense size—with a radius roughly 764 times that of our Sun—and its impending doom have made it a prime target for astronomical observation. For over a century, scientists have noted its perplexing variability. Betelgeuse brightens and dims on multiple overlapping cycles, including a primary period of about 400 days and a longer, more mysterious secondary period of about six years.
Fluctuations in the star’s measured velocity and brightness led to the long-standing hypothesis that a hidden companion might be gravitationally tugging on the supergiant. However, detecting such an object has been nearly impossible. Betelgeuse’s overwhelming glare acts like a brilliant celestial fog, masking any faint objects nearby. Previous attempts to find the companion failed to produce conclusive evidence, leaving the theory unproven. Recent studies, leveraging over 100 years of observational data, provided refined predictions about the companion’s probable location and brightness, reigniting the search and setting the stage for this definitive discovery.
Advanced Imaging Techniques Reveal the Companion
The ‘Alopeke Instrument
The breakthrough was achieved by a team of astrophysicists led by Steve Howell, a senior research scientist at NASA Ames Research Center. They used a high-resolution imaging instrument called ‘Alopeke, which is mounted on the 8.1-meter Gemini North telescope atop Maunakea in Hawai’i. ‘Alopeke, whose name means “fox” in Hawaiian, employs a technique known as speckle imaging. This method involves taking thousands of very short exposures—mere milliseconds long—to freeze the blurring effect of Earth’s atmosphere. Sophisticated computer algorithms then process these thousands of sharp, “speckled” images, combining them to reconstruct a single, diffraction-limited image that reveals fine details that would otherwise be lost.
A Precisely Timed Observation
Timing was critical for the observation. Based on orbital models derived from Betelgeuse’s six-year variability cycle, the team calculated that the companion would be at an ideal position for viewing in December 2024. It was during this window that multiple telescopes were aimed at the star, allowing the team to successfully isolate the faint light of the companion from the overwhelming glare of the supergiant for the first time. The resulting images revealed the companion, officially designated α Ori B but given the more evocative nickname Siwarha. The name is a nod to Betelgeuse’s own name, which derives from an Arabic phrase for “the hand of al-Jawza’,” a figure in Arabian legend; Siwarha means “her bracelet.”
An Unexpected Stellar Partner
The nature of the companion star has surprised many in the astronomical community. Siwarha appears to be a young, Sun-like star, a stark contrast to its colossal and aging partner. It has an estimated mass of about 1.5 times that of the Sun and is significantly fainter than Betelgeuse in visible light. This pairing is unusual because current models of binary star formation suggest that stars born together should have more similar masses. The vast disparity between the supergiant Betelgeuse, at 16.5 to 19 solar masses, and its small companion creates what astrophysicist Anna O’Grady of Carnegie Mellon University calls an “extreme mass ratio binary.” This discovery opens up a new area of research, as such systems are incredibly difficult to find and have not been extensively studied.
The companion orbits Betelgeuse at a remarkably close distance of about four astronomical units (AU)—roughly four times the distance between Earth and the Sun. This places it well within the tenuous outer atmosphere of the bloated supergiant, making it the first time a stellar companion has been detected orbiting so close to a star of this type. This proximity proves the incredible resolving power of the ‘Alopeke instrument and provides a unique laboratory for studying the interactions between a supergiant and a much smaller stellar partner.
Implications for Stellar Evolution
The confirmation of Siwarha has profound implications for understanding Betelgeuse and other red supergiants. The companion’s six-year orbit aligns perfectly with Betelgeuse’s secondary brightness cycle, strongly suggesting the smaller star’s gravitational pull is the cause. As Siwarha moves through Betelgeuse’s extended atmosphere, its gravity likely perturbs the gas and dust, influencing the formation of dust clouds that periodically block some of the supergiant’s light, causing it to dim. This mechanism could also explain the long-term variability observed in other similar red supergiant stars.
Furthermore, the discovery provides a clearer picture of the Betelgeuse system’s past and future. The two stars were likely born together, but their fates will be dramatically different. The companion itself is expected to have a shortened lifespan. Pulled by the immense gravity of Betelgeuse, strong tidal forces will eventually cause Siwarha to spiral inward, where it will be consumed by the supergiant in an estimated 10,000 years. Studying this interaction will provide invaluable data on the final phases of massive star evolution, just before the cataclysmic supernova that will one day mark the end of Betelgeuse itself.
Future Observations Planned
The research, published in *The Astrophysical Journal Letters*, marks the beginning of a new phase of study for the Betelgeuse system. While the companion has been detected, many of its properties are still being constrained. Scientists plan to continue observing the system to refine their understanding of Siwarha’s mass, trajectory, and influence on its massive partner. The next prime opportunity for observation will come in November 2027, when the companion is expected to once again reach its greatest apparent separation from Betelgeuse, making it easier to detect against the supergiant’s glare. These future observations will be crucial for solidifying the models of this unique binary system and unlocking more secrets of our universe.
