Recent astronomical research, based on nearly eight years of observations, has confirmed the presence of a companion star, named Siwarha, orbiting the red supergiant Betelgeuse. This discovery provides an explanation for a periodic dimming and brightening cycle of approximately 2,109 days (5.77 Earth years) observed in Betelgeuse's light. The findings contribute to a greater understanding of how giant stars evolve and shed material before potentially undergoing supernova events.
Discovery and Confirmation
The confirmation of Siwarha stems from investigations conducted by two independent teams in 2024 and 2025. This research utilized data from NASA's Hubble Space Telescope and ground-based telescopes at the Fred Lawrence Whipple Observatory in Arizona and the Roque de Los Muchachos Observatory in the Canary Islands, Spain. Astronomer Andrea Dupree of the Center for Astrophysics | Harvard & Smithsonian, lead author of a study on the topic, stated that the companion star creates a ripple effect observable in the data, providing direct evidence of its influence.
Siwarha, whose name means 'Her Bracelet' in reference to the origin of Betelgeuse's own name, is believed to orbit within Betelgeuse's expanded atmosphere. Its presence is cited as the likely cause of the 'long secondary periods' in brightness variations commonly observed in red giants. The binary system theory has also been proposed to explain Betelgeuse's 'great dimming' event between 2019 and 2020. This 2,100-day cycle is distinct from a previously identified 400-day cycle attributed to internal pulsations within Betelgeuse.
Mechanism of Influence
Researchers report detecting a high-density trail of gas, referred to as a 'wake,' within Betelgeuse's atmosphere. This wake is attributed to Siwarha's movement. The companion's gravitational pull focuses the surrounding stellar wind into this trailing wake of denser, shocked gas, which expands laterally at approximately 6 kilometers per second.
While Siwarha does not directly dim Betelgeuse's visible light, its wake alters the red supergiant's spectrum, particularly affecting ultraviolet (UV) wavelengths emitted by ionized iron. A strong peak in the light emitted by iron is observed when Siwarha is positioned in front of Betelgeuse. As Siwarha moves past Betelgeuse, its trailing gas tail absorbs these specific wavelengths, resulting in a shorter peak in the light signature. This effect recurs with each orbit, approximately every 2,109 days.
Betelgeuse Characteristics and Future Observations
Betelgeuse is a red supergiant star. Its eventual collapse into a supernova is projected to result in a brightness comparable to the Moon for approximately two weeks, potentially remaining visible during daylight hours for 6 to 12 months thereafter. The precise timing of this supernova event remains uncertain. Observations of Betelgeuse have historically shown periods of dimming and brightening, leading to speculation regarding its supernova timeline. The star's expulsion of dust and gas can also obscure observations and influence perceived brightness changes.
Siwarha is currently obscured by Betelgeuse's disk. Astronomers plan to conduct further observations when the companion star is expected to become visible again in August 2027.
Publication Status
The study detailing these findings has been posted on the preprint server arXiv and accepted for publication in The Astrophysical Journal.