Researchers have detected a close stellar companion orbiting the red giant π1 Gruis, about 530 light-years away. The star, now in its asymptotic giant branch phase, has expanded to ~350–400 times the Sun’s radius and shines thousands of times brighter. ALMA observations combined with refined stellar models reveal the companion likely follows a nearly circular orbit—challenging expectations and suggesting companion interactions may change late-stage stellar evolution.
Close Companion Found Orbiting Red Giant π1 Gruis — A Potentially Disruptive Neighbor
Researchers have detected a close stellar companion orbiting the red giant π1 Gruis, about 530 light-years away. The star, now in its asymptotic giant branch phase, has expanded to ~350–400 times the Sun’s radius and shines thousands of times brighter. ALMA observations combined with refined stellar models reveal the companion likely follows a nearly circular orbit—challenging expectations and suggesting companion interactions may change late-stage stellar evolution.
05:51 AM, 11/19/2025Science
A team of astronomers has identified a close stellar companion orbiting the red giant π1 Gruis, a discovery that may change how we understand late-stage stellar evolution and the fate of planets around sun-like stars.
Located about 530 light-years from Earth, π1 Gruis (pronounced “pi-one-Gru”) has a mass similar to the Sun but has swelled to roughly 350–400 times the Sun’s radius during its asymptotic giant branch (AGB) phase. The star now shines thousands of times more brightly than the Sun, making nearby companions difficult to detect.
How the companion was found
To probe the environment around the luminous giant, the team used observations from the Atacama Large Millimeter/submillimeter Array (ALMA). By combining ALMA data with refined stellar models from Monash University — using the star’s luminosity and pulsation behavior to constrain its mass and evolutionary stage — the researchers searched for signatures of an orbiting companion.
What they discovered
The analysis revealed evidence for a stellar companion in a nearly circular orbit around π1 Gruis. This result is notable because companions to AGB stars are often assumed to follow more elliptical orbits after the complex interactions that occur when a star expands. The near-circular orbit implies orbital evolution proceeded faster or differently than many current models predict.
“A key part of understanding the orbit of the companion is knowing the mass of the AGB star,” said Yoshiya Mori of Monash University. “Throwing a close companion into the mix could possibly wreak further havoc on the already complicated processes surrounding these stars.”
Why this matters
As sun-like stars exhaust hydrogen in their cores, they expand into red giants and undergo shell burning, pulsations, and heavy mass loss. A nearby companion can gravitationally interact with the giant, altering mass loss, stripping material, or otherwise changing the star’s late-life evolution. Understanding these interactions is important for predicting the eventual formation of a white dwarf and the structure of the resulting planetary nebula — and for imagining the distant future of our own solar system when the Sun becomes a red giant in roughly five billion years.
The study, led by researchers including Yoshiya Mori (Monash University) and Mats Esseldeurs (KU Leuven), was published on Nov. 10 in the journal Nature Astronomy.