Scientists at Heidelberg University in Germany have identified what researchers describe as the first bare star of intermediate mass that acts as a missing link in the evolution of massive binary systems. The findings appear in Astronomy and Astrophysics, a leading scientific journal. In astronomy, bare giant stars are those that have shed most of their outer layers, exposing a dense core. Most of these rare objects are found in binary systems, where the gravitational pull from a companion star draws away material, shaping the evolution of both stars and the surrounding system.
For many decades, astronomers debated whether intermediate mass bare stars truly exist. Prior observations had confirmed either very light naked stars, or the more extreme naked stars associated with Wolf-Rayet types, leaving a gap in the mass spectrum. The Heidelberg team used state of the art instrumentation to probe these elusive objects, aiming to clarify how common such bare cores are and what triggers the rapid changes that reveal them. The work underscores how binary interactions can reveal nuclear-burning hearts that would otherwise stay hidden inside aging stars.
Employing the formidable capabilities of the Very Large Telescope in Chile, researchers analyzed a stellar object once thought to be a single star. Detailed measurements showed that what appeared solitary is in fact a binary pair. The primary is a star of intermediate mass, and its partner is a rapidly spinning companion. The rotation is sustained by the transfer and redistribution of material sourced from the primary, a process driven by intense gravitational forces that cause the envelopes to peel away. This dynamic exchange leaves each star marked by distinctive rotational and chemical signatures that astronomers can decode.
The binary system sits in the nearby Small Magellanic Cloud, a dwarf galaxy that orbits the Milky Way. Its stars typically contain fewer heavy elements than those in our own galaxy, a consequence of a slower chemical enrichment history. The evolutionary models used by researchers predict that a star with a exposed core can blow apart in a supernova, leaving behind a compact remnant such as a neutron star. In the aftermath, the companion star may lose mass as well, potentially setting the stage for a future system that could become an X-ray binary and, over cosmic timescales, a merger of neutron stars that produces strong gravitational waves and energetic emissions.
Historically, astronomers have pursued signs of stripped atmospheres and cores, including cases where a dying star reshapes the atmosphere of a nearby planet-like body. The new findings add a crucial piece to that narrative, illustrating how binary interactions can unveil hidden stages of stellar life and alter the ultimate fate of both stars involved. With continued observations and refined models, scientists expect to map how common such intermediate bare stars are across different environments and how their precursors evolve into the energetic endpoints observed in the broader cosmos.