A massive star may look like it has spent its entire life alone, but its chemical makeup can tell a very different story.
Scientists have developed a new way to uncover whether a star once had a close companion, potentially revealing hidden events that happened millions of years ago.
More than 70% of massive stars are born in close binary systems, meaning two stars orbit each other.
Their lives can become closely connected. One star may pull material from the other, the two may merge, or one may eventually explode as a supernova.
After these dramatic events, however, the surviving star can appear to be a normal single star. This has made it difficult for astronomers to reconstruct its past.
Now researchers have found that the evidence may be written into the chemicals on the star’s surface.
The study, published in Nature Astronomy, focuses on elements including carbon, nitrogen, oxygen and helium.
When two massive stars orbit close together, one can transfer some of its material to its companion. The receiving star, known as the mass gainer, may first collect material from the donor star’s outer layers. Later, it can receive material from deeper regions that have been changed by nuclear fusion.
This processed material contains more nitrogen and helium but less carbon and oxygen. Once transferred, it mixes into the receiving star’s outer layers and leaves behind a lasting chemical fingerprint.
Researchers discovered that these stars can be identified by comparing their nitrogen, carbon and oxygen levels. Most massive stars follow a particular pattern when scientists plot their nitrogen-to-carbon and nitrogen-to-oxygen ratios. Stars that have gained material from companions, however, follow a separate path.
According to the researchers, this chemical pattern can reveal much more than whether a star once had a companion. It may help scientists estimate how much material was transferred, what that material contained and even the likely masses of the two original stars.
The technique has already changed the story of one well-known star.
Gamma Columbae, a star in the southern constellation Columba with nearly six times the Sun’s mass, was previously thought to be a “stripped” star. In this scenario, the star had lost its own outer layers, exposing material from deeper inside.
But its chemical fingerprint suggests almost the opposite happened. Researchers now believe Gamma Columbae was the star receiving material from a massive companion. Its high nitrogen-to-carbon ratio, helium enrichment and other chemical characteristics match predictions for a former mass gainer.
The finding raises the possibility that other stars classified as stripped stars may also have hidden histories of gaining material from companions.
The technique could eventually be used to investigate many types of massive stars, including runaway stars, supergiants and stars that later explode as supernovae. Researchers have even applied the approach to Supernova 1987A, supporting the idea that its original star formed through a stellar merger and helping reconstruct the two stars involved.
Future surveys such as WEAVE and 4MOST are expected to collect detailed chemical information from thousands of massive stars.
Together, these observations could turn stellar surfaces into cosmic time capsules, allowing astronomers to uncover violent interactions and long-lost companions that can no longer be seen.
