New research on mysterious supermassive stars that 'eat' companion stars

A new study has confirmed the existence of a strange type of supermassive star that grows by "eating" its bloated and dying companion star.

While astronomers have long suspected that barium stars – named after their unusually high levels of the heavy element barium – are born from feeding on material from their companion stars, they have now finally caught a glimpse of these "blood-sucking" stars in action.

Astronomers William P. Bidelman and Philip Keena first discovered barium stars in 1951 after noticing high levels of barium in their atmospheres. All stars are made almost entirely of hydrogen and helium, but they contain small amounts of heavier elements such as barium.

The Hubble Space Telescope has observed the red giant star CW Leonis in the constellation Leo. The nearby star, 58 Leonis, is thought to be a rare barium star. (Image: ESA/Hubble, NASA, Toshiya Ueta (University of Denver), Hyosun Kim (KASI)).

Barium stars are on another level. In addition to barium, they contain large amounts of heavy elements formed in a special way, called the s process .

Astrophysicists have known that the s-process occurs inside massive stars near the end of their lives when neutrons collide with lighter elements like helium and hydrogen, causing them to fuse into heavier elements like carbon, strontium, and barium.

However, barium stars themselves are not always at the end of their lives, so they cannot form these elements on their own.

In a paper published on arXiv on September 4, astronomers confirmed that the strange massive stars are "cosmic leeches".

The team found two new barium stars. Importantly, in addition to measuring the elements in the s-process inside the stars, they ruled out many nuclear processes that could explain how these stars formed.

They also confirmed for the first time that each of these stars was part of a binary system . In one case, they found clear evidence that the companion star was a white dwarf — the leftover remnant of a star like our Sun.

Since barium stars cannot form heavy elements on their own, companion stars are the most likely explanation . In this scenario, to obtain barium, the companion star must go through its entire life cycle. Near the end of its life cycle, it begins the s-process and produces large amounts of barium and other elements, which then migrate into the upper atmosphere. As the companion star expands into a red giant, it loses all of its atmosphere. Some of that atmosphere reaches the surviving star, enriching it and turning it into a barium star.

If previously astronomers could only question the above scenario due to lack of direct evidence, now barium stars in binary star systems are the first evidence for this type of star.