Hot subdwarfs come into focus as AI scans thousands of stars in Gaia data

This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: There are myriad ways of "being a star," and scientists spend years trying to uncover and understand them. One unusual type is the so-called hot subdwarf—small stars that burn mysteriously hot for their size.

Scientists from the Faculty of Physics and the Faculty of Mathematics and Informatics at Vilnius University (VU) are working to explain this strange phenomenon and recently made important breakthroughs. The paper is published in the journal Astronomy & Astrophysics. The space observatory Gaia, launched in 2013, cataloged more than half a billion stars.

Of those, only 60,000 belong to a peculiar class called hot subdwarfs. That's less than one in a thousand. To understand what makes hot subdwarfs so unusual, one has to picture the standard map astronomers use to classify stars, a diagram where temperature runs along one axis and brightness along the other.

"Most stars, including our sun, sit along a broad band called the main sequence: stable, hydrogen-burning objects in the quiet middle of their lives. Hot subdwarfs don't belong there. They're hot and lie below this main branch.

That's where the name comes from," says Dr. Markus Ambrosch, a VU researcher at the Astrospectroscopy and Exoplanets Group. Ambrosch and his colleagues are trying to explain why subdwarfs are the way they are.

Carlos Viscasillas Vázquez, who initiated the research project, hot subdwarfs are among the most fascinating objects in stellar astrophysics because they seem to borrow properties from many different kinds of stars while not fully belonging to any of them. "They are hot and blue like massive OB-type stars, known for their extreme temperatures and brightness, but are much smaller, less luminous and extremely compact. They are exposed stellar cores, in most cases formed when a companion strips away most of the star's outer envelope, changing its evolutionary path and leading it toward the white dwarf stage through a nonstandard route," he explains.