Two massive touching stars in neighbouring galaxy on course to collide as black holes

A known binary star in a neighbouring dwarf galaxy is on course to become black holes that will eventually crash together, according to a new study by researchers at UCL and the University of Potsdam.

The stars, located in the Small Magellanic Cloud about 210,000 light years from Earth, orbit each other every three days and are the most massive touching stars (called contact binaries) yet observed. According to the researchers, one of the two stars is currently feeding off the other and the star that is currently being fed on will become a black hole and will feed on its companion star. The surviving star will become a black hole shortly after.

In only a couple of million years, these black holes will form and start revolving around each other for billions of years until they eventually collide with immense force, creating gravitational waves that can theoretically be observed using Earth-based instruments.

"Our best-fit model suggests these stars will merge as black holes in 18 billion years. Finding stars on this evolutionary pathway so close to our Milky Way galaxy presents us with an excellent opportunity learn even more about how these black hole binaries form," said PhD student Matthew Rickard (UCL Physics & Astronomy), lead author of the study.

The scientists conducted a spectroscopic analysis of the binary star by measuring different bands of light emitted by it, using data collected over several time periods by instruments installed on NASA/ESA Hubble Space Telescope (HST) and the Multi-Unit Spectroscopic Explorer (MUSE) located at ESO's Very Large Telescope (VLT) in Chile, among others, in wavelengths spanning from ultraviolet to optical to near-infrared ranges.

The analysis suggested that much of the outer envelope of the smaller star had been stripped away by its larger companion. The researchers also found that the radius of both stars surpassed their Roche lobe, which represents the area surrounding a star where objects are held in place by its gravitational force. This confirmed that some of the smaller star's material was overflowing and transferring to its companion star.