Two massive stars touching in a neighboring galaxy are on their way to becoming black holes that will eventually collapse together, generating ripples in the fabric of space-time, according to a new study by researchers at UCL (University College London) and the University of Potsdam. .
The study is accepted for publication in the journal Astronomy and astrophysicslooked at a known binary star (two stars orbiting a mutual center of gravity), and analyzed starlight obtained from a combination of ground-based and space-based telescopes.
The researchers found that the stars, located in a neighboring dwarf galaxy called the Small Magellanic Cloud, are in partial contact and exchange material with each other, with one star currently feeding on the other. They orbit each other every three days and are the largest contact stars (known as contact binaries) observed to date.
Comparing the results of their observations with theoretical models of the evolution of binary stars, they found that in the best fit model, the star that is currently being fed will become a black hole and will be feeding on its companion star. The remaining star will become a black hole soon after.
These black holes will form in just 2 million years, but they will then orbit each other for billions of years before colliding with such force that it will generate gravitational waves—ripples in the fabric of space-time—that could theoretically be detected with instruments on Earth.
Ph.D. Student Matthew Rickard (UCL Physics & Astronomy), lead author of the study, said, “Thanks to the gravitational wave detectors Virgo and LIGO, dozens of black hole mergers have been detected in the past few years. But until now we have not observed stars that are expected to collapse into black holes.” of this size and merging on a time scale shorter or even broadly comparable to the age of the universe.”
“Our most appropriate model suggests that these stars will merge into black holes within 18 billion years. Finding stars on this evolutionary path near our own Milky Way provides us with an excellent opportunity to learn more about how these black binaries form.”
Co-author Daniel Pauli, Ph.D. A student at the University of Potsdam said, “This binary star is the most massive contact binary observed to date. The smaller, brighter and hotter star, 32 times the mass of the Sun, is currently losing mass to its larger companion, which has 55 times the mass of our Sun.”
The black holes that astronomers see merging today formed billions of years ago, when the universe had lower levels of iron and other heavy elements. The proportion of these heavy elements has increased as the universe has aged, making black hole mergers less likely. This is because stars with a higher proportion of heavier elements have stronger winds and blow themselves up sooner.
The well-studied Small Magellanic Cloud, located about 210,000 light-years from Earth, has, by nature’s oddity, one-seventh the abundance of iron and other heavy metals in our Milky Way galaxy. In this respect, it mimics conditions in the distant past of the universe. But unlike older, more distant galaxies, they are close enough for astronomers to measure the properties of single and binary stars.
In their study, the researchers measured different bands of light coming from the binary star (spectroscopy), using data obtained over multiple time periods by instruments on NASA’s Hubble Space Telescope and the Multi-Unit Spectroscopic Explorer (MUSE) on ESO’s Very Large Telescope. Chile, among other telescopes, with wavelengths ranging from ultraviolet to light to near infrared.
Using this data, the team was able to calculate the stars’ radial velocity — that is, the motion they made toward or away from us — as well as their mass, brightness, temperature, and orbits. They then matched these parameters to the most appropriate evolutionary model.
Their spectral analysis indicated that much of the smaller star’s outer atmosphere had been stripped away by its larger companion. They also note that the radius of both stars exceeds the Roche lobe — the region around the star where matter is gravitationally bound to that star — confirming that some of the younger star’s material spills over into the companion star.
Speaking about the future evolution of stars, Rickard explained, “The youngest star will become a black hole first, in less than 700,000 years, either through a spectacular explosion called a supernova or it could be so massive that it turns into a black hole without an external explosion.”
“They will be turbulent neighbors for about three million years before the first black hole starts massing off its companion, and retaliating against its companion.”
Pauli, who did the modeling work, added, “After only 200,000 years, an instant in astronomical terms, the companion star will also collapse into a black hole. These two massive stars will continue to orbit each other, spinning and spinning every few days to billions of years.”
“They will slowly lose this orbital energy through the emission of gravitational waves until they orbit each other every few seconds, finally merging together in 18 billion years with a massive release of energy through gravitational waves.”
more information:
MJ Rickard et al, A slow-passing low-metallic bulky contact binary. Mass transfer state: detailed spectral and orbital analysis of SSN 7 in NGC 346 in the SMC, Astronomy and astrophysics (2023). DOI: 10.1051/0004-6361/202346055