February 21, 2024

Researchers from University College London and the University of Potsdam in Germany studied two of the most massive contacting stars in a nearby galaxy, which would eventually become black holes and collide, sending ripples through space-time.

The black holes we observe today formed billions of years ago when there were lower levels of iron and other heavy elements floating around in the universe. As the universe ages, the abundance of these elements increases, making merging black holes less common.

These stars orbit a common center of gravity, known together as a binary star, in the Magellanic Cloud, just 210,000 light-years away in our Milky Way galaxy. These stars orbit one another every three days and are the largest contact stars ever observed (contact binaries). But it’s their mutually disruptive relationship that piques the researchers’ interest.

Using long-term data collected by NASA’s Hubble and the Multi-Unit Spectral Explorer (MUSE) on the European Southern Observatory’s (ESO) Very Large Telescope in Chile, and other telescopes, the researchers measured the distinct bands of light from the binary star Star (spectral analysis). They found that the smaller star had been stripped of most of its outer shell by the larger star.

“This binary star is the largest contacting binary star ever observed,” said study co-author Daniel Pauli. “The smaller, brighter, hotter star, 32 times the mass of our sun, is currently losing mass to its larger companion, which is 55 times the mass of our sun.”

According to the researchers, in the scheme of astronomical evolution, it won’t be long before small stars become black holes and the roles of stars are reversed.

“The smaller star will first become a black hole in as little as 700,000 years, either through a spectacular explosion called a supernova, or it may be so large that it collapses into a black hole without exploding outward,” said lead scientist Matthew Ricard explain. author of the study. “They will be uneasy neighbors for about three million years before the first black hole starts sucking mass from its companion, taking revenge on its companion.”

Their findings are supported by comparisons of gravitational wave observations by the Virgo interferometer and LIGO (Laser Interferometer Gravitational-Wave Observatory) with theoretical models of binary star evolution.

“Thanks to the gravitational wave detectors Virgo and LIGO, dozens of black hole mergers have been detected in the past few years,” Rickard said. “But so far, we have not observed stars that are expected to collapse into black holes of this size and merge on timescales shorter than or even roughly comparable to the age of the universe.”

Gravitational waves are invisible “ripples” in time and space caused by the most violent and dynamic processes in the universe. The strongest gravitational waves are produced by catastrophic events such as black hole collisions, which disrupt space-time, sending cosmic ripples that travel in every direction at the speed of light. These ripples carry information about their origin.

“After only 200,000 years, an astronomical instant, the companion star will also collapse into a black hole,” Pauli said. “These two massive stars will continue to orbit each other every few days for billions of years.”

Based on the findings of this study, we still have a while before black holes collide. But when they do, it produces gravitational waves, which may be detectable here on Earth.

“Slowly, they will lose this orbital energy through the emission of gravitational waves, until they orbit each other every few seconds, eventually merging together in 18 billion years and releasing a huge amount of energy through gravitational waves ,” Pauli said.

Having such stars so close to our own galaxy allows researchers to learn more about our universe.

Ricard added: “Finding stars on this evolutionary path so close to our own Milky Way galaxy offers us an excellent opportunity to learn more about how these black hole binaries form.”

The study has been accepted for publication by the journal Astronomy and Astrophysics.

source: University College London