According to a recent study by scientists at UCL (University College London) and the University of Potsdam, two huge interacting stars in a nearby galaxy are headed toward becoming black holes that will eventually collide, creating ripples in the fabric of space-time.
The study, accepted for publication in the journal Astronomy & Astrophysics, looked at a known binary star (two stars orbiting around a mutual center of gravity), analyzing starlight obtained from a range of ground and space-based telescopes.
The stars are in a nearby dwarf galaxy known as the Small Magellanic Cloud, and the researchers discovered that they are partially in contact, exchanging material, with one star now “feeding” off of the other. They orbit each other every three days and are the most massive touching stars (known as contact binaries) yet observed.
They discovered that the best-fit model predicts that the star that is now being fed on would turn into a black hole and will eat its partner star after comparing the results of their observations with theoretical models of the evolution of binary systems. The surviving star will become a black hole shortly after.
These black holes will arise in just a few million years, but they won’t collide with enough force to cause ripples in space-time that may conceivably be seen by instruments on Earth until after orbiting each other for billions of years.
After only 200,000 years, an instant in astronomical terms, the companion star will collapse into a black hole as well. These two massive stars will continue to orbit each other, going round and round every few days for billions of years.
Daniel Pauli
Ph.D. student Matthew Rickard (UCL Physics & Astronomy), lead author of the study, said, “Thanks to gravitational wave detectors Virgo and LIGO, dozens of black hole mergers have been detected in the last few years. But so far we have yet to observe stars that are predicted to collapse into black holes of this size and merge in a time scale shorter than or even broadly comparable to the age of the universe.”
“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.”
Co-author Daniel Pauli, a Ph.D. student at the University of Potsdam, said, “This binary star is the most massive contact binary observed so far. The smaller, brighter, hotter star, 32 times the mass of the Sun, is currently losing mass to its bigger companion, which has 55 times our Sun’s mass.”
When the universe contained less iron and other heavier elements, the black holes that are merging now created billions of years ago. Black hole mergers are less frequent as the cosmos has aged due to an increase in the concentration of these heavy components. This is due to the fact that stars with a higher percentage of heavier elements have stronger winds and self-destruct sooner.
The well-studied Small Magellanic Cloud, about 210,000 light years from Earth, has by a quirk of nature about a seventh of the iron and other heavy metal abundances of our own Milky Way galaxy.
In this respect it mimics conditions in the universe’s distant past. It is, however, close enough for astronomers to measure the characteristics of individual and double stars, unlike older, more distant galaxies.
In their study, the researchers measured different bands of light coming from the binary star (spectroscopic analysis), using data obtained over multiple periods of time by instruments on NASA’s Hubble Space Telescope (HST) and the Multi Unit Spectroscopic Explorer (MUSE) on ESO’s Very Large Telescope in Chile, among other telescopes, in wavelengths ranging from ultraviolet to optical to near infrared.
The researchers used this information to determine the masses, brightness, temperature, and orbits of the stars as well as their radial velocity, or the movement they made towards or away from us. They then matched these parameters with the best-fit evolutionary model.
Their spectroscopic research revealed that the larger companion had torn away a significant portion of the smaller star’s outer envelope. In order to further prove that part of the material from the smaller star is spilling and moving to the companion star, they also saw that the radius of both stars was greater than their Roche lobes, which is the region around a star where material is gravitationally bound to that star.
Talking through the future evolution of the stars, Rickard explained, “The smaller star will become a black hole first, in as little as 700,000 years, either through a spectacular explosion called a supernova or it may be so massive as to collapse into a black hole with no outward explosion.”
“They will be uneasy neighbors for around three million years before the first black hole starts accreting mass from its companion, taking revenge on its companion.”
Pauli, who conducted the modeling work, added, “After only 200,000 years, an instant in astronomical terms, the companion star will collapse into a black hole as well. These two massive stars will continue to orbit each other, going round and round every few days for billions of years.”
“Slowly they will 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 huge release of energy through gravitational waves.”
