Mutual destruction: Binary stars, black holes, and ripples in space-time
Researchers from University College London and the University of Potsdam, Germany have studied two most massive touching stars in a neighboring galaxy that will eventually turn into black holes and collide, sending ripples through space and time.
The black holes we observe today were formed billions of years ago when there were lower levels of iron and other heavy elements floating around the universe. As the universe aged, the levels of these elements have increased, making the phenomenon of merging black holes less common.
The stars, which orbit around a shared center of gravity and together are known as a binary star, are in the Small Magellanic Cloud, a mere 210,000 light-years away from our Milky Way galaxy. The stars orbit one another every three days and are the biggest touching stars (contact binaries) ever observed. But it’s their mutually destructive relationship that piqued the interest of the researchers.
Using long-term data collected by NASA’s Hubble Telescope and the Multi Unit Spectroscopic Explorer (MUSE) on the European Southern Observatory’s (ESO’s) Very Large Telescope in Chile, as well as other telescopes, the researchers measured different bands of light coming from the binary star (spectroscopic analysis). They found that most of the outer envelope of the smaller star had been stripped away by the larger one.
“This binary star is the most massive contact binary observed so far,” said Daniel Pauli, co-author of the study. “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.”
In the scheme of astronomical evolution, it won’t take long before the small star becomes a black hole and the star’s role will be reversed, according to the researchers.
“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,” said Matthew Rickard, lead author of the study. “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.”
Their findings were backed up by comparing gravitational wave observations from the Virgo interferometer and LIGO, the Laser Interferometer Gravitational-Wave Observatory, with theoretical models of the evolution of binary stars.
“Thanks to gravitational wave detectors Virgo and LIGO, dozens of black hole mergers have been detected in the last few years,” Rickard said. “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.”
Gravitational waves are invisible ‘ripples’ in time and space caused by the universe’s most violent and energetic processes. The strongest gravitational waves are produced by cataclysmic events like colliding black holes, which would disrupt space-time in such a way as to send out cosmic ripples, traveling in all directions at the speed of light. These ripples carry information about their origins.
“After only 200,000 years, an instant in astronomical terms, the companion star will collapse into a black hole as well,” Pauli said. “These two massive stars will continue to orbit each other, going round and round every few days for billions of years.”
According to the study’s findings, we have some time before the black holes collide. But when they do, it will generate gravitational waves that may be detectable on Earth.
“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,” Pauli said.
Having stars like these so close to our own galaxy has enabled researchers to further our knowledge of the universe.
“Finding stars on this evolutionary pathway so close to our Milky Way galaxy presents us with an excellent opportunity to learn even more about how these black hole binaries form," Rickard added.
The study has been accepted for publication in the journal Astronomy and Astrophysics.
Source: University College London
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