The formation of a black hole can be a violent event: a massive star explodes at the end of its life, and some of its remains collapse into an extremely dense object whose gravity is so strong that not even light can escape. But as new observations show, the process can sometimes be much quieter.
Researchers have been tracking a large, bright star that, in its final moments, virtually disappeared from view, apparently turning into a black hole without a supernova explosion. It is detectable today only thanks to the faint glow that is produced when the remaining gas and dust heat up as the newly formed black hole, with its overwhelming gravity, draws them in, Reuters reports.
The star, called M31-2014-DS1, was located in the Andromeda galaxy, a neighbor of the Milky Way, about 2,5 million light-years from Earth. A light-year is the distance light travels in a year – 9,5 trillion kilometers.
Researchers say M31-2014-DS1 could be the strongest evidence yet that black holes can form without a supernova. They tracked how bright the star was over four decades of observations until 2014, then brightened further in 2015, before almost completely disappearing from view - consistent with the scenario of it becoming a black hole.
"This provides observational evidence of black hole formation in real time, suggests that many black holes may form without supernova explosions, and shows that stars with masses up to about 13 times the mass of the Sun can form black holes," said astrophysicist Kishalai De of the Flatiron Institute and Columbia University in New York, lead author of the study published in the journal Science.
Scientists have known for more than 50 years that black holes exist, but they still have "very, very limited observational evidence of how stars turn into black holes," De said. "So this discovery provides important insight into that process."
The star initially had at least 13 times the mass of our Sun. During its relatively short life of about 15 million years, powerful stellar winds ejected about 60 percent of its mass, Reuters reports.
The explosion of a large star usually leaves behind an object known as a neutron star - extremely compact, but still not as massive as a black hole. Such a supernova can produce a black hole, depending on the mass of the star and other factors, although it is difficult to confirm through observations that this actually happened.
"In a supernova scenario, a massive star exhausts its nuclear fuel and its core collapses, briefly forming a neutron star. This collapse creates a shock wave," De explained.
"If the wave succeeds, it completely ejects the outer layers of the star as a brilliant supernova. However, in some cases we think that the remaining core is not ejected, but rather eventually falls back onto the neutron star, causing it to collapse into a black hole," he added.
In the process of thermonuclear fusion, stars fuse hydrogen into helium in their cores, creating an outward pressure that balances the constant gravitational force pulling matter inward. When the nuclear fuel is used up, the balance is upset and gravity causes the core to collapse.
In M31-2014-DS1, the shock wave created by the collapse of the core did not have enough energy to detonate the star.
"We call that a failed supernova," said Flatiron Institute astrophysicist and study co-author Andrea Antoni.
"Gravity therefore took over, leading to the formation of a black hole," De said. "The outer envelope of the star was gently ejected, rather than being explosively ejected. As this material expanded and cooled, it produced a transient boost in infrared radiation. Then the star was left without a central energy source and faded from view across different wavelengths."
The ejection of the outer layers was about a thousand times less energetic than a supernova, Antoni said.
"For the star to disappear and implode so 'quietly', we think the key is that it wasn't rotating too fast before the collapse, so most of the mass fell straight inward, and only the outermost layers were shed in the process," said Harvard University astronomer and study co-author Morgan McLeod.
The newly formed black hole has a mass approximately five times that of the Sun.
Researchers now want to determine how common it is for black holes to form in this silent way. They have already identified another star that appears to have turned into a black hole without an explosion.
"Currently, there is too much uncertainty on the theoretical side to know what percentage of the collapse of the cores of massive stars ends up forming a black hole," Antoni said.
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