Astronomers observe a star that quietly transformed into a black hole

The formation of a black hole can be quite a violent event, with a massive dying star blowing up and some of its remnants collapsing to form an exceptionally dense object with gravity so strong not even light can escape.

As new observations indicate, the process sometimes can be a rather quiet affair.

Researchers have tracked a big and bright star that in its death throes virtually vanished from view as it apparently morphed into a black hole without exploding as a supernova. It is now detectable only because of a subtle glow caused by leftover gas and dust heating up while being sucked inward by the newborn black hole's irresistible gravitational pull.

The star, named M31-2014-DS1, resided in the Andromeda Galaxy, a Milky Way neighbor, about 2.5 million light-years from Earth. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

M31-2014-DS1 may offer the best evidence yet of black hole formation without a supernova, the researchers said. They tracked how the star was luminous in four decades of observations before 2014, then brightened in 2015 before almost disappearing from view, consistent with transforming into 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 as low as about 13 times that of the sun can form black holes," said astrophysicist Kishalay De of the Flatiron Institute and Columbia University in New York, lead author of the research published on Thursday in the journal Science.

Scientists have known for more than 50 years that black holes exist, but still have "very, very limited observational evidence for how stars turn into black holes," De said. "So this discovery provides an important insight into that process."

The star began its existence at least 13 times more massive than our sun. Over its relatively brief lifetime of 15 million years its powerful stellar winds expelled about 60% of its mass.

The explosion of a large star typically leaves behind an object called a neutron star that is highly compact, but not as much as a black hole. Such a supernova can produce a black hole depending upon the star's mass and other factors, though it is tough to confirm through observations that this has occurred.

"In the supernova pathway, a massive star exhausts its nuclear fuel and its core collapses, briefly forming a neutron star. This collapse generates a shockwave," De said.

"If the shock succeeds, it completely expels the outer layers of the star as a bright supernova. However, in some cases we think that the remaining core is not pushed out and eventually falls back into the neutron star, making it collapse into a black hole," De added.

In a process called thermonuclear fusion, stars fuse hydrogen into helium in their cores, generating outward pressure that balances the incessant inward pull of gravity. When the nuclear fuel dissipates, the balance between inward and outward forces is upset and gravity causes the core to collapse.

For M31-2014-DS1, the shockwave generated by the core collapse failed to muster enough energy to detonate the star.

"We call this a failed supernova," Flatiron Institute astrophysicist and study co-author Andrea Antoni said.

"Gravity therefore dominated, leading to the formation of a black hole," De said. "The star's outer envelope was gently ejected rather than explosively expelled. As this material expanded and cooled, it produced a transient infrared brightening. Afterward, the star lost its central power source and faded from view across wavelengths."

The expulsion of the star's outer layers is about a thousand times less energetic than a supernova, Antoni said.

"For a star to vanish and implode as 'quietly' as this one did, we think that the key is that it's not spinning too fast before collapse so the majority of its mass falls straight in and only the outermost layers are sloughed off in the process," Harvard University astronomer and study co-author Morgan MacLeod said.

The newborn black hole has a mass roughly five times greater than the sun.

The researchers are eager to learn how common it is for black holes to form in the quiet way. They already have identified another star that seems to have transformed into a black hole without an explosion.

"Presently, there are too many uncertainties on the theoretical side to know what percentage of core collapse deaths of massive stars lead to black hole formation," Antoni said.

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