Black hole wandering Milky Way galaxy spied by Hubble

When stars massive enough to dwarf our sun die, they explode in a supernova and the remaining core is crushed by its own gravity, forming a black hole.

Sometimes the explosion may send the black hole into motion, hurtling across the galaxy like a pinball. By rights, there should be a lot of roving black holes known to scientists, but they are practically invisible in space and therefore very difficult to uncover.

Astronomers believe that 100 million free-floating black holes roam our galaxy. Now, researchers believe they have detected such an object. The detection was made after dedicating six years to observations — and astronomers were even able to make a precise mass measurement of the extreme cosmic object.

The black hole is 5,000 light years away, located in a spiral arm of the Milky Way galaxy called Carina-Sagittarius. This observation allowed the research team to estimate that the nearest isolated black hole in relation to Earth could be only 80 light-years away.

But if black holes are essentially indistinguishable from the void of space, how did Hubble spot this one?

The extremely strong gravitational field of black holes warp the space around them, creating conditions that can deflect and amplify starlight that aligns behind them. This phenomenon is known as gravitational lensing. Ground-based telescopes peer at the millions of stars dotting the center of the Milky Way and seek out this ephemeral brightening, signifying that a large object has passed between us and the star.

Hubble is perfectly poised to follow up on these observations. Two different teams of researchers studied the observations to determine the mass of the object. Both studies have been accepted for publication in The Astrophysical Journal.

One team, led by astronomer Kailash Sahu, a Hubble instrument scientist at the Space Telescope Science Institute in Baltimore, determined that the black hole weighed seven times the mass of our sun.
The second team, led by doctoral student Casey Lam and Jessica Lu, associate professor of astronomy, both of University of California, Berkeley, arrived at a smaller mass range, between 1.6 and 4.4 times that of the sun. According to this estimate, the object could be a black hole or a neutron star. Neutron stars are the incredibly dense remnants of exploded stars.

“Whatever it is, the object is the first dark stellar remnant discovered wandering through the galaxy, unaccompanied by another star,” Lam said in a statement.

The star-filled sky shown in this Hubble image lies in the direction of the center of the galaxy.

The black hole passed in front of a background star located 19,000 light-years away from Earth toward the center of the galaxy, amplifying its starlight for 270 days. Astronomers had a difficult time determining their measurement because there is another bright star very close to the one they observed brightening behind the black hole.

“It’s like trying to measure the tiny motion of a firefly next to a bright light bulb,” Sahu said in a statement. “We had to meticulously subtract the light of the nearby bright star to precisely measure the deflection of the faint source.”

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Sahu’s team thinks the object may be traveling as quickly as 99,419 miles per hour (160,000 kilometers per hour), which is quicker than most stars in that part of the galaxy, while Lu and Lam’s team arrived at an estimate of 67,108 miles per hour ( 108,000 kilometers per hour).

More data and observations from Hubble and further analysis could settle the argument over the identity of the object. Astronomers continue the needle-in-a-haystack search for more of these invisible oddities, which could help them better understand how stars evolve and die.

“With microlensing, we’re able to probe these lonely, compact objects and weigh them. I think we have opened a new window onto these dark objects, which can’t be seen any other way,” Lu said.

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