Hubble spies on a stellar ‘ghost’ wandering the Milky Way

Hubble spies on a stellar ‘ghost’ wandering the Milky Way

For the first time, the Hubble Space Telescope has detected a lone object drifting through our Milky Way galaxy – the invisible, ghostly remains of a once-bright star. (Digitized Sky Survey, Hubble, ESA via CNN)

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ATLANTA — For the first time, the Hubble Space Telescope has detected a lone object drifting through our Milky Way galaxy — the invisible, ghostly remains of a once-bright star.

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

Sometimes the explosion can send the black hole spinning, hurtling through the galaxy like a pinball machine. By right, there should be plenty of roving black holes known to scientists, but they are virtually invisible in space and therefore very difficult to find.

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 spending six years on observations – and astronomers were even able to make an accurate measurement of the extreme cosmic object’s mass.

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 closest isolated black hole to Earth might be only 80 light-years away.

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

The extremely powerful gravitational field of black holes warps the space around them, creating conditions that can deflect and amplify the starlight that lines up behind them. This phenomenon is known as gravitational lensing. Ground-based telescopes watch the millions of stars dotting the center of the Milky Way and look for this fleeting brightening, signifying that a large object has passed between us and the star.

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

A team, led by astronomer Kailash Sahu, a specialist in Hubble instruments at the Space Telescope Science Institute in Baltimore, has determined that the black hole weighs seven times the mass of our sun.

The second team, led by Casey Lam and Jessica Lu, both of the 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.

“In any event, the object is the first black stellar remnant discovered wandering the galaxy, unaccompanied by another star,” Lam said in a statement.

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

“It’s like trying to measure the tiny movement of a firefly next to a bright light bulb,” Sahu said in a statement. “We had to meticulously subtract the light from the nearby bright star to accurately measure the deviation from the weak source.”

Sahu’s team believe the object can travel as fast as 99,419 miles per hour, which is faster than most stars in this part of the galaxy, while Lu and Lam’s team arrived at an estimate of 67,108 miles per hour.

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

“With the microlens, we are able to probe these solitary, compact objects and weigh them. I think we have opened a new window into these dark objects, which cannot be seen otherwise,” Lu said.

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