For the first time, the Hubble Space Telescope has detected an isolated object floating in our own Milky Way galaxy — an invisible spectral remnant of a once-luminous star.
When stars with enough mass to make our Sun look like dwarfs die, they explode and form supernovae. The remaining core is crushed by its own gravity, forming a black hole.
Sometimes the explosion can set the black hole in motion, pinballing its way through the galaxy. In theory, scientists should be aware of many roaming black holes, but given that they are barely visible from space, they are difficult to identify.
Astronomers believe that there are 100 million black holes floating freely in our galaxy. Now, researchers believe they have found one of those objects. The detection comes after six years of observations — astronomers even managed to accurately measure the mass of this extreme cosmic object.
This black hole is 5,000 light-years away in one of the spiral arms of the Milky Way, known as the Sagittarius Arm. This observation allowed the team to estimate that the closest isolated black hole to Earth may be only 80 light-years away.
But if black holes are basically indistinguishable from the nothingness of space, how did Hubble manage to identify this?
The extremely strong gravitational fields of black holes distort the space around them, creating conditions for the deformation or amplification of radiation from the stars lining up behind them. This phenomenon is called gravitational lensing. Ground-based telescopes peered at the millions of stars scattered across the Milky Way’s center, looking for this fleeting glow that suggested a large object passed between us and the star.
Hubble is perfectly capable of following up on these observations. The observations were studied by two different research groups to determine the mass of the object. Both studies have been accepted for publication in The Astrophysical Journal.
One of the teams, led by astronomer Kailash Sahu, a Hubble instrument scientist at the Space Telescope Science Institute in Baltimore, determined that the black hole weighs seven times the mass of the sun.
A second team, led by UC Berkeley doctoral student Casey Lam and associate professor of astronomy Jessica Lu, identified a smaller range of the object’s mass, between 1.6 and 4.4 times the mass of the sun. According to this estimate, the object could be a black hole or a neutron star. Neutron stars are the extremely dense remnants of stars that exploded.
“Whatever it is, this object is the first dark star remnant found in the Milky Way without another star to accompany it,” Lin said in a statement.
Passing in front of a background star 19,000 light-years from Earth, the black hole extends toward the center of the Milky Way, magnifying its starlight for 270 days. Astronomers had a hard time confirming their measurements because there was another bright star very close to the one they observed emitting light from behind the black hole.
“It’s like trying to measure the tiny movements of a firefly next to a glowing light bulb,” Sahu said in a statement. “We had to painstakingly subtract the light from nearby twinkling stars to accurately measure the deflection of the faint light source.”
Sahu’s team thinks the object may be traveling at 160,000 kilometers per hour, faster than most stars in this part of the Milky Way, while Lu and Lam’s teams estimate the speed to be around 108,000 kilometers per hour.
More data and observations from Hubble, as well as further analysis, may resolve questions about the object’s identity. Astronomers are still searching for these bizarre, invisible needles in a haystack that could help them better understand how stars evolve and die.
“Through the microlensing process, we were able to study these compact, isolated objects and weigh them. I think we opened a new window to these black objects that can’t be seen any other way,” Lu said.