Free Astronomy Magazine

27 JULY-AUGUST 2022 gravity into a black hole. Because the self-detonation is not perfectly symmetrical, the black hole may get a kick, and go careening through our galaxy like a blasted cannonball. Telescopes can’t photograph a way- ward black hole because it doesn’t emit any light. However, a black hole warps space, which then de- flects and amplifies starlight from anything that momentarily lines up exactly behind it. Ground-based telescopes, which monitor the brightness of millions of stars in the rich star fields toward the central bulge of our Milky Way, look for a tell-tale sudden brighten- ing of one of them when a massive object passes between us and the star. Then Hubble follows up on the most interesting such events. Two teams used Hubble data in their investigations — one led by Kailash Sahu of the Space Telescope Science Institute in Baltimore, Maryland; and the other by Casey Lam of the University of California, Berkeley. The teams’ results differ slightly, but both suggest the presence of a com- pact object. The warping of space due to the gravity of a foreground object pass- ing in front of a star located far be- hind it will momentarily bend and amplify the light of the background star as it passes in front of it. As- tronomers use the phenomenon, called gravitational microlensing, to study stars and exoplanets in the ap- proximately 30,000 events seen so far inside our galaxy. The signature of a foreground black hole stands out as unique among other microlensing events. The very intense gravity of the black hole will stretch out the duration of the lens- ing event for over 200 days. Also, if the intervening object was instead a foreground star, it would cause a transient color change in the binary systems or in the cores of galaxies. Stellar-mass black holes are usually found with companion stars, making this one unusual. The newly detected wandering black hole lies about 5,000 light- years away, in the Carina-Sagittarius spiral arm of our galaxy. However, its discovery allows astronomers to es- timate that the nearest isolated stel- lar-mass black hole to Earth might be as close as 80 light-years away. The nearest star to our solar system, Proxima Centauri, is a little over 4 light-years away. Black holes roaming our galaxy are born from rare, monstrous stars (less than one-thousandth of the galaxy’s stellar population) that are at least 20 times more massive than our Sun. These stars explode as supernovae, and the remnant core is crushed by T his is an artist’s impression of a black hole drifting through our Milky Way galaxy. The black hole is the crushed remnant of a massive star that exploded as a supernova. The surviving core is several times the mass of our Sun. The black hole traps light because of its intense gravitational field. The black hole distorts the space around it, which warps images of background stars lined up almost directly behind it. This gravitational “lensing” effect offers the only telltale evidence for the existence of lone black holes wandering our galaxy, of which there may be a population of 100 million. The Hubble Space Telescope goes hunting for these black holes by looking for distortion in starlight as the black holes drift in front of background stars. [ESA/Hubble, Digitized Sky Survey, Nick Risinger (skysurvey.org) , N. Bartmann]