Astronomers estimate that there should be 100 million black holes roaming among the 100 billion stars in our galaxy. But since black holes emit no light of their own, they are extremely difficult to detect. Now, astronomers have at last come up with clear evidence for finding one in a needle-in-a-haystack search among a blizzard of stars seen toward the galactic center. The light from a star far behind the black hole was momentarily brightened and deflected by the black hole passing in front of it. This was a long and painstaking measurement that the Hubble Space Telescope’s exquisite resolution is well-suited for. The black hole’s powerful gravitation left a unique fingerprint on the deflection of starlight, eliminating other potential gravitational lensing candidates.
No need for us to worry because the black hole is 5,000 light-years away. But, statistically, this detection means that the nearest wandering black hole to Earth could be no more than 80 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 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 wayward black hole because it doesn’t emit any light. However a black hole warps space, which then deflects and amplifies starlight from anything that momentarily lines up exactly behind it. NASA’s upcoming Nancy Grace Roman Space Telescope will discover several thousand microlensing events out of which many are expected to be black holes, and the deflections will be measured with very high accuracy.
In a 1916 paper on general relativity, Albert Einstein predicted that his theory could be tested by observing the Sun’s gravity offsetting the apparent position of a background star. This was tested by a collaboration led by astronomers Arthur Eddington and Frank Dyson during a solar eclipse on May 29, 1919. Eddington and his colleagues measured a background star being offset by 2 arcseconds, validating Einstein’s theories. These scientists could hardly have imagined that over a century later this same technique would be used – with unimaginable precision of a thousandfold better — to look for black holes across the galaxy.
The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.