Illustration of a solitary black hole in the Milky Way galaxy. © FECYT/IAC

The Hubble Space Telescope has discovered an isolated, solitary black hole in the Milky Way galaxy for the first time. It is a black hole that is about seven times as massive as the Sun, at a distance of about 5000 light-years. Such ‘stellar’ black holes form during supernova explosions of massive stars. Astronomers suspect that there must be about 100 million of those solitary black holes floating around in the Milky Way galaxy (the closest one could be about 80 light-years away), but so far none had been discovered – black holes do not emit any form of radiation. All stellar black holes discovered in the past half century are part of a binary star system and can be observed (albeit indirectly). because the black hole sucks matter from its companion. This matter accumulates in a whirling ‘accretion disk’, and becomes so hot that visible light and X-rays are emitted, among other things.

Hubble’s new discovery was made possible by the fact that massive objects slightly amplify and bend the light rays from distant background stars – what’s known as microgravity lensing. In recent decades, several tens of thousands of these ‘microlenses’ have been detected with terrestrial telescopes; in most cases there is a foreground star affecting the light of a background object. Two teams of astronomers have now analyzed Hubble observations of a special microgravity lens, and have concluded that the light-bending object is almost certainly a black hole.

The first team, led by Kailash Sahu of the Space Telescope Science Institute, concludes based on the observations that the black hole is about seven times as massive as the Sun. The second team, led by Casey Lam and Jessica Lu of the University of California at Berkeley, arrives at a slightly lower mass: 1.6 to 4.4 times the mass of the Sun. According to Lam and Lu, it could therefore also be a so-called neutron star.

Hubble recorded not only the slow increase and decrease in the background star’s brightness (over a period of 270 days), but also the minute deflection of the star’s light, by an angle of about one thousandth of arcsecond. The measurements not only show the mass and distance of the black hole, but also the speed at which it moves through the Milky Way galaxies: more than 150,000 kilometers per hour. That high speed is most likely the result of a certain asymmetry in the supernova explosion that left the black hole behind.

NASA’s future Nancy Grace Roman space telescope is expected to detect many more of these microgravity lenses, undoubtedly leading to the discovery of new solitary black holes. (GS)