Black holes are objects so massive that not even light, let alone physical matter, is supposed to escape its gravitational pull. Yet sometimes one inexplicably spews jets of radiation and ionized matter into space. Miller-Jones and his team wanted to investigate how matter is sucked into and expelled from black holes, so they took a closer look at Cygnus X-1.
They observed the black hole for six days using the Very Long Baseline Array, a network of 10 radio telescopes sited across North America from Hawaii to the Virgin Islands. The resolution is comparable to what would be required to spot a 10-centimeter object on the moon, and it’s the same technique that the Event Horizon Telescope used to snap the first photo of a black hole.
Using a combination of measurements involving radio waves and temperatures, the team modeled the precise orbits of both Cygnus X-1’s black hole and the massive supergiant star HDE 226868 (the two objects orbit each other). Knowing the orbits of each object allowed the team to extrapolate their masses—in the case of the black hole, 21 solar masses, which is about 50% more than once thought.
The mass of black holes depends on a few factors, particularly the size of the star that collapsed into the black hole and the amount of mass that erodes away in the form of stellar wind. Hotter and brighter stars tend to produce more volatile stellar winds, and they also tend to be heavier. So the more massive a star is, the more prone it is to losing mass via stellar wind before and during its collapse, resulting in a lighter black hole.
But in general, scientists thought stellar winds in the Milky Way were strong enough to limit the mass of black holes to no more than 15 solar masses, regardless of how big the stars were originally. The new findings clearly upend those estimates.
“Finding a black hole that was significantly more massive than this limit tells us that we have to revise our models of how much mass the largest stars lose in stellar winds over their lifetimes,” says Miller-Jones. It may mean the stellar winds that move through the Milky Way are less powerful than we thought, or that stars hemorrhage mass in other ways. Or it could mean black holes behave in more erratic ways than we’re able to anticipate.
The team plans to follow up with more observations of Cygnus X-1. Other instruments, such as the planned Square Kilometer Array in Australia and South Africa, could provide better views of this and other nearby black holes. There could be anywhere from 10 million to a billion black holes in the Milky Way, and studying at least a few more of them might help clear up this mystery.