The Milky Way has a lot of black holes — about 100 million of them, a new study suggests.
But there’s no reason to fear. “It may sound like a big number, but by astronomical standards, it’s a pretty small number,” says Daniel Holz. He is a physicist at the University of Chicago in Illinois. For comparison, our galaxy has a thousand times more stars.
Scientists from the University of California, Irvine calculated the Milky Way’s black hole population as part of a new census. The team estimated the number of black holes that have masses tens of times that of the sun. These are called stellar-mass black holes. They form when massive stars collapse. The census estimates the numbers of these black holes in galaxies big and small. The results will be published in Monthly Notices of the Royal Astronomical Society.
Galaxies are enormous communities of stars. To estimate the number of black holes, the researchers looked at the properties of stars and galaxies. A star’s size and what it is made of determine whether the star can form a black hole. Those factors also determine how big the black hole will be. A galaxy’s size is important, too. From its size, scientists can estimate the number and properties of stars inside it. That allows researchers to determine the number of black holes and their sizes.
Based on the properties of the Milky Way’s stars and our galaxy’s size, the researchers estimate that our galaxy has 100 million stellar-mass black holes. About 90 million have less than 30 times the mass of the sun. About 10 million are biggies, with 30 times the mass of the sun or more.
Stellar-mass black holes are a target of LIGO, the Advanced Laser Interferometer Gravitational-Wave Observatory. This facility has detected three sets of gravitational waves from colliding black holes. Gravitational waves are ripples in the fabric of space. LIGO made its first detection of these ripples in 2016. The ripples appeared to come from the collision of two black holes that were about 30 times the mass of the sun. At the time, some physicists thought that mass of the black holes seemed surprisingly large. The stellar-mass black holes that scientists previously knew about were smaller than that. Some scientists started to propose exotic origins for LIGO’s black holes. Maybe those black holes didn’t form from collapsing stars. Maybe they formed during the universe’s infancy instead.
The new result contradicts that idea. In the Milky Way alone, there are about 10 million black holes with masses 30 times the sun’s mass or more. That means there should be a lot of them in other galaxies, too. “You don’t have to do anything particularly odd or unusual in order to explain the LIGO signal,” says James Bullock. He is a physicist at UC Irvine and a coauthor of the study.
Richard O'Shaughnessy is an astrophysicist at the Rochester Institute of Technology in New York. The abundance of bigger stellar-mass black holes doesn't surprise him, he says. But, the new work may come to satisfy researchers who thought LIGO’s large black holes were an oddity, he says. Hopefully, he says, it will make even the skeptics recognize that it’s logical to see larger stellar-mass black holes merging.
astronomy The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.
astrophysics An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.
black hole A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.
gravitational waves (also known as gravity waves) Ripples in the fabric of space that are produced when masses undergo sudden acceleration. Some are believed to have been unleashed during the Big Bang, when the universe got its explosive start.
gravity Schools tend to teach that gravity is the force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity. But Einstein’s general theory of relativity redefined it, showing that gravity is not an ordinary force, but instead a property of space-time geometry. Gravity essentially can be viewed as a curve in spacetime, because as a body moves through space, it follows a curved path owing to the far greater mass of one or more objects in its vicinity.
laser A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.
LIGO (short for Laser Interferometer Gravitational wave Observatory) A system of two detectors, separated at a great geographical distance, that are used to register the presence of passing gravitational waves.
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
Milky Way The galaxy in which Earth’s solar system resides.
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study which emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in that field is known as a physicist.
star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
universe The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.
Journal: O.D. Elbert et al. Counting black holes: The cosmic stellar remnant population and implications for LIGO. In press. Monthly Notices of the Royal Astronomical Society. doi: 10.1093/mnras/stx1959.