A black hole is a region of spacetime where gravity is so intense that nothing, no matter how much energy it has, can escape. Inside the event horizon, gravity supersedes all other forces of nature.
Why Do Black Holes Form?
Black holes form when a huge amount of matter is squeezed into a very small space. Cram enough matter into a given radius, and it can create a gravity well so deep that nothing can get out. This can happen in many ways, including the titanic explosion of a dying star or the collapse of primordial hydrogen into the supermassive black holes at the core of most large galaxies. The latter, called active galactic nuclei, appear to form at the same time as the galaxies that condense around them. The Milky Way has its own supermassive black hole called Sagittarius A*.
How Big Is a Black Hole?
Main sequence stars (like our Sun) make up about 90% of the stars in the sky. When they die, main sequence stars up to eight times the mass of the Sun tend to blow themselves apart, expanding into red giants that eventually create a planetary nebula as they eject their outer layers. The explosion leaves behind a naked stellar core called a white dwarf, barely the size of Earth but with hundreds of thousands of times that much mass crammed into the same volume. Stars with a mass between eight and twenty times that of the Sun tend to explode in a supernova and then collapse into a neutron star when they burn out, which is how we get exotic stellar objects like magnetars and pulsars. But if the core that remains, after all the fireworks, is greater than three or four times the mass of our Sun, it appears inevitable that the core will collapse into a "stellar-mass" black hole.
Supermassive black holes have millions or even billions of times the mass of our Sun. Why are they so huge? One leading hypothesis is direct collapse from the primordial gluon soup that filled the early Universe. Another is the collisions of hundreds or thousands of black holes, such as might be formed by the collapse of an entire star cluster, most of whose stars had become mature stellar-mass black holes.
There are "stellar-mass" black holes formed by the deaths of stars of up to twenty times the mass of our Sun; there are also supermassive black holes, millions of times more massive than the Sun. A black hole can get bigger, more massive, as it absorbs matter trapped in its gravity well. But one trend that scientists are still working to explain fully is that there seem to be scarcely any black holes of "intermediate" mass. NASA estimates that there are tens of millions, perhaps billions, of black holes just within the Milky Way itself. And we're just one galaxy within our entire Local Group!
However, in the entire history of astronomy, we have found just a bare handful of candidate sky objects that might—maybe—prove to be the elusive middle-sized black holes scientists would love to find and study. Where are the black holes of intermediate mass? We simply don't know. However, scientists are hunting intermediate black holes with tools like the Sloan Digital Sky Survey, which can detect the type of light released when black holes ingest something.
How Do We Find Black Holes?
Because nothing, not even light, can escape a black hole, they're invisible. What we can see, though, is the "stuff" falling into them. Black holes devour anything that crosses their event horizon, including entire star systems. For example, Sagittarius A* (Sgr A*, pronounced "sadge A-star," for short) was ingesting a star when scientists started imaging it directly with radio telescopes. Debris streaming into the black hole orbits so fast that it heats up and starts to glow, and the stars a black hole eats are glowing to begin with. So, with long exposures, we can see the paths traced by light from infalling material surrounding a black hole.
Another way we can find black holes is the way they alter the light nearby. Black holes have such strong gravity that they warp the paths of everything passing near them, including light. This produces a characteristic lensing effect that distorts and magnifies things behind the black hole from the viewer's perspective. Lensing can even flip things over, like when you look at your upside-down reflection in a spoon. Gravitational lensing has enabled humans to find not just black holes themselves but small deep-sky objects behind black holes, things we would never have been able to see without that telltale optical distortion.
Gravitational waves are also an important tool for identifying black holes. Einstein's theory of relativity describes a universe in which everything, no matter how small, leaves a kind of gravitational ripple in its wake as it moves through spacetime. Also, like waves in water, they get bigger as they spread out farther and farther away. These ripples are called gravitational waves. Like a sonic boom, they can be detected with an interferometer, a measuring device that uses two perfectly calibrated lasers whose beams are both perfectly synced in phase. Gravitational waves from faraway black holes that pass through Earth will make one laser shudder, ever so slightly, at a slightly different time than the other, knocking them out of phase.
In 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO) detected the invisible energy fluctuations of gravitational waves created by a pair of black holes merging almost a billion light-years away. This discovery led to the 2017 Nobel Prize in Physics.
When massive objects like black holes collide, they crash into one another with such force that it vibrates the fabric of spacetime, making it ring like a bell. As the newly merged black hole settles into a more stable equilibrium, it twists spacetime around it, producing a highly recognizable pattern like the one at the borders of a sonic shockwave. This "ringdown" creates a clear, recognizable oscillation in gravity. LIGO used ringdown echoes to identify gravitational waves from a collision between black holes, and then about eighteen months later, the waves created by two neutron stars colliding.
What Was the First Black Hole Ever Discovered?
The first accepted black hole was Cygnus X-1. Discovered during a 1971 rocket flight, Cygnus X-1 is one of the strongest X-ray sources detectable from Earth. The stellar wind from a nearby blue supergiant star is streaming into the black hole, forming an accretion disc about the black hole's axis of rotation. Some of that accreted material is then cast off by relativistic jets that release bursts of X-rays from its poles.
Current estimates calculate the mass of Cygnus X-1 at 21.2 solar masses, all crammed into a sphere about as wide as New York City is long. Cygnus X-1's event horizon, meanwhile, appears to lie about 300 km (185 miles) out from its center of mass.
What Is an Event Horizon?
A black hole's event horizon, also called the Schwarzschild radius, is the radius within which its gravity supersedes all other forces of nature. Inside a black hole's event horizon, gravity is so strong that nothing can escape. The event horizon is an unstable gravitational boundary, similar to the L2 Lagrange point where we park our space telescopes: a gravitational eddy that sheds matter away from itself, instead of accumulating it like Jupiter's Trojan asteroids.
The laws of physics predict that there will exist a radius, larger as the mass of the black hole increases, where the math turns itself "inside out." Terms in physics equations change signs at that radius, from negative to positive, or asymptote away to infinity. One quantity that behaves this way is the velocity vector for anything attempting to leave a black hole—starting at the event horizon, all paths point in toward the middle of the black hole.
At the event horizon, the answer to the question of whether anything can escape from the black hole's gravity well changes from negative to positive: Inside the event horizon, nothing can escape. Outside the event horizon, it's possible to escape the gravity of a black hole using "conventional means" like rocketry.
However, this subtle topological shift in the math has wider effects. Inside the event horizon, because all paths point in toward the black hole, nothing inside it can affect anything outside the event horizon. Information about the matter that enters a black hole is lost when it crosses the event horizon.
What Direct Evidence of Black Holes Do We Have?
Many, if not most, galaxies have a supermassive black hole at their core, including the Milky Way. Our galaxy's supermassive black hole, Sagittarius A*, has about 4.3 million times the mass of our Sun. In May 2022, the Event Horizon Telescope (EHT) collaboration released the first direct image ever taken of Sgr A*.
To resolve the black hole, they used the Event Horizon Telescope, a global radio telescope array with an effective aperture the size of the Earth. One participating EHT telescope, highlighted here, is the Atacama Large Millimeter/Submillimeter Array (ALMA), one of the powerful telescopes at the European Southern Observatory in the Chilean Andes.
That image built on previous work by the same EHT collaborative that had, two years earlier, released the first direct image ever captured of any black hole: M87*, the black hole at the center of Messier 87. Messier 87 is a nearby supergiant elliptical galaxy, one of the largest in the local Universe.
These images show us the silhouettes of the black holes themselves—eerie voids blacker than night, perfectly round, warping everything around them. Sgr A* was tougher than M87* to resolve because Sgr A* is much smaller than M87*. M87*'s radius is as wide as Voyager 1 is far from Earth. Sgr A*, by comparison, would fit within the orbit of Mercury, accretion disc and all.
Where Is the Nearest Black Hole to Earth?
The closest black hole to Earth may be just 1,500 light-years away from the Sun. V723 Monocerotis is a variable star within the constellation Monoceros, "the Unicorn," and a unicorn indeed it may be. In 2021, astronomers identified the Unicorn as a binary system with a black hole that would be the closest to Earth. But with just three solar masses, and a Schwarzschild radius of just 9 km, it would also be among the smallest black holes ever found.
