An Introduction to Black Holes

black hole captured by Event Horizon Telescope
APRIL 10: In this handout photo provided by the National Science Foundation, the Event Horizon Telescope captures a black hole at the center of galaxy M87, outlined by emission from hot gas swirling around it under the influence of strong gravity near its event horizon, in an image released on April 10, 2019. A network of eight radio observatories on six mountains and four continents, the EHT observed a black hole in Messier 87, a supergiant elliptical galaxy in the constellation Virgo, on and off for 10 days in April of 2017 to make the image.

 National Science Foundation / Getty Images

Black holes are objects in the universe with so much mass trapped inside their boundaries that they have incredibly strong gravitational fields. In fact, the gravitational force of a black hole is so strong that nothing can escape once it has gone inside. Not even light can escape a black hole, it is trapped inside along with stars, gas, and dust. Most black holes contain many times the mass of our Sun and the heaviest ones can have millions of solar masses.

computer simulation of a supermassive black hole
This computer-simulated image shows a supermassive black hole at the core of a galaxy. The black region in the center represents the black hole's event horizon, where no light can escape the massive object's gravitational grip. The black hole's powerful gravity distorts space around it like a funhouse mirror. Light from background stars is stretched and smeared as the stars skim by the black hole. NASA, ESA, and D. Coe, J. Anderson, and R. van der Marel (Space Telescope Science Institute), Science Credit: NASA, ESA, C.-P. Ma (University of California, Berkeley), and J. Thomas (Max Planck Institute for Extraterrestrial Physics, Garching, Germany).

Despite all that mass, the actual singularity that forms the core of the black hole has never been seen or imaged. It is, as the word suggests, a tiny point in space, but it has a LOT of mass. Astronomers are only able to study these objects through their effect on the material that surrounds them. The material around the black hole forms a rotating disk that lies just beyond a region called "the event horizon," which is the gravitational point of no return.

The Structure of a Black Hole

The basic "building block" of the black hole is the singularity: a pinpoint region of space that contains all the mass of the black hole. Around it is a region of space from which light cannot escape, giving the "black hole" its name. The outer "edge" of this region is what forms the event horizon. It's the invisible boundary where the pull of the gravitational field is equal to the speed of light. It's also where gravity and light speed are balanced.

The event horizon's position depends on the gravitational pull of the black hole. Astronomers calculate the location of an event horizon around a black hole using the equation Rs = 2GM/c2R is the radius of the singularity, G is the force of gravity, M is the mass, c is the speed of light. 

Black Hole Types and How They Form

There are different types of black holes, and they come about in different ways. The most common type is known as a stellar-mass black hole. These contain roughly up to a few times the mass of our Sun, and form when large main sequence stars (10 - 15 times the mass of our Sun) run out of nuclear fuel in their cores. The result is a massive supernova explosion that blasts the stars outer layers to space. What's left behind collapses to create a black hole.

stellar mass black hole
An artist's conception of a stellar-mass black hole (in blue) hat likely formed when a supermassive star collapsed, feeding from material ejected by a nearby star. ESA, NASA and Felix Mirabel)

The two other types of black holes are supermassive black holes (SMBH) and micro black holes. A single SMBH can contain the mass of millions or billions of suns. Micro black holes are, as their name implies, very tiny. They might have perhaps only 20 micrograms of mass. In both cases, the mechanisms for their creation are not entirely clear. Micro black holes exist in theory but have not been directly detected.

Supermassive black holes are found to exist in the cores of most galaxies and their origins are still hotly debated. It's possible that supermassive black holes are the result of a merger between smaller, stellar-mass black holes and other matter. Some astronomers suggest that they might be created when a single highly massive (hundreds of times the mass of the Sun) star collapses. Either way, they are massive enough to affect the galaxy in many ways, ranging from effects on starbirth rates to the orbits of stars and material in their near vicinity.

NASA Galaxy Hunter: Huge Black Holes Stifle Star Formation
Many galaxies have supermassive black holes at their cores. If they are actively "eating", then they give off huge jets and are known as active galactic nuclei. NASA/JPL-Caltech

Micro black holes, on the other hand, could be created during the collision of two very high-energy particles. Scientists suggest this happens continuously in the upper atmosphere of Earth and is likely to happen during particle physics experiments at such places as CERN. 

How Scientists Measure Black Holes

Since light can not escape from the region around a black hole affected by the event horizon, nobody can really "see" a black hole. However, astronomers can measure and characterize them by the effects they have on their surroundings. Black holes that are near other objects exert a gravitational effect on them. For one thing, mass can also be determined by the orbit of material around the black hole.

A model of a black hole minus its surrounding disk of material.
A model of a black hole surrounded by heated ionized) material. This may be what the black hole in the Milky Way "looks" like. Brandon DeFrise Carter, CC0, Wikimedia.   

In practice, astronomers deduce the presence of the black hole by studying how light behaves around it. Black holes, like all massive objects, have enough gravitational pull to bend light's path as it passes by. As stars behind the black hole move relative to it, the light emitted by them will appear distorted, or the stars will appear to move in an unusual way. From this information, the position and mass of the black hole can be determined.

This is especially apparent in galaxy clusters where the combined mass of the clusters, their dark matter, and their black holes create oddly-shaped arcs and rings by bending the light of more distant objects as it passes by. 

Astronomers can also see black holes by the radiation the heated material around them gives off, such as radio or x rays. The speed of that material also gives important clues to the characteristics of the black hole it's trying to escape.

Hawking Radiation

The final way that astronomers could possibly detect a black hole is through a mechanism known as Hawking radiation. Named for the famed theoretical physicist and cosmologist Stephen Hawking, Hawking radiation is a consequence of thermodynamics that requires that energy escape from a black hole.

The basic idea is that, due to natural interactions and fluctuations in the vacuum, the matter will be created in the form of an electron and anti-electron (called a positron). When this occurs near the event horizon, one particle will be ejected away from the black hole, while the other will fall into the gravitational well.

To an observer, all that is "seen" is a particle being emitted from the black hole. The particle would be seen as having positive energy. This means, by symmetry, that the particle that fell into the black hole would have negative energy. The result is that as a black hole ages, it loses energy, and therefore loses mass (by Einstein's famous equation, E=MC2, where E=energy, M=mass, and C is the speed of light).

Edited and updated by Carolyn Collins Petersen.