An Introduction to Black Holes

Computer artwork of black hole
We can't see a black hole directly, but we can see how it affects matter around it, or light traveling past it. Science Photo Library - MARK GARLICK/ Brand X Pictures/ Getty Images

Most everyone has heard of black holes; science fiction books and movies use them as effective plot devices. However, does everybody know what they actually are? These real-life objects are scattered throughout the universe, particularly at the centers of galaxies. So, they're common, but nobody has really explored them. There's a good reason for that: once you get inside a black hole, there's no escape.

That makes them intriguing and frightening all at once. However, that hasn't stopped astronomers from studying them from the outside and using the laws of physics to understand them.

Black holes are objects in the universe with so much mass trapped inside their boundaries that they have incredibly strong gravitational fields. That gravity is so strong that nothing can escape a black hole once it has gone inside. Most black holes contain many times the mass of our Sun and the heaviest ones can have millions of solar masses.

Despite all that mass, the actual singularity that forms the core of the black hole has never been seen or imaged. The way astronomers know about them is by their effect on the material surrounding a black hole and the light that passes by.

The Structure of a Black Hole

The basic "building block" of the black hole is that singularity. It's a pinpoint region of space that contains all the mass of the black hole.

Then, there is the region of space surrounding the black hole from where light cannot escape, giving the "black hole" its name. The "edge" of this region is called the "event horizon". This is the invisible boundary where the pull of the gravitational field is equal to the speed of light. It's where gravity and light speed are balanced.

 

The event horizon's position depends on the gravitational pull of the black hole. You can calculate the location of an event horizon around a black hole using the equation Rs = 2GM/c2.  R is the radius of the singularity, G is the force of gravity, M is the mass, c is the speed of light. 

How Do Black Holes Form?

Since there are different types of black holes, the answer to how they form can be complicated. The most common type of black holes are known as stellar mass black holes, and they are roughly up to a few times the mass of our Sun. These types of black holes are formed 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, leaving a black hole core behind where the star once existed.

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. 

What about those micro black holes? They could be created during the collision of two very high-energy particles. It is thought that this happens continuously in the upper atmosphere of Earth and is likely to happen in particle physics experiments such as CERN. But no need to worry, we are not in danger.

How Do We Know Black Holes Exist If We Can't "See" Them?

Since light can not escape from the region around a black hole affected by the event horizon, we really can't "see" a black hole.

However, we 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. In practice, astronomers deduce the presence of the black hole by studying how light behaves around it. They, like all massive objects, will cause light to bend—due to the intense gravity—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 really apparent in galaxy clusters where the combined mass of the clusters, their dark matter and their black holes create very oddly shaped arcs and rings by bending the light of more distant objects as it passes by. 

We can also see black holes by the radiation the heated material around them gives off, such as radio or x rays. 

Hawking Radiation

The final way that we 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 very fabric of space time if you will), 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. Meaning, 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.