Black Holes are some of the strangest objects in the universe. This means searching for and observing them for a deeper understanding is a must.

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It is Impossible to Directly Observe Black Holes

Light can’t escape black holes, this means observing them directly is impossible. Scientists must be more clever than that to find them.

Indirect Observation

Since we can’t observe black holes directly we must find a way around this. Luckily, black holes leave a distinct signature, everything around them is affected heavily by the gravity. This causes stars to seemingly orbit around nothing, clumps of hot gas to shoot off x-rays, and the paths of electromagnetic rays to be highly bent.

Orbits of Stars
Stars are often found in clusters. Predicting the movements of groups of massive objects is complicated, especially if there are hundreds of them. When the simulations and the real data don’t add up we can assume there is a black hole.
In this video stars are seemingly orbiting around nothing, but it is really just a black hole.

Emitted Radiation
The disks around a black hole have massive amounts of energy, because of the gravity of the black hole. This energy causes the matter in the disk to emit x-rays. These x-rays are easily detected by x-ray telescopes.

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Gravitational Lensing
Black holes bend space around them, changing the path of electromagnetic waves. This can cause electromagnetic waves emitted from an object bend just enough that two distinct images of the object.

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The Concept of Black Holes

After Einstein's theory of relativity many ideas took form. One of the more important ones was the existence of black holes, thought up by Karl Schwarzschild a year after the original papers on relativity. He found exact solutions to the equations in general relativity and got many different types of measurements named after him. The most famous is the Schwarzschild radius, the radius at which a collapsing object becomes a black hole.

Before this modern concept of light, black holes looked different. When light was thought of as only a particle John Michell theorized that any object sufficiently massive would slow down and eventually reverse the path of light. This idea excited scientists at the time, until new discoveries about the nature of light were made.

Categorizing Black Holes

Like all things, black holes can be categorized. Black holes come in two main categories: stellar mass and supermassive. Scientists have only found intermediate mass black holes recently, and few of them at that.
Smaller black holes can theoretically be created by anything with a high enough energy density, colliding particles in the LHC are one example of this. These black holes, if created, would last milliseconds at best before Hawking Radiation caused them to evaporate.

Stellar Mass

Stellar mass black holes are the most common. These ones are formed out of the supernova of giant stars. These black holes often form in star clusters and have multiple stars orbiting them.

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Supermassive

A supermassive black hole is found at the center of every galaxy. These black holes formed from the first stars. Over time massive amounts of dust started orbiting these black holes, feeding them and attracting more dust. This eventually led to galaxy a formation.

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The Middle Ground

Intermediate mass black holes are by far the rarest. Unlike stellar mass or supermassive black holes, intermediate mass black holes have no direct way to form. The most likely way is the merging of stellar mass black holes in star clusters. This chain reaction can bring a black hole into the lower intermediate mass range. Evidence of these black holes has been found but it is not conclusive.
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Black holes are some of the weirdest things out there. It's amazing how we can learn so much about something we can't even see.

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