# Black Hole: Definition & Types

Instructor: Richard Cardenas

Richard Cardenas has taught Physics for 15 years. He has a Ph.D. in Physics with a focus on Biological Physics.

In this lesson you will learn the definition of a black hole, how black holes form, and what types of black holes are out there. You will also learn about the two types of classifications for black holes which depend on their basic properties.

## Definition

There are many exotic and fascinating objects in the universe, but none that capture the imagination more than black holes. However, not many people really know what a black hole is or that there are actually different types of black holes. To get an idea of what a black hole is, consider the following thought experiment. Let's say that I can squeeze the Earth so that it's as big as an orange. What you have done is increase the density of the earth and create a region of space with such a strong gravitational pull that nothing, not even light, can escape that region. That is a simple description of a black hole. The gravitational acceleration on Earth depends on the mass and the radius of the Earth. Let's calculate that gravitational acceleration.

We see the familiar acceleration due to gravity. Note that the constant G is the universal gravitational constant. What if we shrink the Earth down to the size of a 4 cm orange? We get the following acceleration due to gravity.

This new gravity is so strong that it is 2,500,000,000,000,000 times stronger than our current gravity. This is in the quadrillion range. This gravity is so strong that even light will not be able to escape that region of space. That is how a black hole is defined.

## Types of Black Holes

The first classification of black holes has to do with whether they are rotating or not. Non-rotating black holes are called Schwarzschild black holes. These black holes have non-rotating cores and they have two main properties: a singularity and an event horizon. A singularity is simply the name for the collapsed core of the black hole. The event horizon is an imaginary sphere surrounding the black hole and is also defined as the opening of the black hole. Rotating black holes are referred to as Kerr black holes. These black holes rotate because the object that collapsed into a black hole was originally rotating. These black holes have the following properties: a singularity, an event horizon, the ergosphere, and the static limit. The ergosphere is the region of distorted space around the rotating black hole and is usually egg-shaped. As the black hole rotates, it drags the space around it forming this region. The static limit is the boundary between the ergosphere and normal space.

There is another classification of black holes that contains three types: stellar, supermassive, and micro black holes. Stellar black holes are formed from massive stars. As a star ages, there is a battle between gravity pushing inward and the radiation pressure due to nuclear reactions inside the star pushing outward. When these stars die, their outer region is mostly made up of iron. The radiation pressure causes a massive explosion which we call a supernova. This explosion sheds the outer shell of the dying star leaving the core. Gravity then collapses this core into a singularity but only if the core was massive enough (greater than 10 times the size of the sun). The result is what we call a stellar black hole.

Supermassive black holes mainly exist at the centers of galaxies. This includes our own galaxy, the Milky Way. They can have masses equivalent to billions of suns. Unfortunately, scientists still do not know how these supermassive black holes form, although there is speculation that they formed from the collapse of large stellar clusters. It's also possible that they may result from the formation of galaxies. Since they are at the centers of galaxies and close billions of stars, these black holes get bigger as they absorb surrounding matter. The figure below shows an example of a stellar black hole (on the left) and a supermassive black hole (on the right).

Finally, there are micro black holes. Never actually observed, these black holes have masses that are smaller than the sun. The speculation is that these black holes were formed during the Big Bang.

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