To many, black holes are the most fascinating objects in space. The idea of a collapsed star with a gravitational field so strong that not even light can escape is one of the great mysteries of astrophysics, although we have gained a much greater scientific understanding of them in recent years.
In this two part series, we will explore how black holes are formed, the types of black holes, how we detect black holes, and how they work.How are black holes formed?
There are many theories that explain the formation of a black hole, but the most common one is as follows: When a colossal star, with a mass of a least 5 times the Sun, reaches the end of its' life, it gets crushed under its own overwhelming gravity, collapsing and forming a black hole.
During its normal life, stars are in a constant tug-of-war between gravity pushing in and pressure pushing out. For most of the star's life, nuclear reactions in its core can produce enough energy and pressure to balance the inward force of gravity. However, inevitably, the star runs out of nuclear fuel, and that's when gravity does its job. The more massive the star's core, the greater the force of gravity that compresses and collapses it under its own weight.
Normal stars die relatively peacefully. When their nuclear fuel is exhausted, they evolve into a red giant,
a fiery ball of burning hydrogen, slowly lose mass, and subside into a white dwarf.
It's a little bit different for massive stars. When they run out of fuel, it creates a giant explosion event, known as a supernova.
The outer parts of the star are violently expelled into space, and the core collapses under its own weight. If that core is massive enough, i.e. 2.5 times the mass of the Sun, then no internal force can combat gravity's influence, and the core collapses into a black hole.
Next week, we'll explore how these newly-formed black holes do their job, with a whole slew of jargon!What are the types of black holes?
Before we explore the different types of black holes, let's go over some terminology, which will be important in the future.
- Singularity - Singularity is essentially the collapsed core.
- Event Horizon - The Event Horizon is the opening of the hole - At this point, the force of the black hole is inescapable
- Ergosphere - An egg-shaped region of "distorted" space - This distortion is caused by the rotation of the black hole dragging the surrounding space along with it
- Static Limit - The boundary between the ergosphere and normal space
Black holes can be classified by rotation
and by mass.
Let's start with the former, since it involves the terms we just reviewed.
- Schwarzschild - The Schwarzschild Black Hole is the simplest type of black hole, in which the core is non-rotating. This type of black hole has only a singularity and an event horizon. They are less common in nature.
- Kerr - Kerr Black Holes, the most common type in nature, is a rotating black hole. It rotates because the star that preceded it rotated, and its collapsed core did the same. The rotating core is carried over the black hole, because of a basic principle of physics: Conservation of angular momentum. The Kerr Black Hole exhibits all of the terminology above, and is the type conventionally studied by astrophysicists.
Although the above classification is valid, astrophysicists also classify black holes by mass, and thus method of formation. Let's take a look at that as well.
- Stellar Black Holes - Stellar Black Holes are what we normally associate with black holes, and are the most commonly found in nature. These black holes are of "average" mass and form when a massive star collapses under its own weight.
- Supermassive Black Holes - Supermassive Black Holes can have a mass equivalent to billions of suns. They likely exist in the center of galaxies, including our own - We don't know exactly how they form, but they seem to be a byproduct of galaxy formation. Because of their central location, supermassive black holes are fed a steady diet of matter and continue to grow over billions of years.
- Miniature Black Holes - The existence of miniature black holes has been hypothesized, but has yet to be confirmed by experiment or observation. These black holes would have a mass much smaller than that of the Sun, and might have formed soon after the Big Bang, about 13.7 billion years ago. Early in the life of the universe, the faster expansion of some matter might have condensed slower-moving matter enough to form a powerful, albeit small, black hole.
Great! Hope you learned a little bit about black holes. Stay tuned for Part 2 next week!
 HubbleSite - Reference Desk - FAQs
 HowStuffWorks "How Black Holes Work"