Learn about black holes, their myths and their reality. Learn how black holes form after stars undergo supernovae and create singularities. Discover how big black holes grow, how scientists find black holes and where black holes are located in the universe.

Black Holes – Myth vs. Reality

Black holes – they are what you think, and they aren’t. There are a lot of general misconceptions about black holes. First of all, our sun will never become one – it just isn’t big enough. Black holes aren’t giant vacuum cleaners sucking matter out of space, we cannot detect black holes visually, and time travel is not possible by falling into them.

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Scientists have theorized about the existence of black holes since way back in the 1700s. It wasn’t until Einstein’s work on general relativity in the early 1900s that scientists had support for the relationship between gravity and light. Further work was performed by other scientists, and it was 1967 when a physicist named John Wheeler first publicly used the words ‘black hole.’ Although these scientists couldn’t see a black hole, they theorized its existence from the effect of the black hole’s gravity on everything around it.

How a Black Hole Forms

A star collapses when it runs out of fuel, causing its density to become greater.
Star Collapse

Black holes form from stars of a specific size at the end of their life. A smaller star, like our sun, forms a white dwarf at the end of its life. A medium-size star ends its life as a neutron star. A huge star, many times bigger than our sun, becomes a black hole.

When a star runs out of fuel in its core, it collapses and its density becomes greater. When a star starts big enough, it will collapse, making its density very strong, nearly infinite. Density is defined as mass divided by volume; this means that the more massive something is but the less space it takes up, the more dense it is. Picture the sun collapsing and becoming so dense that the entire sun fits into an area the size of New York City.

The density in a black hole is so strong that not even light can escape it. In doing so, the star creates what is known as a singularity. A singularity is an infinitely dense point of zero volume. Have you ever seen them crush a car at the junkyard? They squish it down from a huge volume to just a little flat pancake. This is essentially what a singularity is, only even more so – a huge volume of matter that is squished into an itty-bitty, zero-volume point.

Surrounding the singularity is an event horizon. The event horizon can be thought of as the line of no escape in the black hole from which nothing that passes over it can ever leave. Matter that passes this point cannot overcome the pull of gravity from the black hole and falls into it. Think of it like the security checkpoint at the airport. Once you pass that point, you can’t turn back.

Little and Big Black Holes

The largest black holes form when a neutron star collides with an existing black hole.
Neutron Star

Black holes come in many sizes, from micro to massive. In fact, there is no theoretical limit on either end of the size scale. Micro black holes, the size of only a pound or two, were around at the beginning of the universe, but scientists are unsure if these still exist. The largest black holes form when an existing black hole collides with a neutron star, producing another, even bigger black hole. Black holes can also grow as they gather, or accrete, matter into them. Astronomers believe that supermassive black holes are at the center of nearly all large galaxies, including our own Milky Way.

Lesson Summary

Black holes, which generally range in size from several kilometers to several times the size of our sun, are the result of supernova explosions of dead stars. Black holes earned their name because they don’t allow light to escape their gravitational pull. They have been theorized to exist since the 1700s and were discovered in the 1960s through observance of the effect they have on their surroundings. Black holes are stellar remnants of massive stars that have collapsed in on themselves, creating a point of zero volume and infinite density, called a singularity. The defining feature of the black hole is the event horizon, a boundary of no return. If anything passes over this horizon, it will never escape the black hole.

Learning Outcomes

After watching this lesson, you should be able to:

  • Describe black holes and how scientists know they exist
  • Understand how a black hole forms
  • Define singularity and event horizon