Facts, and Some Myths, about Black Holes

FROM THE LECTURE SERIES: Understanding the Misconceptions of Science

By Don Lincoln, Ph.D., Fermi National Accelerator Laboratory

Black holes are real. They might be invisible but the evidence for their existence is overwhelming. We still do not have complete details of what happens inside black holes. Researchers continue to investigate, explore and discover the mechanics inside them. However, that doesn’t mean we do not know anything at all, so let us peek into these bottomless pits and try to understand them.

Black hole in a nebula gravitational field.
The baffling bottomless pits in the nebula are called black holes where matter is compressed into a tiny space and a massive gravitational pull is experienced. (Image: Jurik Peter/Shutterstock)

A black hole is a place in space where matter is compressed in a tiny space and the gravitational pull is so strong that even light cannot escape. When a gigantic star collapses in a catastrophic supernova, a huge amount of stellar remnant is left behind. The remnants collapse to the point of singularity where all of the mass of the black hole is concentrated at the center in a sphere of zero radius and zero volume.

Learn more about why black holes get such a bad rep.

Formation of Black Holes

We know that black holes are formed from the death of a star. When huge quantities of hydrogen and helium gas condense under gravity, nuclear fusion occurs. After millions of years, the pressure and density of these gases build up so much that the temperature is high enough to generate fusion. Hydrogen is converted to helium during the fusion process and once the hydrogen is exhausted, other types of fusion involving heavier elements begin.

The relativity theory concept showing a sketch of a 
wormhole and the relation between 
time, energy, mass and light speed
The relativity theory concept describes a wormhole and the relation between
time, energy, mass, and light speed.
(Image: udaix/Shutterstock)

Initially the fusion heats up the star but once the fuel drains, the core of the star collapses. When gravity pulls the mass of the star to the center, the matter that was flowing inwards bounces off the core and finally the star explodes in a supernova that is visible across the cosmos.

Every galaxy has a huge black hole at its center. These black holes are enclosed in clouds of non-rotating hydrogen gas. As the clouds are not rotating, the gas would drop into the hole analogous to a ball falling to the ground. Once the gas entered the hole, it would start heating up due to friction with other gases. As a result, the black hole would seem to be glowing and consuming gas.  Over millions of years, the holes would grow larger and seldom would an unlucky star be gobbled by the hole.

This is a transcript from the video series Understanding the Misconceptions of Science. Watch it now, on The Great Courses Plus.

The Schwarzschild Radius

After the explosion, there is some mass left over at the center of the star and that mass depends on the mass of the original star. For instance, if the original star was more than 20 solar masses, the black hole formed is at least three times the size of the sun. Every galaxy has an enormous black hole at its center and the biggest black hole ever measured has a mass of about 66 billion solar masses. There is a monstrous black hole at the center of our Milky Way too with a mass of four million suns.

Illustration of a supermassive black hole devouring matter in the event horizon,  from which radius even light cannot escape.
The closer someone is to the black hole, the greater would be the speed required to escape the massive gravitational pull.
(Image: XMM-Newton, ESA, NASA/Public domain)

The size of black holes can be described using the concepts of event horizon and singularity. The event horizon is the radius of a sphere defining the space around the black hole which does not allow even light to escape. It is the sphere with a special radius around a black hole from which nothing can escape, not even light. This special radius was calculated by German physicist Karl Schwarzschild.

The Schwarzschild radius as it came to be known was calculated as a function of the object’s mass. Though the Schwarzschild radius of typical black holes are in the range of 10 to 50 kilometers or 6 to 30 miles, this radius for any astronomic body is minuscule in comparison to its size. For instance, the Schwarzschild radius of the Earth is only about a centimeter while that of the sun is about three kilometers or two miles.

Based on the distance from the center of the black hole, the regions near the black hole can be divided as the distances much larger than the Schwarzschild radius, distances close to but outside the Schwarzschild radius, distances inside the event horizon but not close to the singularity and finally areas near singularity. The closer someone is to the black hole, the greater would be the speed required to escape the massive gravitational pull. At a certain point close to the black hole, the speed to escape the gravitational pull would have to exceed the speed of light, which is impossible. 

Learn more about untangling how quantum mechanics work.

Myths about Black Holes

Black holes are badly misunderstood and there are several misconceptions with respect to these cosmic bodies. One such common myth is that black holes are ravenous monsters that capture anything in their gravitational field. This is partly true but closer to being false. Similarly, it is believed that black holes are everlasting but it is false. It is also incorrect that black holes can connect to one another and can travel in the universe as wormholes. Another common misconception is that black holes are attached to white holes, with the black holes acting like cosmic vacuum cleaners. However, the idea that black holes can evaporate and that they are remnants of gigantic stars are true.

Common Questions about Facts about Black Holes

Q: What are the different types of black holes?

The four different types of black holes include stellar, intermediate, supermassive, and miniature.

Q: What are active galactic nuclei?

The enormous black hole at the center of the galaxy is consuming gas, which means that the central region is more luminous then the rest of the galaxy. These nuclei in the central region are called active galactic nuclei. However, the process of galactic black holes consuming gas seems to be common at great distances, the closest being about 13 million light years away.

Q: What was the misconception about black holes in the 1960s?

In the 1960s and 70s, the black holes were ill-conceived as white holes. It was thought that these were places where matter shot up to the universe. It was even misunderstood that these might be the other side of black holes or the place where matter falling into black holes settled down.

Q: What is the heaviest object in the universe?

Black holes are the heaviest objects in the Universe, so heavy that all known laws of physics do not seem to work for black holes. The heaviest known black hole has a mass of 66 billion solar masses.

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