You’ve probably heard of black holes, those mysterious cosmic vacuum cleaners that tear apart and suck up everything around them. These exotic objects make for excellent science fiction and have a reputation for being incredibly complicated. While they can live up to their complex reputation, at a basic level they are actually not too difficult to wrap your head around!
The black hole from the blockbuster Interstellar, which hired astrophysics guru Kip Thorne as a consultant to keep scientific accuracy through much of the movie.
Jumping up in the air on Earth is a short-lived journey. An average person starts their upward flight with a speed of about 7 miles per hour. Our planet’s gravity quickly overwhelms that momentum, and the jumper lands. However, if someone could jump at 25,000 miles per hour, they could escape Earth’s gravitational pull and continue into space! Every planet and star has a special speed requirement to escape its gravity. We call this speed the escape velocity. Larger objects have higher escape velocities; it would take a monstrous 133,000mph takeoff to break free of Jupiter’s gravitational pull. The sun would shut down any jump slower than 1.4 million mph!
A black hole has so much gravity that not even light, the fastest thing in the universe, can escape it. Light travels at a whopping 670,000,000 mph. As we have seen, the bigger the planet or star, the faster something has to go to overcome its gravity. So black holes must be HUGE, right? Well, sort of.
Everything you have ever seen on Earth is made out of atoms. While many people are aware that atoms are made up of protons, neutrons, and electrons, it might be more accurate to say they are made up of nothing. The most common atom in the universe is hydrogen. It is made up of one proton and one electron and is 99.9999999996% completely empty space. To think of it another way, if a hydrogen atom were the size of our planet, the proton would be just over a tenth of a mile wide, the electron would be about three inches across, and they would be separated by about 4000 miles. Most of our universe is empty!
Classic diagram of an atom. All of the parts are drawn FAR too large, which makes sense because if they were to scale they would all be too small to see! Image credit: Sparkfun
Black holes have a LOT of mass, which is why they have so much gravity. So much, in fact, that atoms are actually crushed to fill in the empty space. Sometimes a dying star has enough mass (and gravity) to crush atoms, but not quite enough to keep light from escaping. In these cases, a neutron star is born. These strange objects contain as much mass as the sun, but are squeezed into a space smaller than New York City. Put another way, a soup can of the stuff would weigh about as much as the mountain that AstroCamp lives on!
Black holes are so massive that not even light can break free from their gravity. Inside a black hole, the immense gravitational pull crushes atoms and even neutrons themselves down into a tiny speck called a singularity. This tiny point of matter is even smaller than an atom. It can range tremendously in mass, from about twice as heavy as the sun for a smaller black hole, to millions of times the mass of the sun!
Everything that we know about space comes from the light that galaxies and stars and other things give off. However, black holes don’t let light escape, so how do we find them? Well, there are a couple of ways. One is to wait for the black hole to get in between us and a distant object. Since gravity can bend light, this results in gravitational lensing, where the black hole distorts images of the things behind it, a bit like a carnival mirror.
Simulation of a black hole causing gravitational lensing on the Milky Way. Note that it is not actually moving the stars, just bending the light to change how we see it! Credit: Andrew Hamilton
The black hole at the center of our galaxy was found another way: by looking at how stars in its neighborhood are moving. They whiz around in circles as if pulled by an immense central object, but we can’t see anything there. Calculating the mass needed to move the stars that fast reveals the invisible culprit: a black hole! See for yourself: