Everything in space seems to be orbiting something. Moons orbit planets and planets orbit stars. But what are all the stars circling around? The answer: extremely powerful objects called black holes. Black holes are regions of space that have such strong gravity, nothing can escape them. They just pull things close and then suck them in. What happens after, no one can say for sure.
Why are Black Holes Black?
When we say that nothing can escape a black hole, we really mean it. Not even light waves can get out once they’re inside (light waves are the fastest thing in the Universe). This is what makes black holes black.
Note that the color black is just the absence of reflected light. We see it when something absorbs all of the visible light coming at it. A black hole does just that. As a result, it appears as a big black dot. In the blackness of space, that makes them pretty much invisible.
And what enables a black hole to capture light? Simply, gravity. Gravity is a force associated with massive objects. The more massive something is, the harder it tugs on objects around it.
Well, black holes have masses equal to many, many times the mass of our sun. What’s more, all of it is compressed into a single point. We call that point a singularity. Singularities are immeasurably dense with matter. Nothing could ever come close to escaping their gravitational pull. It’s like if all the gravity from stars across many solar systems got put into a single point.
How Do We Know Black Holes Exist?
Now, you might be wondering: If we can’t see black holes, how do we even know they’re real? To be clear, we didn’t know for sure until just a few years ago.
Astronomers could prove mathematically that they should exist. But, they had no direct evidence of them. There was one particular thing they kept looking for, but couldn’t find. That thing was the existence of what are called gravitational waves.
In 1916, the famous physicist Albert Einstein predicted the existence of these waves. He said that they would be given off by the collision of very massive objects. Two, colliding black holes obviously qualify. Still, we weren’t able to observe these waves for a very long time. That changed in 2015.
3 years ago, scientists at the LIGO lab in California claimed to have measured gravitational waves. What’s more, they said, the two things that had collided had gotten very close together. Only black holes could’ve done this because they’re so dense (and so small for their mass).
Since this discovery, the same thing has been observed at a separate laboratory. This is, more or less, a direct confirmation of what Einstein predicted over 100 years ago.
Why Do Black Holes Exist?
While it took us a while to prove that black holes exist, we’ve had a good idea of why they form for a long time. And what could produce these extremely powerful objects? The answer is the second most powerful thing in the Universe: stars.
To be clear, most stars do not become black holes. It’s only those which are very, very massive (about 20x the mass of our Sun). These stars evolve super fast, burning all their fuel just to stay together. Not long after birth, they become what are called neutron stars.
Neutron Star Collapse
Neutron stars are a special type of star with an extremely dense core. The particles that make it up, neutrons, form a very dense mass. It’s just about as dense as things can get, aside from forming a singularity.
And actually, that’s what ends up happening a lot of the time. The outside of the neutron star grows over time, pushing harder and harder on the core. Eventually, the force is too much and it crushes the core into a single point. This is how a black hole is born.
Did You Know?
There’s actually a black hole at the center of the Milky Way. We call it Sagittarius A*, and it’s huge! But don’t worry. It’s also 26,000 light years away.
Other Great Resources:
Black Holes Explained – From Birth to Death:
The ESA on Black Holes: https://www.esa.int/esaKIDSen/SEM0V1BE8JG_OurUniverse_0.html
Even More Black Hole Facts: https://easyscienceforkids.com/all-about-black-holes/
NASA Simple Explanation: https://www.nasa.gov/audience/forstudents/k-4/stories/nasa-knows/what-is-a-black-hole-k4.html