This article will cover just six of the many interesting aspects of black holes that are waiting for you to discover.
For this article, I won’t be talking about the supermassive black holes which are found at the centers of galaxies. I’ll be talking about the much smaller (and counterintuitively much stronger) stellar black holes. But hey, if you’re interested in learning about the big bruisers, here’s a link.
By the way, there’s some pretty hairy technical jargon flying around in the more academic circles of black hole theory.
The links I provide here will all be to Wikipedia articles, only because they don’t bombard you with advertisements. However, their articles tend to be on the technical side. So if you want less nerdgasm and a more readable explanation of the topics I link to here, Google is your friend!
It’s my aim with this article to simplify the topic to make it more approachable for a wider, less nerdy audience. So nerds (you know who you are), please forgive me for any errors of omission or less than precise turns of phrase!
1) How a Black Hole is Created
All stars are not created equal. For a star to be capable of becoming a stellar black hole, it has to be of a certain minimum mass. There are multiple opinions in the astrophysics community concerning what that minimum is.
You don’t have to worry about our Sun becoming a black hole. Sorry, but our nearest star doesn’t qualify. Too small. The most old Sol can expect is to expand into a red giant in 5 or 6 billion years before it collapses into a white dwarf. The minimum size for a star to become a black hole is roughly 4-5 solar masses.
Basically, a star needs to be massive enough to become a supernova when its fuel runs out and the star’s core collapses under its own gravity.
Except for a Type Ia supernova, all supernovae are caused by the gravitational collapse of a very massive star’s core, which happens at the end of the star’s life and it can no longer sustain the nuclear fusion that keeps it from collapsing under its own weight. Depending on other factors, the star will then become either a neutron star or a black hole.
2) Accretion Disk – All The Best Black Holes Have The Hottest Bling
An accretion disk is often found surrounding a massive celestial body, such as a star. Black holes, being pretty darn massive, are no exception.
The disk consists of diffuse material, such as gas and dust that has fallen into the central body’s gravitational influence.
As the material in the disk loses some of its angular momentum due to friction caused by rubbing against and bouncing off of other nearby particles, it begins to heat up as it falls lower in its orbit around the black hole.
Though scientists are unsure why, the accretion disks of some bodies, such as black holes, emit jets of radiation along their polar axes.
Less dense bodies, such as young stars and protostars radiate in the infrared band of the electromagnetic spectrum. But the gravitational forces of black holes are so intense that they radiate in the X-ray band. This is one of the tell-tale signs used to find a black hole.
One of the better known components of a black hole is its Event Horizon.
A black hole’s event horizon is either a spherical or an oblate spheroid region of space which surrounds the mass of the black hole. It doesn’t have any physical properties, per se.
The radius of the event horizon is determined by how massive the original star was, minus whatever mass was thrown off by the supernova explosion.
An event horizon’s radius is a property of the amount of mass contained within the black hole. But in a sense, its not physically “there”.
You can’t “see” it as you approach it. But it certainly has a gravitational effect on any matter which has the audacity to cross that line!
The beaten-to-death cliche for black holes is that “its gravity is so strong that nothing can escape it, not even light!”
Granted, it’s a reasonable description if not completely accurate. But its definitely a cliche.
Actually, how close you get to the black hole determines whether you’ll return home to tell the tale. When matter crosses the event horizon, then there’s no turning back.
At the point you cross the event horizon, to escape the black hole’s gravitational attraction, you’d have to be traveling faster than the speed of light, because at the event horizon, the escape velocity is the speed of light. And the Relativity Cops say that’s a speed limit you can’t break.
But before you get to that point, you’ll have other things to worry about.
4) Spaghettification – Believe Me, The Sauce is Gross!
The late great astrophysicist Stephen Hawking popularized the term “spaghettification”.
As an object approaches a very strong gravitational field, it is subjected to extreme tidal forces which stretch the object vertically and compress it horizontally.
A solid object will, of course, resist this attempt to stretch it into a noodle. But in the case of black holes, as the object gets closer to it, the tidal force increases to such an extreme that it is stretched and compressed beyond its ability to resist.
For example, if you were approaching a black hole, feet-first in your spaceship, the gravity gradient (the difference in gravitational force exerted between one side of an object and the other) would pull on your feet with more force than it pulled on your head, thereby stretching you out like a string of spaghetti!
Eventually, you’ll get to the point where your body can’t resist the stretching and you (and your ship) would be torn apart, long before you got anywhere near the event horizon.
5) Singularity – The Devil in the Dark
The culprit causing all this mayhem is the singularity at the black hole’s center. It’s the very reason the black hole exists at all! As I mentioned earlier, the mass of the original star determines the radius of the event horizon, but what happens to that mass is mind-blowing!
The reason a singularity is called “a singularity” is because all of the mass that was once contained in the star is now collapsed into a single point. I’m not talking about a point the size of a house or even the size of the period at the end of this sentence. I’m talking about a zero-dimensional geometric point!
This is the aspect of black holes that have astrophysicists pulling their hair out. They know that singularities exist, but they don’t know how they exist! All of the star’s mass has been squeezed into a point of infinite density by infinite gravity while spacetime has reached infinite curvature.
In other words, at the singularity, time and space do not exist as we know them and our current understanding of the laws of physics do not apply.
Unlike stars, planets, galaxies, nebulae, etc., black holes don’t shine or reflect any light. (They are called black holes, after all!) They can only be found indirectly by finding their accretion disks, or by how they affect other nearby celestial objects.
They were predicted to exist by Einstein’s General Theory of Relativity in 1915, but none were discovered until 1971.
So, next time you take a peek through a telescope or binoculars, don’t expect to see one. But if you’re as fascinated by black holes as many professional and amateur astronomers are, that shouldn’t stop you from researching them and enjoying all the fascinating details about the latest discoveries concerning black holes!
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