Black holes are some of the most fascinating celestial bodies in the universe. Most of us have probably heard of black holes before, but hardly anyone knows what they really are, even scientists! In this blog we will go over the basics needed to understand black holes such as how they are measured, how big they really are, and the differences between mass, size, and density! This will be the background information for part two of this blog which will explain how black holes are created and their anatomy. Enjoy!
Mass vs. size
It is important to understand that mass does not equal size. Mass is the amount of matter within an object, which is also not the same as density. Density is the amount of mass inside a given volume. To visualize it, imagine a pound of gold and a pound of feathers; they have the same mass because they both weigh one pound on Earth, but they take up different amounts of space, so they have different densities. It is also important to note that more mass equals more gravitational pull. Then there's size which is simply how big something is.
To measure the extent to which a celestial object would need to be compressed to become a black hole, we use something called Schwarzschild radius. In order to understand this, you must know what escape velocity is; the amount of speed necessary for an object to escape another's gravitational pull.
Schwarzschild radius is a theoretical radius applied to any object with mass that determines when the escape velocity of the object would be the speed of light. If the actual radius of the object is larger than the Schwarzschild radius than it is not a black hole, if it is smaller or equal to its Schwarzschild radius, then the gravity becomes so extremely intense that the speed of light cannot escape it, making it a black hole, and this is also partially where black holes get their name.
The first photo of a black hole, M87. A supermassive black hole at the center of another galaxy. Source
When an object is at or within its Schwarzschild radius, the object cannot omit or reflect any light, making it "black", and this is not so much because the escape velocity is faster than the speed of light, scientists are quite certain that is impossible, it is because a black hole bends time and direction so intensely, causing light to flow straight to the theoretical center, where it cannot reflect, so a black hole isn't really black, it's actually invisible!
It should also be noted that not every object can become a black hole, it is only objects, typically dying stars, with large enough masses. The Earth, for example, could not become a black hole, even though a Schwarzschild radius can be theorized because it lacks the mass necessary to form one.
How big really is a black hole?
We've established that black holes are very massive(containing a lot of mass), but it is also important to keep in mind that they are very dense. This means that a black hole with the mass of the sun would only have about a 1.9 mile radius! Many people think of black holes as incomprehensibly large, and while they are indeed very big, they aren't quite as big as you might think.
The largest black hole we've discovered is called TON 618. It is 66 billion times the mass of the sun and its radius is around 67 times the orbit of Uranus, which is about 1,300 astronomical units, and one astronomical unit is 93 million miles(the distance from the Earth to the sun)! This is definitely pretty mind blowing, but TON 618 is actually quite far from being the largest thing in the universe!
Black holes can vary in sizes greatly. So there is no definitive answer for how large or small they can be, especially considering how there are still types of black holes that are purely theoretical! We'll discuss that more in part two.
Why do we use the sun's mass as a baseline measurement?
The final bit of information that is important to understand before you know more about black holes is why we use the sun as a baseline measurement for mass. Typically we use kilograms when measuring the mass of an object, but when referring to celestial objects we use solar mass, which is the mass of the sun. We do this because the sun has the strongest gravitational pull in the solar system, and its mass is already a huge number in kilograms, so it makes far more sense to use the sun as a baseline rather than counting, say, the mass of a black hole in kilograms.
Conclusion
Black holes are incredible objects that carry extreme power. Hearing all of this can understandably cause some anxiety, so I'll leave you with this last bit of information: the black hole closest to us is called Gaia BH1, which is 1,560 light years away in the constellation Ophiuchus. This is more than far enough away to not be a threat to our solar system in any way.
Learning about outer space can be overwhelming, there's a lot of information to take in and it can sound scary at times, but it's important to understand that we are safe, and we will be for a very long time. So feel free to enjoy space as much as you can! there's an infinite amount of incredible possibilities out there, and we are in a great position to explore them. I hope you found this information fascinating! Till next time, stay curious!
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