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SciShow Space explores the supermassive black hole spinning at the center of our galaxy, and how we’ve all learned to live with it in harmony.

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Sources:
http://www.sciencemag.org/content/345/6202/1330.abstract
http://science.nasa.gov/astrophysics/focus-areas/black-holes/
http://eagle.phys.utk.edu/guidry/astro490/lectures/lecture490_ch13.pdf
(Intro)

Twenty-six thousand light years away a supermassive black hole is spinning at the center of our galaxy. A black hole as wide as the orbit of mercury. A black hole more than four million times as massive as the sun. And, everything in the galaxy is swirling around it. So, does that mean that the black hole is dragging everything into it until there's nothing left, like a cosmic drain in the great space bathtub that we call the Milky Way?

In short, no. Black holes have gotten a bad rap, so let's set some things straight. A black hole is just an object that is so dense that as you approach it, the velocity you need to break free of it's gravitation--known as escape velocity--exceeds the speed of light. This means, that within a certain distance from the center of the black hole, not even light travels fast enough to escape, and nothing can travel faster than light, so nothing can escape. 

And while that might sound a little bit terrifying to you, the first thing you should know about our resident black hole, which is named Sagittarius A*, is that it's not terribly big, for it's kind. Black holes are generally divided into two classes that are hugely different in size. First, there are stellar-mass black holes, which have the mass of several, to maybe, a few dozen suns. These are formed from supernova explosions of giant stars at the end of their life times. Then, there are supermassive black holes, which include the one at the center of our galaxy.

Astronomers don't know for sure how supermassive black holes form, but ours is actually pretty small, just four million times the mass of the sun, compared to those at the centers of some other galaxies, which are more than 100 million times the mass of the sun. So, that makes our black hole sound nice and non-threatening, right?

But the much more important fact is a black holes gravity doesn't just suck things in like some sort of cosmic vacuum cleaner. Most things in our galaxy are so far away from it that they barely even feel the black hole's gravitation. In fact here on earth the gravitational pull of a person standing 30 feet away from you is 10,000 times stronger than the gravitation pull of our galaxies distant supermassive black hole. And even objects that are a lot closer to it will most often just fall into an elliptical orbit around it.

so the fact is our galaxy is orbiting Sagittarius a* just like we are orbiting the sun, but we're not getting drawn into it. But, I'm not saying that hanging out near a black hole is gonna be pleasant or even comfortable. In fact I don't recommend it at all.

This is partly because to the effect created by clouds of gas that form around black holes called accretion discs. These discs of gas become super-heated producing strong emissions at x-ray and radio wavelengths. So in the immediate area of a typical super massive black hole there is a lot of rather unfriendly high energy radiation flying around. 

And another concern if you happen to find yourself in the metro area of a black hole is tidal forces. These forces are exerted because one side of a body like a planet or moon is closer to the massive object than the other side. That closer side feels a stronger gravitational pull than  the far one does and the closer that body gets to the massive object, the bigger the difference becomes. Eventually this difference becomes so great that it causes the body to start stretching out.

Now the boundary that stakes out the maximum danger zone around any black hole is its event horizon. This is the distance from the center where the escape velocity equals the speed of light. 

For our black hole the event horizon is about 30% the average distance between the earth and the sun. So outside that range and object orbiting a black hole would be far enough away that the tidal forces wouldn't do too much harm.

But, if the object manages to cross the event horizon then, basically, all bets are off. The side that's closer to the black hole would begin to accelerate faster than the farther side, creating an inescapable stretching force that physicists rather delightfully refer to as spaghettification.

So yes if you found yourself inside the event horizon, your noodle would essentially be cooked. 

None the less, you don't have to worry your intelligent little head about any of that because our Galaxy is orbiting Sagittarius A* you can rest assured that the sun, the earth and everything else is going to keep on turning, blissfully unaware of the super massive black hole at the heart of our galaxy.

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