In the center of our galaxy, there is a Supermassive black hole. We were told in early astronomy units of school that black holes are giant vacuums that will absorb/consume the area around it. My question is, how do remain in balance with the black hole so that it does not consume us, but that we actually orbit around it?
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3$\begingroup$ How does the earth stay in "balance" with the sun, and is not consumed by it? $\endgroup$– ACuriousMind ♦Commented Feb 2, 2016 at 22:26
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$\begingroup$ Sagittarius A* has about 4.3 million solar masses. The total mass of stars, gas clouds and dark matter in the galaxy between us and this black hole is much, much larger than that, probably by a factor of 10,000 or so. Therefor we are not actually rotating this supermassive black hole but the center of mass of this matter distribution. One could even say that it's not Sagittarius A* that captures the galaxy, but it's the galaxy that is holding Sagittarius A* captive in its gravitational minimum. All Sagittarius A* "can do" is to locally feed on a few stray stars and hydrogen once in a while. $\endgroup$– CuriousOneCommented Feb 2, 2016 at 22:37
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$\begingroup$ Related: physics.stackexchange.com/q/12140/2451 , physics.stackexchange.com/q/9049/2451 and links therein. $\endgroup$– Qmechanic ♦Commented Feb 2, 2016 at 23:37
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2 Answers
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The picture of a black hole being like a huge vacuum is pretty misleading, not to say wrong. The gravitational field around the black hole is exactly the same as around any other object of the same mass. Like any other object, it's perfectly possible to orbit a black hole, just like we orbit the Sun and don't fall in. In fact, if you were to magically replace the Sun with a black hole of the same mass, the Earth's orbit wouldn't change at all.
There are a couple of reasons the vacuum analogy is used. One is that black holes usually have accretion disks, big disks of glowing matter being sucked in. Again, this is no different than what would happen if you had a star instead of a black hole. The other, probably more important reason is that when you cross the event horizon of a black hole, it's impossible to get out. But that's about it. A planet orbiting a black hole (never mind one at the edge of the galaxy) would be in no real danger of being sucked in.
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$\begingroup$ I think the Earth's orbit might be slightly altered if the black hole were spinning at a physically significant speed (i.e., near c), though I am not sure if the effects would still be negligible... $\endgroup$ Commented Feb 4, 2016 at 23:08
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You've the answer in your statement:
...black holes are giant vacuums that will absorb/consume the area around it.
The key is that the area around it is a few times the event horizon of the black hole and we are very far from the event horizon of the supermassive black hole at the center of the Milky Way. The event horizon for Sag A* (the name of the SMBH) is about 0.0041 light years and we sit around 25,000 light years away from it. Since we're so far away, the "giant vacuum" action simply doesn't exist--you may want to read this short article Is a black hole a giant cosmic vacuum cleaner? over at "Hubble Site."
answered Feb 2, 2016 at 22:49
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$\begingroup$ Oh, wow! I didn't realize how small (well in comparison) the event horizon is. Thank you! $\endgroup$– callistoCommented Feb 2, 2016 at 22:53
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1$\begingroup$ Can the downvoters clarify what they feel is wrong with this post so that I can correct it? $\endgroup$ Commented Feb 4, 2016 at 16:17
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$\begingroup$ I up voted for you because I found your answer informational. I feel the down votes may have been the way you answered it. $\endgroup$– callistoCommented Feb 4, 2016 at 20:53
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$\begingroup$ I didn't downvote, but personally I feel your answer is lacking because it doesn't address OP's misconception (like I tried to do with my answer). $\endgroup$– JavierCommented Feb 5, 2016 at 1:47