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Black Hole Misconception: The Cosmic Vacuum Cleaner?

March 16, 2014 Astrophysics 10621
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Many people view black holes as a massive cosmic recycling centers. I know that, when I was younger, I always thought of a black hole as being the universe’s vacuum cleaner, sucking up everything within its path. As dramatic and as cool as it may sound, it is not quite true. I am afraid that this is a common misconception about black holes.

When we look out into the universe, we find that most galaxies have a supermassive black hole at their cores. The galaxies that don’t are mostly dwarf galaxies or irregular galaxies. Although there are plenty of cases where a dwarf galaxy has a black hole at its core, a general rule of thumb is that an irregular galaxy doesn’t, though there are some exceptions.

So should we expect to be eventually sucked into our supermassive black hole some day? Well, I wouldn’t hold your breath or wait around. Our sun and some 200 billion other stars have been in a pretty well defined orbit around the center of the Milky Way for billions of years and would continue for indefinitely if it weren’t for the eventual collision with Andromeda.

The reason for this is that the same physics that determines the orbit of planets around the sun also governs the movements of stars around the center of the Milky Way. Everything in our solar system orbits around the sun, and has been doing the same for billions of years. This is because the sun is pretty much the same mass as it was 4.7 billion years ago. If you were to replace our sun with a black hole of the same mass the Earth and all the other planets would continue to do what they’ve been doing, the only real difference would be that life wouldn’t be able to exist because it would be very cold without the sun’s warmth. In short, a black hole is just a mass like any other.

When two galaxies collide, however, the supermassive black holes at their centers eventually merge. It may take a billion or more years, but it’ll eventually happen. Two enormous masses just aren’t going to happily orbit one another. After the eventual merge, however, things settle down again, stars resume a stable orbit around the now larger supermassive black hole and the galaxy goes on.

Although stars and gas that aren’t near the black hole are quite safe, that is not always the case for things that are near the black hole. Stars can and do have quite stable orbits near the center of the galaxy, but the active supermassive black holes do have an accretion disk that is very hot and emits a lot of energy. We can generally say that anything in the accretion disk around a black hole is eventually going to fall into it. The reason for this is that it is losing gravitational energy, where as a star orbiting outside the accretion disk isn’t.

In the accretion disk atoms are moving very quickly and are constantly colliding into other atoms quite violently. This causes the accretion disk to become very hot. This heat means that the atoms are releasing a lot of energy, a lot in the form of X-rays and radio waves. When atoms release this energy they lose some gravitational energy and move in closer to the black hole.

As you can see, black holes in general are not giant vacuums that will eventually suck everything caught in their gravitational influence. Most of the galaxy is in a nice stable orbit and would continue to be if it weren’t for galactic collisions. Black holes are just a mass like any other, the difference being that if you get too close to them, you may lose too much gravitational energy to continue your nice stable orbit.

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22 Comments

  1. Evan Conas R. Allen January 20, 2013 at 8:59 am -

    But i have read that eventually, if the sun remained constant, we and all the other planets would be sucked into it, over billions and billions of years. Kind of like those funnels that you put the coins in, they roll around it but eventually fall in the center. If this were true would that not meant the same for a sun orbiting a black hole? Only on a much larger and slower scale?

    Reply
    • Miguel August 31, 2013 at 4:21 pm -

      Good point but it simply won’t happen. Imagine there’s any given object and any given orbit around it. The object going around it gets caught in its gravity, and is always being sucked towards it, but its forward momentum means that as it’s being sucked towards it, it is constantly curving (thus forming an orbit). If that forward momentum were to cease (for example, with an object moving in the opposite direction), it would move in closer. In space there is no medium such as we have on Earth to break up this momentum, so our sun will continue to orbit the black hole unless something drastically reduces its relative speed. In the case of the accretion disk mentioned, collisions with other atoms are the culprit.

      Reply
  2. Lorenzo Cervantes March 16, 2014 at 9:21 am -

    Instead of vacuum cleaners, I rather prefer to think of blackholes as organizers of matter,…and storage for information.

    Reply
    • Raptor March 17, 2014 at 9:09 am -

      I quite agree, they are at work reducing entropy of the universe, and the universes best data source might be the event horizon, imagine all the information ever since the conception of the black hole will exist there, which could be extracted by evaporising the hole.

      Reply
  3. Travis Besst March 16, 2014 at 9:24 am -

    They are objects, not holes or portholes. Its really very simple. All the matter is right there in plain view, just really compact and dense. Like an excellent fudge.

    Reply
  4. Travis Besst March 16, 2014 at 9:24 am -

    They are objects, not holes or portholes. Its really very simple. All the matter is right there in plain view, just really compact and dense. Like an excellent fudge.

    Reply
  5. Travis Besst March 16, 2014 at 9:24 am -

    They are objects, not holes or portholes. Its really very simple. All the matter is right there in plain view, just really compact and dense. Like an excellent fudge.

    Reply
  6. go hawks March 18, 2014 at 11:05 am -

    Super dimple in the fabric of space. They might not suck in the entire universe, but once you fall in, nothing gets out.

    Reply
  7. sfg March 23, 2014 at 12:16 pm -

    gfgf

    Reply

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