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According to List of Common Misconceptions by Wikipedia, under the heading of astronomy, this line can be found:

A black hole can act like a "cosmic vacuum cleaner" and pull a substantial inflow of matter, but only if the star it forms from is already having a similar effect on surrounding matter.

This is referenced to a paper written in Yale university: "Frontiers And Controversies In Astrophysics Transcript 9", although that's a dead link. By linking so, I believe that it is a reliable source.

This got me thinking that is there a black hole that doesn't suck in surrounding matter? And is there a star that don't take in surrounding matter?

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This is the full quote

If, for example, the Sun were replaced by a black hole of equal mass, the orbits of the planets would be essentially unaffected. A black hole can act like a "cosmic vacuum cleaner" and pull a substantial inflow of matter, but only if the star it forms from is already having a similar effect on surrounding matter

It's badly written, but what it's basically saying is that it doesn't matter the form of the massive object that's being orbited by a planet or other object. A stable orbit would remain stable, so, if our sun was to collapse into a black hole and nothing else changes, Earth's orbit would remain the same. It would get dark and cold, but the orbit would be unchanged.

The misconception is, if the sun became a black hole than Earth would be sucked into the sun. That's 100% false and that's all they're saying.

Gravity is a function of mass and distance. Black holes have very high gravity, but a big part of the reason for that is because when they form out of dying stars, they become very small, a few solar masses squeezed into only about 10 miles across so the distance to the center of mass gets very small.

For a planet to be in any danger from a black hole, it would probably need to be inside the Roche Limit, perhaps only a few million miles away, which is several times closer than Mercury is to the sun, for example. The safe distance, of-course, varies with the density and solidity of the orbiting object.

Here's a Q on Roche Limits and Black holes if interested.

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  • $\begingroup$ So basically it is bad wording and I have misinterpreted it. Am I right? $\endgroup$ – CipherBot Nov 27 '15 at 9:29
  • $\begingroup$ I think so, but I'm not 100% clear on your wording. Gravity always attracts (always sucks if you like) and whether an object orbits, escapes or falls into depends on tangential velocity vs gravitation. If there's no tangential velocity the object would fall in from a very far distance, but there's almost always some. That's why orbiting is a common outcome between 2 objects, even around a black hole. When objects get quite close to a black hole, then the rules start to change due to relativity, but I'm not expert in that area. I'm sorry if I'm not explaining it well. $\endgroup$ – userLTK Nov 27 '15 at 9:43
  • $\begingroup$ Ah ok I get it. It is just that that wording of the quote says: 'A black hole can act like a...' just got me confused that it has other types of black holes that don't take in matter. $\endgroup$ – CipherBot Nov 27 '15 at 10:26

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