Thursday 13 May 2010

star - A black hole that doesn't take in matter?

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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