Black hole misconception

Nope, a black hole is not a cosmic vacuum cleaner.

Wikipedia explains, in a list of common misconceptions:

"The gravity of a black hole is slightly weaker than, not stronger than, the gravity of the star which formed it (at distances greater than the star's radius). Isaac Newton's laws of gravitation state that, for an object with a spherically symmetric distribution of mass, two things affect how much gravitational force is felt by an observer: the mass of the object and the distance between the observer and the object's center of mass. A black hole has slightly less mass than the star which formed it, because when a star becomes a supernova, some of the star's mass is converted into energy according to Einstein's equation E = mc², and a great deal of the star's mass is returned to the interstellar medium. Only when a distance of (slightly less than) the star's original radius is passed does the force of gravity become greater. The event horizon is usually much smaller than the original star's radius. As such, black holes are not similar to "cosmic vacuum cleaners". Objects can settle into stable orbits around them just as they would around any other mass in space, including stars."

The entire list of common misconceptions on various subjects can be found here.

[Image: Wikimedia Commons]

black hole, cosmology, relativity

Comments

[Elizabeth Joseph]

I'm sorry but shouldn'tthe gravity of the black hole eventually become greater than the original star as black holes are of infinite mass as its volume des not change and whatever goes in never comes out?

[anonymous]

Black holes are not of infinite mass, but their density would approach infinity (the exact mathematics to work out these values is beyond current science, a theory of quantum gravity would fix that).
Their mass is less than the star that collapsed (assuming it formed through a star going super-nova). It's gravity can increase as objects fall into it, but this happens rather slowly (at the near same rate objects previously fell into the star).