A Low Density Black Hole

By Anupum Pant

Imagine you have two equal sized balls of clay. You mix both of them together to get a bigger ball of clay. Now as simple math dictates, the mass of this new ball will be twice the mass that you had in each ball initially. However, the radius of this new ball is clearly only slightly more than the initial ball. Had you wanted to make a ball with the radius twice as much as the initial ball, you would have needed to club 8 of the small balls.

Black holes do not work that way. The event horizon, a kind of one-way membrane from which not even light can escape is like a boundary for the black hole. Let’s say this defines the size of a black hole.

The funny thing about the size of a black hole is that if you double the mass of a black hole, the size doubles. The physics of it is complex, unlike anything of a clay ball. So, just believe me when I say that doubling the mass doubles the radius.

When such things happen, there are weird results. That means, if you keep adding mass to a black hole, and at some point it reaches a really massive mass, say as much as Billion times that of our sun – That’s not unheard of, it happens. If you get your calculations right, you’ll find that in a black hole like that one, the density will be really low. A black hole of that size would span from the center of the sun to the orbit of Neptune. And the density of it would be around 1/1000 of a gram per cc. That, if you did not know, is also the density of air. That’s how low density black holes can be. Probably even lower, if they are bigger…

A very very big sphere of a radius 2.7 billion miles filled with air would be a black hole. Hard to believe, but it’s true.

via [BadAstronomy]

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