Tag Archives: stellar explosion

Black Holes and Density

 A question asked of me by klcrace earlier inspired this article. 

Black Holes and Density.

Here is a good source of information on black holes:
http://en.wikipedia.org/wiki/Black_hole

Our atoms are mostly empty space, lots of room for things to fit into. Normally, the fields/forces around atoms keep them relatively far apart and the spaces in between remain mostly empty.  All mass have gravitational effects on surrounding masses and the more mass the higher those effects, but as long as the density remains below a certain point there is no black hole. It is not so much the mass that makes a black hole but the density – mass in a tiny space.

As a star gains mass, it’s outer mass compresses the inner material but the internal pressure keeps the atoms apart and the interior spaces empty. When a star grows too much or its internal pressure decreases because its fuel is depleted, it can collapse and when the density reaches a certain point, it technically becomes a “black hole”, one in which the gravitational pull is too great for light to escape. This is often accomplished by a stellar explosion that implodes the mass toward the center of the star, greatly increasing its density.

The mass is still the same but it is concentrated in such a small space (essentially zero space) that the density is enormous (essentially infinite), and the space around it is so severely warped that light does not get out. The density is self sustaining because the gravity of the masses within it is high enough to keep it together, except after much evaporation.

It is thought that much smaller masses can also be compressed by outside forces to the point that the small mass achieves black hole density, but none are known to exist. It would take enormous energy to accomplish this.

For example if you were able to compress a baseball enough, it would become a micro black hole. But it would have to be compressed so much that its outer radius would be essentially zero (much smaller than an atom).  The mass would still be that of a baseball, but the density would be so high that light could not get away from it. Space would be severely warped around this very tiny black hole, but only very very near its center, probably too small to be detected.  Gravity from such a source at the distance you would normally pick up the ball would be no more than for a regular baseball because the mass is still a baseball mass. If you could weigh it, (a real problem) it would weigh the same. It is the ratio of the mass to the radius that is important. Make the mass high enough and/or compress to an extremely small radius and you have a black hole.

Known and predicted black holes contain mass greater than our sun. Sometimes thousands and even billions of times greater. Yet the size of the space occupied by the mass in the black hole is still essentially zero. The gravity around such large masses is extremely high and will capture all light out to a certain radius, the event horizon. The event horizon can be far from the central mass depending on the amount of mass. The capture range is usually further out than the size of the central mass, and grows as the black hole captures more and more mass while the central region does not grow measurably, if at all.

Black holes can evaporate and if there is no nearby mass that it can capture to replenish itself with, a black hole could evaporate to a smaller mass than what was required to establish it. Smaller in mass than our own sun.

Black holes do typically have very high gravitational pulls, proportional to the mass inside and inversely to the distance to the center.  But a micro black hole could theoretically be floating around a lab that created it (such as with a high energy accelerator) and never be noticed as it would likely evaporate before it hit anything and even if it did encounter a part of the lab, it would be so tiny and have such small gravitational pull at atomic-size distances that it would not capture more “stuff” needed to grow. Likely it would just poof out of existence unnoticed.

At least I hope so.

What I want to emphasize it that any given mass has a certain gravitational pull at any given distance, black hole or not. Super-massive black holes at say 1000 light years away have the same gravitational effects as that of a group of stars of the same total mass that are close together (but not close enough to be a black hole) at the same 1000 light year distance. The difference is the black hole has much higher concentration of mass, occupies much less volume, and also warps space much more tightly than the group of stars with the same mass. The stars shine brightly and the black hole is, …well, black.

Oldtimer