Size of a BH

[obsidianjoy]

This question has been bugging me for awhile. How big are regular black holes? I read that a Super Massive black hole is the size of a speck of dust. Which would make sense since there has to be so much gravity that not even light can escape. But whenever I see shows and movies they show regular black holes as being big. I know tv and movies isnt a good place to get ideas but even some of the educational ones show them that way. So, like I said, it's been bugging me and I'm curious.

 

[doubletruncation]

Typically when you talk about the "size" of a black hole, people refer to something called the "Event Horizon" or "Schwarzschild Radius". If anything gets closer than this distance to the center of the black hole then it can't come back out. There is, however, no surface here (you could pass by it without noticing). The size of the event horizon depends on the mass of the black hole, if m is the mass of the black hole as fraction of the mass of the sun, then the radius in kilometers would be:<br /><br />r ~ 3m<br /><br />so a black hole the mass of the sun would have an event horizon radius of about 3 km (meaning if you came within 3 km of a solar mass black hole you couldn't get out again). Supermassive black holes can be millions of times as massive as the Sun (for the one in our own galaxy) to as much as a few billion times the mass of the Sun (for black holes at the center of very large elliptical galaxies). Those black holes will then have event horizon radii of millions to billions of kilometers.<br /><br />The mass of the black hole is not distributed uniformly within the event horizon (the event horizon is a location that's out in free space), but is instead in an infinitely dense region called the singularity (whether there really is an inifinitely dense region or something else happens is not actually known). If the black hole is not spinning or charged, then formally the singularity does not take up any volume and is instead the precise point at the center of the black hole regardless of its mass. <div class="Discussion_UserSignature"> </div>