Re: I was wondering what shape a black hole was, are they always round, 2D/3D?

Actually, a black hole doesn't have any shape at all!

But that's getting ahead of myself. Let me first say a bit about the
anatomy of these strange creatures. 

A black hole is a point of infinite density that arises from the collapse
of a supermassive star. What happens is that the force holding up the star
vanishes when the star's fuel runs out. When that occurs, gravity squeezes
the star from all sides, causing a type II supernova explosion when the
upper layers of the star bounce off the superdense core. 

The ensuing physics is pretty dicey and not well-understood, but
astronomers believe that if the star core has a mass greater than 8 times
that of the Sun, no force in the universe is sufficient to keep it from
collapsing all the way to a singularity. This a mathematical abstraction, a
point with a finite mass, but zero volume. 

Remember your high school physics classes, where you learned that density
is the mass of an object divided by its volume?  That equation applies
here. Finite mass divided by zero volume gives infinite density. That's a
singularity, the core of a black hole. And that's why it has no shape. A
point is a point.

But singularities don't just sit around in space all by themselves. Each
one is surrounded by a sphere called the event horizon, which is the region
around a black hole from which nothing, not even light, can escape. The
radius of the event horizon is defined by the mass of the black hole. 

If our Sun were to turn into a black hole (aside: this is impossible,
because the Sun's mass is far too small, but let's say some particularly
nasty aliens came along and zapped the Sun with a black-hole-producing ray)
the event horizon would only be 3 km in radius. What's more, all the
planets would continue in their same orbits. None of them would get "sucked
in". 

[Moderator's Note:
Denise is completely correct; a stationary black hole is a single point 
surrounded by a spherical event horizon. If the star that forms the black
hole is rotating, though, the black hole will also spin (because of something
known as the conservation of angular momentum). In this case, the event horizon
would be shaped more like a flattened sphere.

Jim O'Donnell, MadSci Admin]