To answer this question, first we need to look at what black holes are and how they work. A black hole is what remains when a massive star dies. A massive star usually has a core that's at least three times the mass of the sun. Stars are huge, amazing fusion reactors. Because stars are so large and made out of gas, an intense gravitational field is always trying to collapse the star. The fusion reactions happening in the core are like a giant fusion bomb that's trying to explode the star. The balance between the gravitational forces and the explosive forces is what defines the size of the star.
As the star dies, the nuclear fusion reactions stop because the fuel for these reactions gets used up. At the same time, the star's gravity pulls material inward and compresses the core. As the core compresses, it heats up and eventually creates a supernova explosion in which the material and radiation blasts out into space. What remains is the highly compressed and extremely massive core.
This object is now a black hole. It literally disappears from view. Because the core's gravity is so strong, the core sinks through the fabric of space-time, creating a hole. What was the core of the original star now becomes the central part of the black hole -- it's called the singularity. The opening of the hole is called the event horizon.
Types of Black Holes
You can think of the event horizon as the mouth of the black hole. Once something passes the event horizon, it's gone for good. Once inside the event horizon, all events (points in space-time) stop, and nothing -- not even light -- can escape. There are two types of black holes:
The Schwarzschild black hole is the simplest black hole, in which the core doesn't rotate. This type of black hole only has a singularity and an event horizon.
The Kerr black hole, which is probably the more common form in nature, rotates because the star from which it was formed was rotating. When the rotating star collapses, the core continues to rotate, and this carries over to the black hole. The Kerr black hole has the following parts:
- Singularity - the collapsed core
- Event horizon - the opening of the hole
- Ergosphere - an egg-shaped region of distorted space around the event horizon (caused by the spinning of the black hole, which "drags" the space around it)
- Static limit - the boundary between the ergosphere and normal space
Black holes won't consume everything around them. If an object passes into the ergosphere, it can still be ejected from the black hole by gaining energy from the hole's rotation. However, if an object crosses the event horizon, it will be sucked into the black hole and never escape. What happens inside the black hole is unknown.
Black Hole Sun
So, what if the sun were to become a black hole? It turns out that the chances of this actually happening are pretty much nil. The sun's core isn't large enough for it to become a black hole at all. When the sun dies, about 5 billion or so years from now, scientists believe it will expand into a red giant. As this happens, the sun will increase in size and most likely consume Mercury and Venus and possibly Earth. Eventually, millions of years later, the sun will quite literally run out of gas. When this happens, a planetary nebula will form, leaving behind a very dense, mostly carbon core about the size of Earth. At this point, the sun will then be a white dwarf. As its temperature continues to cool, it will eventually become a black dwarf.
Now, just for argument's sake, suppose the sun did become a black hole and the Earth and other planets managed to survive the transformation. Since the sun is a rotating star, its core would continue to rotate, making it a Kerr black hole with an ergosphere. Because the sun's core is very small, the ergosphere would also be small -- so small, in fact, that the planets would probably just continue to orbit in their usual manner. The black hole would have the same mass, and therefore the same gravity, as the original sun. Orbiting planets wouldn't notice any difference.
Obviously, if this were to happen, life as we know it would be changed dramatically -- but for a different reason. A black hole emits no light. Darkness would engulf Earth and it would be extremely cold. The oceans would probably freeze and any existing life forms would die off rather quickly. If humans could get underground with a good way to generate electricity and heat, they might survive. But it would be pretty bleak out on the surface.