The following questions were answered by astronomer Dr. Cathy Imhoff of the Space Telescope Science Institute.


How does a black hole form?

We think that a black hole can form when a very massive star collapses at the end of its lifetime. Stars become like our sun because of energy from nuclear reactions going on deep inside the star. They convert hydrogen to helium, and that gives off energy, which becomes light. Once that hydrogen is used up, they can start using helium in their nuclear "furnaces." But eventually any star will run out of "fuel." Stars like our sun will collapse and become a "white dwarf." But very massive stars — ten times heavier than our sun — have such strong gravity that when they collapse they can't stop. Everything in the star collapses to a tiny point that still has the mass and gravity of the star but no longer shines. In fact the gravity is so strong that even light can't escape. That is why we call it a black hole. If you could visit a black hole (don't get too close!), you would probably see a black "ball" a few miles across.


Does a black hole "suck" down everything in its vicinity?

The gravity of a black hole is the same as the gravity of a star with the same mass. For instance, if the sun suddenly became a black hole (couldn't happen, but just for instance), Earth would continue to orbit the black sun as usual. The danger is that if you get too close to the black hole, like only a few miles away, the gravity would be extremely strong and it would be hard to escape.


Have astronomers found any black holes?

Yes, we think so. Of course a black hole itself would be hard to see! But we can see when hot gases are "sucked" into the black hole. The collisions and explosions that occur when that happens generate huge bursts of light, including X-rays. We know of several stars that appear to have invisible but very massive companion stars. They look as if the invisible companion is pulling gas from the regular star. The only thing we know of that could do that is a black hole. We also see incredible explosions in the centers of some galaxies. The only thing we know of that can make such spectacular explosions is a very big black hole — a black hole with the mass of millions of stars! Such a huge black hole can pull in whole stars!


How do black holes have energy if they form from dead or burned-out stars?

Black holes form from large stars that collapse when they run out of fuel for their nuclear reactions. There are many forms of energy — heat, light, energy of motion, and the energy of gravity. Black holes have LOTS of gravitational energy!


Are there any black holes in our galaxy, and if so, are there any plans to explore them?

Fortunately for us there are no black holes near our solar system. Unfortunately this means that we can't send a space probe to explore one. We believe that there are several black holes in our galaxy. Some are about the size of stars. We can tell that a black hole is there by how it affects a companion star. Of course we can't see the black hole — it's black! But we can see that a star is going around in orbit around something else that we can't see, and we can see gases that the black hole has "sucked off" from the other star. We also think that most if not all galaxies have a big black hole at each center. Our own Milky Way probably has a big black hole at its center, but it's hard to tell for sure because there are so many stars in the way when we look toward the center. (The center is in the constellation Sagittarius, which you can see during the summer, towards the south). I'm not sure what would happen to someone who went into a black hole, but I don't think it would be pleasant!


Could a beam of light orbit a black hole?

That's an interesting idea. I think that it could, but only if it happened to come in at just the right angle so it would be in a perfectly circular orbit. That seems unlikely, so it would be more likely that it would either pass by the black hole in a curved path or go into the black hole.


What is meant by the term associated with black holes called the "event horizon"?

What you are describing we usually call the "event horizon." Imagine a black hole, and a beam of light passing by. The gravity of the black hole is tugging on the light (and matter) nearby, but the beam of light has energy so it keeps trying to go on by. The gravity bends the light beam, but if the light isn't too close it can still pass by the black hole. If it gets too close, the gravity of the black hole bends the light into the hole and it can't escape. This would be true no matter what direction the light is coming from around the hole. So there would be a sphere around the black hole where, if light went in, it couldn't come out. That sphere is the "event horizon." I guess it's called that because it's the limit at which you could see an event occur, and a horizon is the limit of which you can see (usually we mean the sky).


What is at the end of a black hole?

Umm, more black hole? In science fiction, people travel through black holes. Some people have suggested that you could go into one black hole and come out another. But that is still fiction. As far as we know, if you go into a black hole, that's it.


Why do we not send a probe down a black hole in order to find out information and see what is at the end?

There are two problems in doing that. The first is that the farthest we have sent a space probe so far is within our own solar system. The nearest black hole is many light-years away! With our current technology, it would take thousands of years to get there. The second problem is how to get information back from the probe if we did send one into a black hole. Radio waves are a form of light. Light can't escape from the black hole because of the huge gravity. Nothing can escape the black hole's gravity! So getting any kind of information back would be tough! We could track the probe as it goes into the black hole though, up until the point that light can no longer escape the black hole's gravity.


What happens to time in a black hole?

As far as I know, time is no different inside a black hole. But if your spacecraft fell into one, things would seem very strange. As you fell faster and faster, your speed would approach the speed of light. It would seem to you that you are falling forever. So perhaps it would seem that time had stopped. But falling into a black hole would be pretty painful — the gravity would pull a spacecraft apart.


Will the sun become a black hole?

Fortunately for us, the sun will never become a black hole. We believe that only a very big star, several times more massive than our sun, will become a black hole.


Are there pictures of the insides of a black hole?

Nope! To do that, we would have to send a camera into the black hole. Then we would have to get it back out again, which we can't do! We couldn't even send the picture out using radio waves. Why? Because a black hole has such strong gravity that even light (and radio is a form of light) can't escape!


How close can something get to a black hole before it is sucked in? Could it suck in another black hole?

That depends on how much matter has been pulled into the black hole. The more matter, the stronger the gravity (just like everything else). It also depends on how something is moving near the hole. A planet could be in orbit around the black hole and not be pulled in. For instance, suppose the sun suddenly became a black hole (couldn't happen, but just suppose). It has the same amount of matter, so it has the same gravity. All the planets would continue to orbit it. It just would get pretty dark!!!

If a black hole were inside a cloud of gas, the gas near it would probably be pulled in. Well, since the hole then has more matter, it has more gravity. So it can pull in more gas. More gas, more gravity. Or maybe it's in a cluster of stars and some stars fall in. The hole could start growing. If there were a smaller black hole nearby, it could get pulled in too.


What are white holes?

A "white hole" is supposed to be the opposite of a black hole. If matter and light can be trapped inside a black hole, then a white hole is where the matter and light come spewing out.


What is a wormhole?

Some people think that two black holes could be connected — they call that a wormhole. If you could somehow survive the trip into one black hole, you would end up at the other. People writing science fiction like to use wormholes because it makes a quick way to get around the universe. But if wormholes exist (we don't know if they do or don't), it's not clear how you would survive the trip in or how you would escape from the second black hole once you got there!