By Dr. Marc Rayman
Asked by our friends at the Paulucci Space Theatre in Hibbing, Minnesota.
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Black holes are just about the strangest objects in the universe. They are regions of space where gravity is so powerful that even light can't escape. Scientists call them holes, but they are anything but empty! They contain matter so compressed that their gravitational pull will not let even light leave them to reach our eyes and telescopes. That is why we cannot see them directly. But we are confident they exist because of the odd behavior of the matter we can see near them. For example, gases swirl around these regions at nearly the speed of light, giving off lots of high-energy radiation.
Black holes come in vastly different sizes. Some contain huge amounts of matter, some contain much less.
Some black holes form after a very large star uses up all its fuel and blows off its outer parts in a gigantic explosion (called a supernova). Then, what is left collapses under the weight of its own gravity to become a super-dense object called a black hole. Our Sun is actually too small to end up as a black hole. It simply does not contain enough matter to exert that kind of gravitational force on itself. A star has to be more than about 10 times the mass of our Sun to become a black hole.
Just remember that a black hole is any amount of matter squeezed into a very, very dense package. Imagine all of planet Earth squeezed into the size of a marble. Earth would then be a black hole! But, the gravitational pull of a black hole, or anything else for that matter, depends only on mass and distance, not how large or small the object is. Even I could be a black hole if I were compressed to be more than one thousand billion billion times smaller than a grain of salt. But, you could still stand as close to me as you would for a normal conversation and you would not fall into the "Dr. Marc" black hole, because I would exert no more gravitational force than I do now.
If the Sun were somehow compressed enough to become a black hole, it would be less than 6 kilometers (well under 4 miles) across. It would exert no more gravitational force on Earth or the other planets in the solar system than it does now. Why? Because it would contain no more matter than it does now and it would be no closer to the planets than it is now.
In the last few years scientists have discovered that many galaxies have supermassive black holes at their centers. These enormous monsters may contain the mass of 100 million or more Suns! Scientists are still trying to understand how common they are and how they may have formed.
At the other extreme, tiny black holes may have formed in the first few seconds of the universe.
Black holes are fascinating objects and astronomers have a lot more questions about them. One way that will help scientists to learn about them will be to detect and study the gravitational waves some of them create in the fabric of space. Find out more about gravitational waves and solve an online crossword puzzle about them.