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. To find out more about gravitational waves and solve an online crossword puzzle about them, go [here] to The Space Place at spaceplace.nasa.gov and click on Dr. Marc's Amazing Facts. Then go to "What space mission will give Earthlings a sixth sense?"
Thanks again for calling Dr. Marc here at The Space Place.
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