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What if the Sun became a black hole?


Narrator: Welcome to Space Place Musings, where an expert answers questions from our Space Place museum partners across the nation. I'm Diane Fisher of the New Millennium Program, and our expert is Dr. Marc Rayman, a scientist at the Jet Propulsion Laboratory.

Marc, today we have a question on a subject that often fascinates people: Black holes. It comes from the Paulucci Space Theatre in Hibbing, Minnesota. Here it is: If the Sun became a black hole, would Earth get pulled inside?

Rayman: Interesting question. Black holes are just about the strangest objects known in the universe. Scientists call them holes, but they are anything but empty! They contain matter so compressed that their gravitational pull will not let even light escape them to reach our eyes and telescopes. That's why we can't see black holes directly and so we call them black.

Narrator: So if the Sun were a black hole, we would have no sunlight. But what would its gravity do to us?

Rayman: The gravitational pull of the Sun, a black hole, or anything else for that matter depends only on its mass and distance from us, so even if the Sun were compressed to become a black hole, it would pull only as hard on Earth as it does now. Earth wouldn't be pulled inside if the Sun were compressed to become a black hole.

Narrator: So Earth would be safe! Now if we can't see black holes, how do we even know they exist?

Rayman: Because of the odd behavior of the matter near them that we can see. For example, gases swirl around these regions at nearly the speed of light, giving off lots of high-energy radiation.

Narrator: What does this swirling gas and radiation tell us?

Rayman: Well, for one thing, black holes come in vastly different sizes. Some contain huge amounts of matter, some contain much less.

Narrator: How do they form?

Rayman: 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's left collapses under the weight of its own gravity to become a super-dense object–a black hole. Our Sun is actually too light-weight 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.

Narrator: So, do black holes always contain at least this much matter?

Rayman: Well, not always. The key is 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! 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 wouldn't fall into the "Marc" black hole, because I would exert no more gravitational force than I do now.

Narrator: If the Sun could be compressed enough to become a black hole, how big would it be?

Rayman: 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.

Narrator: So how big can black holes be?

Rayman: 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! A black hole that massive would be so large that if it were in the center of the solar system where the Sun is, it would extend beyond the orbit of Mars. Scientists are still trying to understand how common they are and how they may have formed.

At the other extreme, microscopic black holes may have formed in the early moments of the universe.

Narrator: So, if no light escapes them, how can scientists study black holes?

Rayman: One way will be to study the gravitational waves some of them make in the fabric of space. NASA and the European Space Agency are working on some new technologies that will make it possible to detect gravitational waves using a space-based laser interferometer. The New Millennium Program's Space Technology 7 project will develop and test one of the technologies that will be essential for making space-based gravitational wave astronomy possible.

Narrator: Gravitational waves are a pretty exotic idea.

Rayman: Yes, but not that difficult to understand. One of the articles on The Space Place website gives a simple explanation, along with a neat, related activity for the classroom. To find it, our listeners can visit, and search on "listening for rings from space."

Narrator: Great suggestion, Marc. The Space Place also has an interactive crossword puzzle on the subject. Our time's up for now. Thanks to our subscribers for joining us, and we'll be back soon with more answers to great questions.