What is the size of a black hole?
I have read that a black hole has very strong gravitation force and nothing can escape from the gravitational force of a black hole not even light. Now my question is what is the size of the black hole?
When you read about black holes, you would have read about something called the event horizon. That is the point from which light can no longer escape from the black hole. Nobody knows as to what really happens inside the event horizon. One needs to have a theory on quantum gravity to explain that. Classically, matter collapses to a point, called a singularity (infinite density), but what really happens is not known.
When astronomers refer to the "size" of the black hole, they are talking about the event horizon. The event horizon is refers to the location from the black hole where the escape velocity equals the speed of light. In other words, no particle (even light) can escape from within the event horizon.
Mathematically, the size of the black hole is given by GM/c2 where G is the gravitational constant, M is the mass of the black hole and c is the speed of light. So, when one says that the black hole has a size of 5 km, it means that the event horizon is at a distance of 5 km from the center of the black hole. If the Sun were to become a black hole, then its size would be about 3 km.
Still Curious?
Get More 'Curious?' with Our New PODCAST:
- Podcast? Subscribe? Tell me about the Ask an Astronomer Podcast
- Subscribe to our Podcast | Listen to our current Episode
- Cool! But I can't now. Send me a quick reminder now for later.
Related questions:
More questions about Black Holes and Quasars: Previous | Next
How to ask a question:
If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.
Main Page | About Us | For Teachers | Astronomy Links | Ask a Question | View a Random Question | Our Podcast
URL: http://curious.astro.cornell.edu/question.php?number=160
This page has been accessed 47696 times since August 23, 2002.
Last modified: November 13, 2002 3:42:25 PM
Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.
Warning: Your browser is misbehaving! This page might look ugly. (Details)