Astronomy/Black Holes

Advertisement


Question
How do stars and planets orbit a black hole without being destroyed by it

Answer
Hello Kaitlyn,

This is a common misconception about black holes. Black holes do not have any more gravity than any other body of the same mass!

To explain, suppose our sun suddenly collapsed to become a black hole (this is actually impossible, because you'd need a mass about 3 times that of our sun to collapse all the way to a black hole). But let's pretend it happened.

What would happen to the earth and the other planets? Would they be sucked in? No - they would continue in their orbits! They would feel the same gravity from the sun, whether it was a black hole or not.

Where things start to get sucked into the black hole is when they get close to the "event horizon". For a mass of the sun, the event horizon has a radius of about 3 km. Earth is (on average) about 150 million km from the centre of the sun - a LOT farther than 3 km! So earth will experience no great effects.

If a comet approached the black hole sun and got approximately 3 km from the centre of the sun, then it could very well be sucked in and never heard from again.

So remember - black holes are only "dangerous" if you get very close to their event horizon (which is usually small for stellar black holes).

Hope that helps,

Prof. James Gort  

Astronomy

All Answers


Answers by Expert:


Ask Experts

Volunteer


James Gort

Expertise

Questions on observational astronomy, optics, and astrophysics. Specializing in the evolution of stars, variable stars, supernovae, neuton stars/pulsars, black holes, quasars, and cosmology.

Experience

I was a professional astronomer (University of Texas, McDonald Observatory), lecturer at the Adler Planetarium, professor of astrophysics, and amateur astronomer for 42 years. I have made numerous telescopes, and I am currently building one of the largest private observatories in Canada.

Publications
StarDate, University of Texas, numerous Journal Publications

©2016 About.com. All rights reserved.