B What is the gravitational force like near a black hole?

[Timothy Schablin]

Found this interesting interactive experiment.
http://hubblesite.org/explore_astronomy/black_holes/encyc_mod3_q14.html

In the interactive, it looks like the black hole is orbiting around something. What would it orbit around? Or am I looking at it wrong?

 

[berkeman]

The center of mass of the BH + Star system.

 

[davenn]

The center of mass of the BH + Star system.

yup, or more commonly known as the barycentre

https://en.wikipedia.org/wiki/BarycenterDave

 

[berkeman]

yup, or more commonly known as the barycentre

Show-off!

 

[davenn]

Show-off!

hahaha

 

[DaveC426913]

It's orbiting around a plan ol' star.

Black holes are not magical star-gobbling machines. They are compact masses, subject to gravity, just like any other mass. More to the point, they act on other things like a regular mass does.

If the Sun were replaced by a black hole of the same mass, Earth and the other planets - even Mercury would all happily continue on their merry way as if nothing had changed.

Black holes can easily have a mass similar to a large star, and so will behave like a star in every way. The only tricky bit to dealing with them is that you can get a lot closer to them than you can to a regular star.

For example, you cannot get closer to the sun than its surface - which is 400,000 miles from its centre. But a BH of one solar mass would be less than 13 miles in radius. So you could get 399,983 miles closer to that same one solar mass. And gravitational force squares as you halve the distance - making it about 10 million times greater - when you're that close.

That's where you start to run into problems.