Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

Ive been watching a video about dark mater and Joe al lot of the mass is missing in our universe. And astronomers got to this by measuring the speed that stars orbit he Center of the galaxy and when they did this I didn't match the amount of gravity/mass in the galaxy.

However I thought that the mass of the black hole in the Center of the galaxy was measured by the stars orbiting it? To me it sounds like a catch 22? But I dot understand it very well. So could someone explain it further?

share|improve this question
add comment

1 Answer 1

For the most part the mass of the super massive black hole (SMBH) at the galaxy center is estimated in much the same way the solar mass is estimated by Newtonian mechanics. In fact general relativistic elements are not used much, where GR contributes heavily for orbits with a few number of Schwarzschild radii, or semi-major axis length. The motion of stars in the galactic center have been observed over a number of years and the velocities and accelerations known. It is then straight forwards to use Newtonian mechanics or $v~=~\sqrt{2GM/r}$.

A good source for this is the UCLA Galactic Center web site:

http://www.astro.ucla.edu/~ghezgroup/gc/

enter image description here

The data animation of stellar trajectories:

http://www.astro.ucla.edu/~ghezgroup/gc/pictures/orbitsOverImage10.shtml

is interesting, just click on the animation hyperlink below. A couple of stars are on highly eccentric orbits which take then somewhat close to the SMBH.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.