**Ask a monkey a physics question thread**

[MadMojoMonkey]

How quickly would the dramatic changes in the appearance of the Milky Way happen?

You wouldn't see anything for 27,000 years. Same as the time it takes to notice anything at all that is 27,000 light years away.

I've looked into some numbers and I'm now a bit skeptical how dramatic the changes would be. In fact, let me take back all the stuff I said about this earlier. I was comparing it to the "solar system w/o a sun" scenario and that's just no good here.
mass of Milky Way: ~10^12 solar masses
mass of Sagitarius-A: ~4.3(10)^6 solar masses

Sagitarius-A is the name of the suparmassive black hole in the center of the Milky Way. It's only 4 millionths the mass of the Milky way. This is really not even close to our previous discussion. The sun is well over 99% of the total mass of the solar system. Given this, I think most of the objects in the galaxy would remain gravitationally bound to each-other as a galaxy.

The objects bound tightly to the core would likely be flung out, but it seems not very likely that many of them would have escape velocity of the now less massive galaxy. They would have to be quite near escape velocity already.

My current guess as to what we'd actually see in the night sky is that the bulge in the Milky Way toward it's center would become slightly bigger and less bright as the objects orbiting the center would acquire more eliptical orbits as they responded to the change in gravitational potential.

[Eric]

You wouldn't see anything for 27,000 years. Same as the time it takes to notice anything at all that is 27,000 light years away.

Right, I understand that we won't see a change in something that is 27,000 light years away for another 27,000 years. I guess the question should be, how quickly would the changes in the geography of the Milky Way happen? In other words, suppose a given star is 15,000 light years from the center of the Milky Way. Will it start drifting out more then 15,000 light years from the center the very second the black hole disappears or will it take longer for it to lose its gravitational attraction and drift out?

[MadMojoMonkey]

Right, I understand that we won't see a change in something that is 27,000 light years away for another 27,000 years. I guess the question should be, how quickly would the changes in the geography of the Milky Way happen? In other words, suppose a given star is 15,000 light years from the center of the Milky Way. Will it start drifting out more then 15,000 light years from the center the very second the black hole disappears or will it take longer for it to lose its gravitational attraction and drift out?

It will take 15,000 years for it to lose the gravitational attraction.

[Eric]

It will take 15,000 years for it to lose the gravitational attraction.

Makes sense, thanks for the clarification. Today I re-read a 2003 New Scientist article that reports on the 2002 measurement involving Jupiter:

The speed of gravity has been measured for the first time. The landmark experiment shows that it travels at the speed of light, meaning that Einstein’s general theory of relativity has passed another test with flying colours.


Ed Fomalont of the National Radio Astronomy Observatory in Charlottesville, Virginia, and Sergei Kopeikin of the University of Missouri in Columbia made the measurement, with the help of the planet Jupiter.