What makes the world go round

[cactuswatcher]

Apod this morning has a picture (here, if you don't subscribe http://antwrp.gsfc.nasa.gov/apod/image/0812/mwcenter_eso.jpg ) of the center of the galaxy. The text from today's cover page says "...astronomers patiently measured the positions of the stars over time, following one star, designated S2, through a complete orbit as it came within about 1 light day of the center of the Milky Way. Their results convincingly show that S2 is moving under the influence of the enormous gravity of a compact, unseen object -- a black hole with 4 million times the mass of the Sun. "

Several things are undeniable. Whatever is at the center is enormous, it does not continually give off infra-red light as a star might, and it's far too small to keep the galaxy together by means of gravity alone. I would have expected the object at the center to be at least a thousand times more massive, and I never assumed like the current theory that gravity was the end all and be all of galactic forces. With this result expect to hear more and more about 'dark energy' and searches for 'dark matter' which are a polite way of summarizing, 'we don't know what the heck we're talking about yet.'

Comments

[anomster.livejournal.com]

Well, the "typo" speculation was a joke.

So I did read that about the neutrino hypothesis!

And how do you do those calculations? What do you plug into them besides the mass of the black hole?

[cactuswatcher.livejournal.com]

You have to estimate the mass of the star. In this case it is tiny compared to the black hole, which fortunately helps simplify things. The rest is a standard formula or two. No doubt because of relativity the velocity numbers you get from the formulas for this kind of extreme case are too high. But it does give a ballpark figure. Incidentally working the same formulas backwards is how they estimate the size of the black hole.