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

[cactuswatcher.livejournal.com]

'Dark matter' as it is currently used refers only to those things that couldn't be seen in a telescope or at least predicted from what we see directly. The reason for that is that it has been long established that stars and clouds of dust and gas are many, many times the mass of everything else we can predict from knowing about our own solar system. So planets, moons, asteroids, comets associated with stars are accounted for in normal matter as well as anything part of the visible dust and gas clouds whether they shine from their own light or not. Throwing all those kinds of things into the mix for 'dark matter' effectively doesn't add much, at least in terms of what these scientists are looking for. Dark matter is what is beyond that - individual molecules and atoms, dust outside of large clouds, and any else from the size of a large planet to infinitesimal particles floating around in what looks like empty space.

Personally I think the idea that there are enough neutrinos to make up the mass difference is grasping at straws. They pick something that's difficult to detect and assume that must be the answer. If we are 'swimming' through that many particles (enough to total tens of billions of suns worth just within the galaxy) then other, much stranger theories of how the galaxy holds together are just as likely to be correct.

On the other hand a central black hole billions of times the mass of the sun would have gone a long way towards vindicating Einstein.

I did the calculations once for a black hole 1 billion times the mass of the sun and determined that a star in orbit at one light year from the black hole would be traveling around 3/4 the speed of light. The effects of relativity on such a star would be very noticeable. The star they found is about one light day away and relativity is surely noticeable in its orbit. The fact they were not jumping up and down with glee about observed effects of relativity on it makes me guess the quoted mass of the central black hole is not a typo.

[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.