What Is At The Centre Of A Galaxy?

[naomi24]

A question arising in my home from the BBC programme ‘Stargazing Live’ tonight. Moons revolve around planets, planets revolve around suns, and the whole lot revolves around …. what?

[AB Editor]

"The Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 8.33±0.35 kpc (~27,000±1,000 ly) from the Earth[1][2][3][4][5] in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest. There is strong evidence that supports the existence of a supermassive black hole at the Galactic Center of the Milky Way."

http://en.wikipedia.org/wiki/Galactic_Center

So something massive.

[Old_Geezer]

Few and probably no analogy is perfect mibn2cweus. The one you describe is not really for considering in 3D. It is to show that on the surface of the balloon all dots are moving away from each other (as it grows) and there is no special centre which is stable and all else moves away from it. Wherever you are on the surface (or in space) everthing seems to be rushing away from you, so you could erroneously think you were special, in the centre. But not so. It's the same everywhere.

[Old_Geezer]

Yes, if one has the imagination one can contemplate further dots on further surfaces on balloons inside (or outside) the one first considered.

[Old_Geezer]

Ah pdq1 rather than mibn2cweus . The older I get the more mistakes I make and the more the site is content that I can't correct them. :-(

[mibn2cweus]

Old_Geezer
Ah pdq1 rather than mibn2cweus . The older I get the more mistakes I make and the more the site is content that I can't correct them. :-(
18:52 Fri 11th Jan 2013

Ah, well at least you got me name right, which is a pretty significant accomplishment in itself . . . even if it was the wrong name. ;o)

[naomi24]

Ooer! When I see things like //8.33±0.35 kpc (~27,000±1,000 ly)// I almost regret asking – or at least I almost regret abandoning the sciences before A-level! :o)

Brilliant answers! We thought it was probably a black hole – honest we did! Thank you.

[mibn2cweus]

Gravity and inertia, these two opposing forces, cause order to arise out of chaos. Having a huge chunk of mass at the centre, which by virtue of its mass defines the centre, orchestrates the symphony of motion it creates around it, just as our Sun brings order and stability to the solar system we inhabit.

♪♫ And I think to myself, what a wonderful . . . galaxy.♪♫ :o)

[naomi24]

^^ Lovely voice. ;o)

[beso]

Orbits are not actually of one thing around another. Systems orbit around their common centre of gravity. For example the Sun and Earth have a common centre that is not in the centre of the Sun but is within the radius of the Sun surface where it would appear to orbit the centre of the Sun from the Sun's perspective.

Like many things in astrophysics that perspective is a matter of frame of reference. Where you are standing governs what you perceive as a force.

Mibs gives us a good example with Inertia. Inertia isn't really a force but a manifestation of the Law of Conservation of Momentum. Its characteristics depend on whether one views it from afar or while standing on the object that is being subjected to a force.

Gravity itself is the same. It isn't really a force but a consequence of the warping of SpaceTime that causes both mass and the attraction between masses. The familiar concept that mass causes SpaceTime to warp is quite arbitrary. Like magnetism and electricity, mass and gravity are two sides of the same thing.

When the choice of frame of reference is between frames that are moving at a significant fraction of the speed of light relative to each other it all gets very weird. Time and distance (and hence speed) become dependent on the motion of the observer.

Even the observed mass is dependent on the frame of reference. However there is one thing that remains constant across frames of reference. The Momentum. Momentum is the fundamental currency of motion. Mass, speed and distance and time can all change but momentum is always conserved.

Anyway back to the galaxy.

Unlike the mass ratio of the Sun to the planets, the mass of the black hole is relatively trivial despite its enormous mass in human terms. Compared to the mass of our galaxy's hundreds of billions of stars it is relatively small.

Moreover, the vast majority of the mass (and momentum) in galaxies is due to the Dark Matter. Despite the apparent similarities in the name this ins't the Black Hole. Most of the mass in the Universe is Dark Matter.

Nobody really knows what Dark Matter is and it can't be seen. But without it galaxies would fly apart because what we can see does not provide enough gravity to hold it together at the observed speeds of galactic rotation.

[naomi24]

Thanks Beso. I was aware of some of that, but all very interesting.