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It seems fairly certain there is a large black hole, about 25,000 ly from Earth roughly in the center of our galaxy, in the direction of the Sag. A radio source.

• In fact, is it thought to be literally at the center of gravity of our galaxy?

Does we know, or is this uncertain?

• Indeed, do we actually know where the CG of the galaxy is?

I observe that the term "Galactic Center" seems to be an astronomical term. (Example.) Can an astronomer tell me: does that term mean what we believe to be the CG, or rather, is it a nominal mapping point?

• If the black hole IS exactly at the CG of the galaxy ... why?

It would seem to me a big BH could form any old place, and perhaps still be orbiting or oscillating the CG - right? If the SBH is literally the CG of there galaxy, is there a reason for that? (Like, "SBH can only form at the CG" .. or "stellar collisions can only happen at the CG" .. or whatever, those are only wild guesses.)

Or indeed, is the SBH the "main player" in the galaxy - just as the solar system formed around the sun (so it would be silly to ask "why is the sun at the center"), did the SBH sort of co-form with the galaxy (or something?)

In any event, is the SBH at the actual CG of our galaxy? Do we know?

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    $\begingroup$ Very similar question here: astronomy.stackexchange.com/questions/7861/… $\endgroup$
    – userLTK
    Commented Jun 12, 2016 at 9:33
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    $\begingroup$ You know what, LTK .. that QA does a superb job of (a) explaining the basic newtonian physics of shell orbits (and (b) "what would happen if they flew off.." sort of thing) and (c) that we can "see" these things by looking at the velocities towards/away from us on each side. However! Note in fact RobJeffries final comment: "That's a good question and I do not have the information to hand." indeed, that's just what I want to know here! $\endgroup$
    – Fattie
    Commented Jun 12, 2016 at 12:26
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    $\begingroup$ What do you mean by "CG"? $\endgroup$
    – Peter Erwin
    Commented Jun 13, 2016 at 18:40
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    $\begingroup$ Dear @PeterErwin - sorry: Center of Gravity. (I guess, that is somewhat common abbreviation in say engineering or video game engineering), cheers.... $\endgroup$
    – Fattie
    Commented Jun 13, 2016 at 18:52
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    $\begingroup$ I'd answer the question with a question. Is the "center" of the sun the center of mass in the solar system? I would bet that the CG of the Milky Way is not at the location of the supermassive black hole, but pretty close by galactic standards. Maybe, one day, our instruments will be able to narrow down its location so accurately that we can detect the "wobble" (if we have a few hundred million years to watch). $\endgroup$
    – Jack R. Woods
    Commented Jun 14, 2016 at 0:52

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In simple terms: yes, the Milky Way's supermassive black hole (SMBH) is at the center of the galaxy, we know approximately where the center is (but not terribly precisely), and we should expect the SMBH to be there.

You can define the Galactic Center using the orbits of stars and gas: what is the (average) center of their orbits? You can also use the spatial distribution of the same (which is probably a bit closer to what you're thinking of as a "center of gravity" or "center of mass"). This is never going to give you an exact answer, and since the galaxy is not perfectly symmetric or unchanging, there will always be some uncertainty and variation.

There's a 2D coordinate definition, which is the point on the sky where Galactic latitude and longitude both = 0. This is intended to approximate the direction toward the GC; but since it was defined back in the 1950s, no one is worried if modern attempts to measure the (3D) center of the galaxy might be slightly different.

As far as we can tell, yes, the SMBH is at the Galactic Center. If an SMBH did form outside the central region of a galaxy, dynamical friction would make it spiral into the center of the galaxy (and damp any radial orbital oscillations as well). This would also cause an SMBH from another (smaller) galaxy that merged with the Milky Way to spiral into the GC and merge with the SMBH there; see my answer to this question.

The SMBH is not the "main player" in our galaxy: it has a mass of about 4 million solar masses, which you can compare with the total stellar mass of about 50 billion solar masses. (And a dark-matter mass of roughly one trillion solar masses.) Nonetheless, there is evidence that SMBHs "co-form", in some sense, with their host galaxies. In particular, the mass of SMBHs is fairly tightly correlated with several properties of the host galaxy (e.g, the mass of the "bulge", or the velocity dispersion of stars in the inner part of the galaxy).

[Edited to fix a typo.]

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    $\begingroup$ awesome answer!! You know, this dynamical friction issue (I indeed first heard about the issue in your other answer) seems to be incredibly important, rather an elephant in the room issue, in dynamics. (Indeed I wonder for example, in studying and thinking about solar system formation: is dynamical friction that the central process that results in "1 or 2 big clumps" coming together in the early stages; ultimately forming the dominant body(s), the stars?) $\endgroup$
    – Fattie
    Commented Jun 14, 2016 at 12:57
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    $\begingroup$ Dynamical friction is particularly important when other forms of "friction" and energy loss aren't. In star formation, you have lots of "gas friction", turbulence, and (in gas disks) spiral arm, so classical dynamical friction is less relevant. (I'll admit I'm not that current on the details of star and solar system formation, so perhaps I'm missing something.) $\endgroup$
    – Peter Erwin
    Commented Jun 14, 2016 at 16:36
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    $\begingroup$ Thanks a million for this answer, which changed my understanding of the world around me. $\endgroup$
    – Fattie
    Commented Jun 19, 2016 at 0:57

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