<p>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.</p>
<p>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 &quot;center of gravity&quot; or &quot;center of mass&quot;). 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.</p>
<p>There's a 2D <i>coordinate</i> 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.</p>
<p>As far as we can tell, yes, the SMBH is at the Galactic Center. If an SMBH <i>did</i> 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 <a href="https://astronomy.stackexchange.com/questions/13013/why-do-we-believe-that-the-super-massive-black-holes-at-the-centers-of-two-mergi?rq=1">this question</a>.</p>
<p>The SMBH is not the &quot;main player&quot; 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 &quot;co-form&quot;, 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 &quot;bulge&quot;, or the velocity dispersion of stars in the inner part of the galaxy).</p>
<p>[Edited to fix a typo.]</p>

CC BY-SA 3.0
2016-06-14T12:57:34.997Z
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?)
Fattie
CC BY-SA 3.0
2016-06-14T16:36:06.113Z
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.)
Peter Erwin
CC BY-SA 3.0
2016-06-19T00:57:28.477Z
Thanks a million for this answer, which changed my understanding of the world around me.
Fattie
CC BY-SA 4.0
2016-06-14T10:00:19.907Z
2024-09-15T00:46:45.103Z
2024-09-15T00:46:45.103Z
Peter Erwin
<p>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.</p>

<p>• In fact, <strong>is it thought to be literally at the center of gravity of our galaxy?</strong></p>

<p>Does we know, or is this uncertain?</p>

<p>• Indeed, do we actually know where the CG of the galaxy is?</p>

<p>I observe that the term "Galactic Center" seems to be an astronomical term. (<a href="https://en.wikipedia.org/wiki/Galactic_Center#Distance_to_the_Galactic_Center" rel="noreferrer">Example</a>.) Can an astronomer tell me: does that term mean what we believe to be the CG, or rather, is it a nominal mapping point?</p>

<p>• If the black hole IS exactly at the CG of the galaxy ... why?</p>

<p>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.)</p>

<p>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?)</p>

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

CC BY-SA 3.0
2016-06-12T09:33:27.620Z
Very similar question here:   http://astronomy.stackexchange.com/questions/7861/orbiting-supermassive-black-hole-or-galactic-center-of-mass?rq=1
userLTK
CC BY-SA 3.0
2016-06-12T12:26:34.490Z
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!
Fattie
CC BY-SA 3.0
2016-06-13T18:40:56.600Z
What do you mean by "CG"?
Peter Erwin
CC BY-SA 3.0
2016-06-13T18:52:51.340Z
Dear @PeterErwin - sorry: Center of Gravity.  (I guess, that is somewhat common abbreviation in say engineering or video game engineering), cheers....
Fattie
CC BY-SA 3.0
2016-06-14T00:52:12.703Z
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).
Jack R. Woods
CC BY-SA 3.0
2016-06-14T09:51:52.987Z
@JackR.Woods -- the center of mass of the Solar System is very slightly displaced from the exact center of the Sun in the direction of Jupiter (with a smaller displacement in the direction of Saturn, etc.), though it's still well inside the Sun. If we observed the Sun from a nearby star with an accurate enough telescope + spectrograph, we could measure the Doppler shift from the Sun's wobbling around this center; this is how most of the first few hundred exoplanets were detected (and is still the gold standard for confirming and measuring exoplanets).
Peter Erwin
CC BY-SA 3.0
2016-06-14T12:51:05.837Z
Hi Jack - hmm, that sort of missed the point: in our solar system,the sun is indeed at the center.  (Obviously there is a tiny displacement, due to the other miniscule bodies in the system.)  However, alpha centauri is *not* at the center of it's system, it's a completely different type of system.  Our sun is not at the center of the galaxy (kids often assume this is the case); our planet is not at the center of our system (ditto).  It would seem completely ordinary to me if I learned that the SBH orbited at (just an example) the bulge radius - or whatever.
Fattie
CC BY-SA 3.0
2016-06-14T12:53:24.433Z
Just to clarify my question for anyone reading, I can see no immediately compelling reason why, particularly, a SBH in a galaxy should be at the middle.  For example, say I learned that a SBH forms when (this is just a random-sounding example) "a big star cluster collapses".  That could certainly happen anywhere in a galaxy as "big star clusters" are all over; the SBH would then orbit at that radius.  I observe that some folks seem to "naturally assume" a SBH would be central: I just don't see that. (By all means someone can tell me different, and *why* would it be in the middle?)
Fattie
CC BY-SA 3.0
2016-06-14T22:58:54.357Z
@PeterErwin The barycentre of the solar system spends most of its time *outside* the Sun.
ProfRob
CC BY-SA 3.0
2016-06-15T11:53:43.323Z
@RobJeffries -- whoops. Yes, of course, you're right; I think I confused myself by thinking of the Earth-Moon barycenter (which is within the Earth).
Peter Erwin
CC BY-SA 3.0
2016-06-17T00:20:36.150Z
I guess all I was saying is that just as random interactions during the formation of the solar system has displaced the center of gravity from the center of the sun, the interaction of matter during and since the formation of the milky way has probably displaced the center of the galaxy from the central SMBH. Also, the galaxy is so large compared to the size of the SMBH that the center is probably not inside the black hole. I could be wrong , but this seems logical.
Jack R. Woods
<supermassive-black-hole><galactic-dynamics>
Is our central black hole actually at the CG of the galaxy?
Fattie
# Is our central black hole actually at the CG of the galaxy?

(Asked by: username_0 on 2016-06-11)

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](https://en.wikipedia.org/wiki/Galactic_Center#Distance_to_the_Galactic_Center).) 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?

**Tags:** supermassive-black-hole, galactic-dynamics

> **Comments:**
> **username_1**: Very similar question here:   http://astronomy.stackexchange.com/questions/7861/orbiting-supermassive-black-hole-or-galactic-center-of-mass?rq=1  
> **username_0**: 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!  
> **username_2**: What do you mean by "CG"?  
> **username_0**: Dear @username_2 - sorry: Center of Gravity.  (I guess, that is somewhat common abbreviation in say engineering or video game engineering), cheers....  
> **username_3**: 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).  
> **username_2**: @JackR.username_3 -- the center of mass of the Solar System is very slightly displaced from the exact center of the Sun in the direction of Jupiter (with a smaller displacement in the direction of Saturn, etc.), though it's still well inside the Sun. If we observed the Sun from a nearby star with an accurate enough telescope + spectrograph, we could measure the Doppler shift from the Sun's wobbling around this center; this is how most of the first few hundred exoplanets were detected (and is still the gold standard for confirming and measuring exoplanets).  
> **username_0**: Hi username_3 - hmm, that sort of missed the point: in our solar system,the sun is indeed at the center.  (Obviously there is a tiny displacement, due to the other miniscule bodies in the system.)  However, alpha centauri is *not* at the center of it's system, it's a completely different type of system.  Our sun is not at the center of the galaxy (kids often assume this is the case); our planet is not at the center of our system (ditto).  It would seem completely ordinary to me if I learned that the SBH orbited at (just an example) the bulge radius - or whatever.  
> **username_0**: Just to clarify my question for anyone reading, I can see no immediately compelling reason why, particularly, a SBH in a galaxy should be at the middle.  For example, say I learned that a SBH forms when (this is just a random-sounding example) "a big star cluster collapses".  That could certainly happen anywhere in a galaxy as "big star clusters" are all over; the SBH would then orbit at that radius.  I observe that some folks seem to "naturally assume" a SBH would be central: I just don't see that. (By all means someone can tell me different, and *why* would it be in the middle?)  
> **username_4**: @username_2 The barycentre of the solar system spends most of its time *outside* the Sun.  
> **username_2**: @RobJeffries -- whoops. Yes, of course, you're right; I think I confused myself by thinking of the Earth-Moon barycenter (which is within the Earth).  
> **username_3**: I guess all I was saying is that just as random interactions during the formation of the solar system has displaced the center of gravity from the center of the sun, the interaction of matter during and since the formation of the milky way has probably displaced the center of the galaxy from the central SMBH. Also, the galaxy is so large compared to the size of the SMBH that the center is probably not inside the black hole. I could be wrong , but this seems logical.  

---

## 1 answers

### Answer by username_2 on 2016-06-14. Score: 7 | Accepted answer

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](https://astronomy.stackexchange.com/questions/13013/why-do-we-believe-that-the-super-massive-black-holes-at-the-centers-of-two-mergi?rq=1).

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.]

> **Comments:**
> **username_0**: 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?)  
> **username_2**: 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.)  
> **username_0**: Thanks a million for this answer, which changed my understanding of the world around me.  

---

