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The super massive black hole at the centre of Milky Way has a mass of about 4 million times that of our sun. Is this enough to keep the entire galaxy together, is it this black hole which keeps the galaxy together? I did read that there are also galaxies that don't have a super massive blackhole, what keeps those galaxies together?

O S
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    I’m voting to close this question because it belongs either on Astronomy stack exchange or Physics stack exchange. (But the answer is: the mutual gravitation of the stars in the galaxy, even in galaxies with SMBHs, since SMBHs are a very small proportion of the mass of most galaxies: certainly of ours). –  Feb 28 '21 at 13:24
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    Could someone transfer this question to the right place? – O S Feb 28 '21 at 13:28
  • I don't have the power to do that I think (or at least I don't know how): sorry! –  Feb 28 '21 at 13:36
  • It's a good question but I’m voting to close this question because it is off-topic here, not about Space Exploration as defined in the help center. Luckily however it's 100% on-topic in Astronomy SE and possibly Physics SE, so I hope it will be migrated soon and find a new home :-) – uhoh Feb 28 '21 at 13:52
  • @tfb we should vote to close because it's off-topic here, not because we think it belongs somewhere else. Recommendations where it might be on-topic should be separate from the close reason. – uhoh Feb 28 '21 at 13:54
  • @OS it's just been closed and I think it is a good candidate for migration, so just sit tight and I think that may happen. Only the site moderators can migrate and they'll review this now that it's closed. – uhoh Feb 28 '21 at 13:55
  • @tfb don't forget the gas and dust. – Christopher James Huff Feb 28 '21 at 14:50
  • @uhoh there is an actual close reason for suggesting a question be migrated to a different SE. It isn't populated with anything useful on this site (yet?), but clearly migration was considered a valid close reason when designing the stack exchange platform. – Starfish Prime Feb 28 '21 at 16:48
  • @StarfishPrime I think there are probably good reasons why it can not be used. I don't know the history of it but it transmits the idea that the community decides where it belongs and the "may be better asked on" folks can start moving questions to different sites. I think it's good that that relic lies mostly dormant. It might be interesting to ask a question in Space Meta to find out why it's not really activated and if we should start down the "better asked on" slippery slope or not. – uhoh Mar 01 '21 at 00:10
  • @StarfishPrime The OP and any user can leave comments about migration and we can also leave flags for the moderator as suggestions. Flagging has a cost associated with it (we don't want to raise flags that are rejected too often) so only seriously considered recommendations to migrate vie for the moderator's attention and time. I'd added such a flag for the moderators when I left this message for the OP. I think this way prevents hasty/creative "better asked on" votes but still allows serious recommendations – uhoh Mar 01 '21 at 00:15
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    @StarfishPrime now I've had to remove the flag because the OP went ahead and cross-posted to Astronomy – uhoh Mar 01 '21 at 02:15
  • @ChristopherJamesHuff: yes, I should have said that, sorry. I think the mass of some galaxies is mostly gas & dust isn't it? –  Mar 02 '21 at 11:02
  • @tfb and whatever it is, most galaxies have more dark matter than visible matter (though some have barely any). – Christopher James Huff Mar 02 '21 at 15:28
  • @uhoh Part of me thinks migration should be an uber-high rep privilege - like, 50k or 100k, or 250k on main SO. No one accidentally gets 50k rep without learning the ins and outs of migration, especially on smaller/beta sites. – corsiKa Mar 02 '21 at 20:45
  • @corsiKa I know what you mean, but high rep does not denote good intentions, interest in maintaining the site's quality and a good experience for other users, nor good will towards others in any way. On the other hand being a moderator comes with a substantial commitment, recognized responsibility, a bit of a vetting process and a great deal of visibility. With higher rep do come some moderation tools, like editing posts (I do that a lot) but those are visible to all and easily reversible. Migration is pretty much permanent & irreversible and requires negotiation w/ moderator at the other site – uhoh Mar 02 '21 at 23:09
  • @uhoh Possibly, if only because of the irreversible factor. But If two or three high rep users all agree with a migration, it is probably going to be alright. I rarely have to negotiate with the mods of a site I migrate to, to be honest. It's pretty clear where it belongs most of the time. – corsiKa Mar 02 '21 at 23:51
  • @corsiKa two or three high-rep users can cabal together and start migrating any entire class of questions that they don't want to be answerable, that for example could allow another user to get as much reputation as they have. There are some that take the gamification aspects of SE very seriously and there are inconvenient rules in place that exist only because people were gaming the system for personal reputation enhancement (e.g. subsequent bounties on the same question must double up to 500). I don't have faith in "it's pretty clear" being a universally shared view with gamification afoot. – uhoh Mar 02 '21 at 23:57
  • @corsiKa I like the "OP decides where to ask, community decides if off-topic here" system the way it is. I think has found a good balance of forces, wills and interests as it currently stands. I don't think two or three users should have scope-narrowing powers for an entire site. That's for discussions in meta. – uhoh Mar 02 '21 at 23:59

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