# Suppose The Earth Was In The CENTER of The Galaxy **URL:** \ **Category:** Factual Questions\ **Created:** [October 16, 2003, 1:17pm UTC](https://boards.straightdope.com/t/suppose-the-earth-was-in-the-center-of-the-galaxy/207774 "2003-10-16T13:17:38Z")\ **Posts on this page:** 12\ **Page:** 2
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [October 17, 2003, 6:21am UTC](https://boards.straightdope.com/t/suppose-the-earth-was-in-the-center-of-the-galaxy/207774/21 "2003-10-17T06:21:33Z")
> [@](#): > > _Originally posted by Achernar \* > \*\* So a 4 MM[sub]SUN[/sub] BH at 1AU will have 4 million times the tidal force of the sun at 1AU, or about 1.8 million times the tidal force of the Moon on the Earth. I don’t know if that’s enough to rip Earth apart, but it would probably make living there a pain._\* It’d be pretty close to breaking apart. The Roche limit (the point at which gravitational binding force of the planet is equal to the tidal force) for the earth orbiting a 4-million M[sub]sol[/sub] black hole is about 0.6 AU. (That’s a first order estimate but I think it’s in the right order of magnitude.) Any guesses on how large and dense the accretion disk would be? I have a feeling that’d be a major obstacle. ---
**Author:** ![Angua](https://avatars.discourse-cdn.com/v4/letter/a/8baadc/32.png) [@Angua](https://boards.straightdope.com/u/Angua)\ **Post date:** [October 17, 2003, 7:45am UTC](https://boards.straightdope.com/t/suppose-the-earth-was-in-the-center-of-the-galaxy/207774/22 "2003-10-17T07:45:19Z")
> [@](#): > > \*Originally posted by Chronos \* > \*\* > > What do you mean by this, **Angua**? Even if it’s closer than the sun (which I see no reason to assume), it won’t fill more than half the sky, and you could still get light from the rest of the sky. And if it’s further away than the planet’s sun, then there wouldn’t be any problem at all. \*\* OK, I was working under the assumption that getting light from t herest of the sky could well be problematic. However, seeing as the density of stars in the disk is greater than out here, I can see this won’t be a problem. ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [October 17, 2003, 10:59am UTC](https://boards.straightdope.com/t/suppose-the-earth-was-in-the-center-of-the-galaxy/207774/23 "2003-10-17T10:59:50Z")
> [@](#): > > \*Originally posted by scr4 \* > \*\*It’d be pretty close to breaking apart. The Roche limit (the point at which gravitational binding force of the planet is equal to the tidal force) for the earth orbiting a 4-million M[sub]sol[/sub] black hole is about 0.6 AU. (That’s a first order estimate but I think it’s in the right order of magnitude.) > > Any guesses on how large and dense the accretion disk would be? I have a feeling that’d be a major obstacle. \*\* I don’t think the accretion disc is would be too major as Sag. A\* is not an active galactioc nuclei which means that there is no large, constant infall of matter otherwise it would be an active galactic nuclei (quasar/blazar/radio galaxy). ---
**Author:** ![Angua](https://avatars.discourse-cdn.com/v4/letter/a/8baadc/32.png) [@Angua](https://boards.straightdope.com/u/Angua)\ **Post date:** [October 17, 2003, 11:20am UTC](https://boards.straightdope.com/t/suppose-the-earth-was-in-the-center-of-the-galaxy/207774/24 "2003-10-17T11:20:44Z")
> [@](#): > > \*Originally posted by MC Master of Ceremonies \* > \*_I don’t think the accretion disc is would be too major as Sag. A_ is not an active galactioc nuclei which means that there is no large, constant infall of matter otherwise it would be an active galactic nuclei (quasar/blazar/radio galaxy). \*\* Indeed, the accretion disc shouldn’t be too much of a problem. However, since we’re in a spiral galaxy, if the nucleus was active, we’d probably be more of a Seyfert galaxy rather than quasar/blazar/radio galaxy, which tend to be hosted by giant ellipticals. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [October 17, 2003, 2:28pm UTC](https://boards.straightdope.com/t/suppose-the-earth-was-in-the-center-of-the-galaxy/207774/25 "2003-10-17T14:28:03Z")
S2 [orbits the central hole](http://www.mpe.mpg.de/www_ir/ir_recent.php#GCS2) with a period of 15.2 years and a semi-major axis of 5.5 light days. Applying simple [centrifugal calculations](http://www.labcentrifuge.com/gforce5.html) (rcf = 0.00001118 r X RPM^2) gives a value of ~0.0012 g. That’s using the semi-major axis in a circular orbit. The value would be higher for an ellipse. I’d expect that 0.0012 g acting on the outer atmosphere of a star would cause some major hydrogen leakage, or am I all washed up here ? ---
**Author:** ![Sarcastro](https://avatars.discourse-cdn.com/v4/letter/s/e495f1/32.png) [@Sarcastro](https://boards.straightdope.com/u/Sarcastro)\ **Post date:** [October 17, 2003, 4:33pm UTC](https://boards.straightdope.com/t/suppose-the-earth-was-in-the-center-of-the-galaxy/207774/26 "2003-10-17T16:33:19Z")
> [@](#): > > \*Originally posted by Mycroft H. \* > \*\*Isaac Asimov wrote a story/novella called ‘Nightfall’. It involved a planet with multiple suns so there was no darkness except, by a certain conjunction with a dark planet (IIRC), every 50,000 (?) years. The planet was closer to the galactic center so there were lots more stars when they did come out. The story line involved the person predicting the impending problem, with evidence of civilization’s collapse every 50,000 years, and the chaos that did ensue when Nightfall occurred. > \*\* FYI, he later expanded it into a full novel, which of course gave a lot more background and fleshed out the story (and the aftermath) quite a bit. In that, it was 2,000 years, but I don’t remember if it was different in the original short story. Worth reading, I think. Though what little I’ve read about orbital mechanics would seem to indicate that the scenario would be wildly improbable - although in an infinite universe, it’s gotta happen somewhere, right? ---
**Author:** ![The\_Bad\_Astronomer](https://avatars.discourse-cdn.com/v4/letter/t/f05b48/32.png) [@The\_Bad\_Astronomer](https://boards.straightdope.com/u/The_Bad_Astronomer)\ **Post date:** [October 17, 2003, 7:32pm UTC](https://boards.straightdope.com/t/suppose-the-earth-was-in-the-center-of-the-galaxy/207774/27 "2003-10-17T19:32:51Z")
> [@](#): > > \*Originally posted by Achernar \* > \*\*This is only true if you’re talking about at the event horizon of each of the two black holes. \*\* Yeah, I wasn’t clear in my post. That’s indeed what I meant. You can get closer to the center of a small black hole than a bigger one, so the tidal force can be bigger for a small black hole. A star would get ripped apart by a stellar mass BH, but not necessarily a supermassive one. A star might fall in whole! I don’t have the wherewithal to calculate the Roche limit for a star. Anyone wanna do the full-up polytropic analysis? 🙂 ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [October 17, 2003, 7:41pm UTC](https://boards.straightdope.com/t/suppose-the-earth-was-in-the-center-of-the-galaxy/207774/28 "2003-10-17T19:41:34Z")
I could be wrong, but in “Nightfall” they were in a cluster, not the galactic core, and there were about 30,000 stars visible to the naked eye, as opposed to the 6,000 we have at Earth. One thing that bugged me was when they had to pick an order of magnitude for the separation between stars, they picked four light years, which is the typical separation in the solar neighborhood. I mean, come on. Not only would this value be too big for a cluster, but why would they even think it’s so vast if they had never even seen a star? ---
**Author:** ![Angua](https://avatars.discourse-cdn.com/v4/letter/a/8baadc/32.png) [@Angua](https://boards.straightdope.com/u/Angua)\ **Post date:** [October 18, 2003, 11:04am UTC](https://boards.straightdope.com/t/suppose-the-earth-was-in-the-center-of-the-galaxy/207774/29 "2003-10-18T11:04:06Z")
> [@](#): > > \*Originally posted by The Bad Astronomer \* > \*\* I don’t have the wherewithal to calculate the Roche limit for a star. Anyone wanna do the full-up polytropic analysis? 🙂 \*\* OK, not the full thing, but my trusty “Cambridge Handbook of Physics Formulas”, aka, my primary bible, gives the Roche limit for a star as R\>~(100M/9pi rho)sup[/sup] So, if we assume M~4 MM[sub]sun[/sub], and rho=rho\_sun~470kg/m[sup]3[/sup] Then, the Roche limit for a star is R~2.6AU (or 3.9x10[sup]11[/sup]m) Comparing this to the Schwartzschild radius (incorrect I know!) of a 4 MM[sub]sun[/sub] black hole, which is 0.08AU, I would think that a star would be ripped apart even by a SMBH. I could be wrong though. ---
**Author:** ![Choarg](https://avatars.discourse-cdn.com/v4/letter/c/b4bc9f/32.png) [@Choarg](https://boards.straightdope.com/u/Choarg)\ **Post date:** [August 10, 2026, 1:27am UTC](https://boards.straightdope.com/t/suppose-the-earth-was-in-the-center-of-the-galaxy/207774/30 "2026-08-10T01:27:40Z")
That is… kind of complicated. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 10, 2026, 2:22am UTC](https://boards.straightdope.com/t/suppose-the-earth-was-in-the-center-of-the-galaxy/207774/31 "2026-08-10T02:22:36Z")
[Moderating] Welcome, @Choarg. Please note that the thread you’re replying to here is likely older than you are, and most of the participants are no longer active on this board. Although we do permit bumping new threads to add substantive new information, what you have posted does not do so, and so I will close this thread. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 10, 2026, 2:22am UTC](https://boards.straightdope.com/t/suppose-the-earth-was-in-the-center-of-the-galaxy/207774/32 "2026-08-10T02:22:43Z")
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