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From my understanding, it is believed that almost every big galaxy and especially spiral galaxies have supermassive black holes (SMBH's) at the center. Also, from what I've read, a SMBH isn't required for a galaxy to exist since and in layman's terms, the matter inside a galaxy such it gas clouds and formation of stars will keep it gravitational-ly in check.

If I am right or even wrong, then would there be significant difference if our galaxy did not have a SMBH at the center? Significant enough for noticeable differences even here on Earth?

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The supermassive black hole (SMBH) in the center of the Milky Way (MW) — called Sgr A* [Sagittarius A-star] — has no direct impact on our galaxy. Its mass is only a few million Solar masses, and if you remove it$^\dagger$, it will only affect the most central stars, which would suddenly continue in straight paths out through the MW. These stars would almost surely not hit any other stars or something like that (since stars are really, really far apart), but some of them have velocities high enough that they may escape the MW.

If Sgr A* weren't there to begin with, things might look a little different. There seems to be a relation between the mass of a galaxy's SMBH and the velocity dispersion of the stars in its central bulge; the so-called M-sigma relation. so MW without Sgr A* would mean a more ordered center. Our Solar system is located in the disk, far from the center, and their is evidence that SMBHs have little impact on the disk (Gebhardt et al. 2001). However, in their early phase (as an active galactic nucleus), their extreme luminosities cause galactic superwinds which blow out gas and may quench star formation (Tombesi et al. 2015).

$^{^\dagger}$Removing Milky Way's SMBH is left as an exercise for the reader.

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  • $\begingroup$ I wondered if the gamma ray bursts that likely emitted from the SMBH when the milky way was young had any effect on star formation, so there might be an effect there, but I'm not at all sure. $\endgroup$ – userLTK Aug 21 '15 at 17:24
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    $\begingroup$ @userLTK: Gamma-ray bursts are massive, dying stars, and doesn't really have anything to do with SMBHs. SMBHs do emit radiation in their QSO phase. The gamma-ray portion of this light is relatively small, but you're on the right track, bacuse the total amount of radiation does cause galactic winds which can deplete the galaxy of gas. I'll edit my answer a bit. Thanks. $\endgroup$ – pela Aug 21 '15 at 22:27

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