What is this bright "glow" in the center of galaxies?

  • It was always my belief that at the center of many galaxies, there are supermassive black holes. If this is the case, then we should not see a "light" coming out from the center since light get's sucked in black holes. Doing a quick search of galaxies on google image, I came across this:



    Androme



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    These are famous galaxies, and real, and so I was wondering what this very white-yellow glow is in the center. If it is a collection of stars, then why are there so many stars in the center of a galaxy? If there is a black hole there, then why is there light?


    I think you might be overestimating the size of supermassive black holes. They're big, but they're not large on galactic scales.

    Short answer, that is the center of the galaxy, where gravity is the strongest (not just the SMBH, but all the stars in the galaxy), so lots of stars get pulled there. Check this out: https://en.wikipedia.org/wiki/Bulge_(astronomy)

    I think it's fairy dust.


  • If this is the case, then we should not see a "light" coming out from the center since light get's sucked in black holes.




    You are overestimating the size and the capabilities of a supermassive black hole. Contrary to pop sci portrayals of black holes, black holes are not giant vacuum cleaners in space that suck up anything and everything close by. While the supermassive black hole at the center of the Milky Way is indeed very massive (about four million times the mass of our Sun), it isn't very large physically. It's less than a couple dozen solar diameters across. It also isn't that hungry, gobbling up perhaps the equivalent of four or so Earth masses over the course of a year.



    On the other hand, the central bulge of a spiral galaxy contains several million stars in a fairly small volume. That central bulge is what you are seeing in those images. The supermassive black holes near the centers of those bulges gobbles only a tiny, tiny fraction of the light emitted by those millions of stars.


    *"The supermassive black holes near the centers of those bulges gobbles only a tiny, tiny fraction of the light emitted by those millions of stars."* This is an emotional answer, not scientific. How do you define *tiny, tiny fraction* in this phenomena? What is used as the measuring reference? What constrains the black hole to "tiny" gobbling or hungry gbbg? Although I didn't exercise the downvote.

    @Bhumishu米殊 There is a very well known mass-velocity relation in astronomy called M-sigma relation, which relates the mass of the SMBH to the velocity dispersion of stars in the bulge. This paper expands on that and looks at the M_bh-M_bulge relation https://arxiv.org/pdf/1411.1438.pdf , which is typically around 0.5%

    @Dean -- as a minor comment: the relation between supermassive black hole mass and bulge mass is almost as old as the M-sigma relation (in its simpler form, which relates the black hole mass to the bulge luminosity, it's arguably older); the paper you linked to is fine as a reference, but it's not an original or new result.

    I'm curious: How much power does the blackhole at the center radiate due to the consumption of all that matter as it heats up and falls in? What's the closest you could get to it before the outgoing power kills you in some fashion (vaporizes you, or perhaps before that, just nukes your cells with ionizing radiation as no doubt it's mostly hard X and soft gamma coming off so it doesn't have to come anywhere close to vaporization)? And what would the intensity of gravity be like at that point, that is, would this kill you before it has enough hold on you to defeat your rockets?

    E.g. would a human be safe at a light year? Or not? What would the hole look like from that distance to some hypothetical invincible observer?

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Content dated before 7/24/2021 11:53 AM