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One "fun fact" that's always been, well fun. Is despite what most assume, our sun does not orbit a supermassive black hole or any object at the center of our galaxy. Recently I read an explanation that our Milky Way is gravitationally bound to our local group. This would explain the predicted collision with Andromeda, and a formation of some super galaxy beyond that. However, I'm sure I've seen estimates of "the Sun's journey around the Milky Way", and local galaxies should have a fairly negligible effect on our sun.

An explanation I found that made sense was Dark Matter. More common than "normal" matter. Dark Matter's gravitational effects were the first hints it existed. Observations of galaxy's showed "normal" matter simply wasn't strong enough to maintain a galaxy's structure. Without Dark Matter galaxies would fall apart.

Here's where I'm unclear. Is it innacurate to say our sun travels around the Milky Way? The spiral structure of the arms seem to imply a general direction. I've been assuming Dark Matter was responsible for structure AND general rotation. Where galaxies containing high concentrations of Dark Matter tended toward spiral structures, while galaxies with less mass tended towards undefined shapes and random star distributions.

Do galaxies tend toward order as total mass is increased, while reverting to complete disorder when a galaxy loses mass?

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  • $\begingroup$ Interesting question, and Welcome to Stack Exchange! "...despite what most assume, our sun does not orbit a supermassive black hole or any object at the center of our galaxy." Yep! When one is embedded within a diffuse mass distribution, we can say we "orbit" anything significantly closer to the center in the sense that we do rotate around it, but at the same time since there is no one single object that is the primary body determining the the Sun's motion around the Milky Way's center, one can also say we don't technically orbit anything, at least not orbit anything in particular. $\endgroup$
    – uhoh
    Sep 16 at 3:30
  • $\begingroup$ It all depends on what the meaning of the word "orbit" orbits :-) $\endgroup$
    – uhoh
    Sep 16 at 3:31
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    $\begingroup$ So I don't mean falling into orbit, the way Earth orbits the Sun. I'm assuming an overall general rotation, where a large percentage of stars are rotating in a shared direction. Obviously there will be many stars with variables influencing múltiple behaviors. However with such well defined structure in our home Galaxy, that would imply a higher concentration of Dark Matter correct? $\endgroup$
    – Troy Dube'
    Sep 16 at 4:30
  • $\begingroup$ Well Wikipedia's Dark matter; Observational evidence first paragraph is about the Galaxy rotation curve. I'm not an astronomer and can't speak to the evolutional aspects of high vs low dark matter galaxy formation, (maybe Wikipedia can) but we can measure our rotation speed around the center of the galaxy and show that it's faster than it should be based on all the observable mass of the galaxy, and use that difference to estimate amount and to some extent distribution. $\endgroup$
    – uhoh
    Sep 16 at 5:10
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    $\begingroup$ So based on observing the movements of stars, we realize an exotic matter is necessary to describe the observed speed. We then can estimate the amount of Dark Matter, by removing observable matter, and gain a limited understanding of the overall structure of the Milky Way. $\endgroup$
    – Troy Dube'
    Sep 16 at 5:23

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The dark matter isn't directly responsible for the rotation of the Galaxy - that is a consequence of the initial angular momentum (or the angular momentum it has accrued during its formation).

Dark matter is required to explain the rotation speed as a function of radius that are observed in the Milky Way (and other spiral/disc galaxies). The dark matter in the Milky Way needs to be distributed pseudo-spherically on a scale that is much larger than the distribution of the normal, luminous matter.

It is correct to say that the Sun "orbits" the Galactic centre, but it is the combined effects of all the mass in the Galaxy that define its orbit, not any central object. As a result, the orbit is not "simple" (like the orbits of planets around the Sun) and it wobbles up and down and in and out and it is not "closed" in the sense that it will not follow exactly the same trajectory each time it laps.

Within the 8 kpc galactocentric radius of the Sun's roughly circular trajectory around the Galaxy, it is the normal luminous matter of the bulge and disk that provide most of the centripetal gravitational force on the Sun; dark matter dominates orbits only at galactocentric radii of $>10$ kpc. Without any dark matter we would still expect the solar "orbital" speed to be about 180 km/s, compared with the 220 km/s it actually has (e.g. see https://physics.stackexchange.com/a/169141/43351 ).

The second part of your question conflates spiral structure with "order". The difference between spiral and elliptical galaxies is indeed that most of the stars and gas in spiral galaxies orbit in a plane with approximately circular orbits, whereas stellar orbits in ellipticals have a much wider variety of orbital inclinations and eccentricities. This difference is not really attributable to differences in mass (whether total or just dark matter), it is a consequence of how those galaxies formed and how star formation occurred in the gas that was present.

In a spiral galaxy, there was plenty of available gas that collapsed to a disc prior to the majority of star formation - this leads to the observed "ordered" orbits of the stars today. On the other hand, if a galaxy forms from the merger of essentially gas-free smaller units, then the stars form a collisionless system and the orbits get scrambled during the merger - leading to an elliptical galaxy.

Measuring how much dark matter there may be in elliptical galaxies is considerably harder than for spirals - precisely because they do not have stars and gas on ordered pseudo-circular orbits. However, measurements of masses using strong gravitational lensing do indicate that most ellipticals do contain significant amounts of dark matter within their luminous regions (e.g. Bovy 2023). i.e. The amount of dark matter present is not what explains the differences between spiral and elliptical galaxies and since elliptical galaxies can be as massive as the most massive spiral galaxies, it isn't total mass that explains their differences either.

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    $\begingroup$ Wow. That was a very good answer. Thank you. I think I was caught up from hearing the overwhelming amount of dark matter compared to baryonic matter, obviously all mass will have an effect on a given system. A galaxy's structure sounds like it's a complicated combination of many variables like formation, collisions, and age. I don't know why I tried to simplify it. Kinda obvious now. The only question I'm left with is something I've heard so many times, I'm gonna have some angry letters to write if it's false. Is dark matter responsible for keeping galaxies from "falling apart"? $\endgroup$
    – Troy Dube'
    Sep 16 at 18:27

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