Wait....help me understand this. Stars are expected to move SLOWER near a galactic center than away from it?

Wait....help me understand this
Stars are expected to move SLOWER near a galactic center than away from it? Isn't that the opposite of Kepler's 3rd law?

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  1. 2 weeks ago
    Anonymous

    galaxies don't have the majority of their mass in the center like the solar system does.

    • 2 weeks ago
      Anonymous

      you memorized some stupid gravity formulas out of a dumb textbook that are all wrong and as a result you now grandiosely and preposterously presume that you understand how everything in the universe works and you don't want to admit that the lame garbage you memorized is clearly wrong because your absurd special snowflake soience genius fantasy life inasely revolves around being proud that you were able to memorize the physics 101 material.

      • 2 weeks ago
        Anonymous

        "grandiosely" the post

      • 2 weeks ago
        Anonymous

        chill out moron.
        Galaxies are like planets where at their center there's equal mass in all directions so the gravity gets cancelled out.

      • 2 weeks ago
        Anonymous

        Dunning Kruger speedrun, any%.

      • 2 weeks ago
        Anonymous

        I swear, if OP wrote this I will be very annoyed, I may even furrow my brow, you came here looking to have the question answered and now you're insulting the first poster. Even if he is wrong. You probably know less about the subject.

        • 2 weeks ago
          Anonymous

          He's not me lol
          My criticism of FP is that his reasoning is circular. The OP pic is widely cited as providing evidence for dark matter, while his post presupposes the existence of dark matter.

          It may be popsci, I need to find the original source for this graph

          • 2 weeks ago
            Anonymous

            Dark matter is irrelevant here.
            The expected curve shows stars also moving slower close to the center of a galaxy for the first 10,000 light years or so.
            FPs explanation applies here.

          • 2 weeks ago
            Anonymous

            That's my main question. Kepler's law says bodies travel FASTER near the center of the galaxy and slower at the edges. So why do both lines on this graph predict the exact opposite?

          • 2 weeks ago
            Anonymous

            because as already pointed out Kepler's laws apply for systems where the vast majority of mass is at the center.
            The distribution of mass in a galaxy, especially throughout the galactic bulge is not like this.

    • 2 weeks ago
      Anonymous

      >galaxies don't have the majority of their mass in the center l
      they literally do

      • 2 weeks ago
        Anonymous

        Mine doesn't.

  2. 2 weeks ago
    Anonymous

    Wait, did I misinterpret this graph?
    When a body reaches "aphelion" (furthest from orbit's center), its orbit slows down as per Kepler.
    But is there a law of conservation of momentum that says a reduction in orbiting speed leads to an INCREASE in rotational speed?

    • 2 weeks ago
      Anonymous

      So, yes, it is true that as a body reaches apoapsis, the conservation of momentum leads to a loss in linear velocity, but this graph, doesn't seem to be sampling a single star's orbit around the galactic center, correct me if I'm wrong, it seems to be sampling multiple stars velocities once or at least each of their averages. Therefore, a body that is in a higher orbit will have to be traveling faster than a body in a lower orbit. Although I will say the graph lacks context.

  3. 2 weeks ago
    Anonymous

    That is actually Kepler's third law perfectly.

  4. 2 weeks ago
    [email protected]

    Kepler laws hold for a body in orbit around a CENTRAL MASS (typically a sun or a planet). A galaxy is a distribution of masses. A star at distance R from the galaxy center effectively "sees" only the masses at distances below R from the center, and this means that stars CLOSER to the center experience LESS gravity than stars further away. This tends to counteract the Kepler law "faster if closer to central mass". The actual velocity of the stars thus depends on the details of the mass distribution in the galaxy. You should expect nearly Keplerian behavior only for galaxies with an huge central black hole accounting for most of the galactic mass.

  5. 2 weeks ago
    Anonymous

    The decreasing portion is pretty obviously the central bulge. That's how all clouds under gravity behave. The centre hardly moves, same reason that you'd feel zero g at the earth's core

    • 2 weeks ago
      Anonymous

      because as already pointed out Kepler's laws apply for systems where the vast majority of mass is at the center.
      The distribution of mass in a galaxy, especially throughout the galactic bulge is not like this.

      Oh...that makes sense lol. Does an uneven mass distribution like this allow for eccentric orbits, or does it favor circular orbits?

      One last question, if two objects of different eccentricities orbit a central mass A, and both intersect through point B, which travels fastest at point B? The circular orbit, or the elliptical? (assume equal mass)

      • 2 weeks ago
        Anonymous

        Not the same anon, but a cloud of objects that are constantly affecting each other's orbits usually result in crazy n-body interactions that might sometimes settle into elliptical orbits, with circular orbits being always relatively more rare in nature, but sometimes you'll get chaotic things like this because all the point masses are interacting with each other. pic related is 3 bodies of equal mass interacting with each other, and thus do not settle into what can be described by a keplerian orbit.
        And yes in pic related, at point B, the object of circular orbit will be traveling faster, you can think about it intuitively by imagining a constant acceleration vector towards the body being added to a differently sized tangential velocity vector.

      • 2 weeks ago
        Anonymous

        I'd say the circular one travels faster at B. Because the elliptical-orbiting body doesn't have enough speed to remain at that distance, but falls back

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