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Thread: Why do so many stars have a higher proper motion than Alpha Centauri?

  1. #1
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    Why do so many stars have a higher proper motion than Alpha Centauri?

    I thought most stars had relatively the same speed, hence the galactic rotation curve.
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  2. #2
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    They do have much the same speed. Including the sun. We see the residual velocities relative to the sun, which are generally an order of magnitude lower than the orbital velocity at the sun's distance from the galactic centre.
    Think of a swarm of bees - they're all headed in the same direction, but all moving relative to each other. Alf Cen's velocity relative to the Local Standard of Rest isn't exceptional.

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  3. #3
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    Let me add that very few stars have higher proper motions than Alpha Centauri, and most of them are inconspicuous red dwarfs near the Sun.

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    The top 9 from Hipparcos are found in https://en.wikipedia.org/wiki/Proper..._proper_motion
    Alpha Centauri is not in it, and Hipparcos data already exclude dim stars like Teegarden.
    Top 20 can be found here
    https://www.cosmos.esa.int/web/hippa...-proper-motion
    and it does reach Alpha Centauri
    Not all are red dwarfs:
    1) Barnard runner 5,96 ly; 10,36创/y; 90 km/s
    2) Kapteyn star 12,8 ly; 8,67创/y; 170 km/s
    3) Groombridge 1830 29,7 ly ;7,06创/y; 315 km/s
    4) Lacaille 9352; ;6,90创/y
    5) Gliese 1; ;6,10创/y
    6) Hip 67593; ;5,83创/y
    7) 61 Cygni; ;5,28创/y
    8) Lalande 21185; ;4,80创/y
    9) Epsilon Indi; ;4,70创/y
    10) Gliese 412; ;4,51创/y
    11) Omicron2 Eridani; ;4,09创/y
    12) Proxima Centauri; ; 3,85创/y
    13) Mu Cassiopeiae; ;3,78创/y
    14) Luyten磗 star; ; 3,74创/y
    15) Alpha Centauri; ; 3,72创/y
    Groombridge 1830, 61 Cygni, Epsilon Indi, Omicron Eridani and Mu Cassiopeiae are not red dwarfs.
    The matter is, Alpha Centauri is the closest star, but does not have highest tangential speed. Higher proper motions are possessed by still nearby stars with a bit larger distances, but higher tangential speeds.
    Last edited by chornedsnorkack; 2018-Dec-02 at 04:39 PM.

  5. #5
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    Those tangential speeds are primarily the result of eccentric orbits, including that of the Sun. The galaxy is not as neat and clean a system as a thought exercise model in which the stars are in circular orbits with equal velocities around the galactic center. We have some stars overtaking us, some lagging behind, some in crossing paths, and combinations thereof.
    Groombridge 1830, 61 Cygni, Epsilon Indi, Omicron Eridani and Mu Cassiopeiae are not red dwarfs.
    Groombridge 1830 and Mu Cas are G-type halo stars with extremely high relative velocities. The other three are nearby K types, not far removed from the M types we commonly call red dwarfs.

    Another bright star with an extremely high relative velocity is Arcturus. It is in a steeply inclined orbit and is plunging through the thin disk near its descending node. If I am not mistaken it is currently classified as a "old disk" star rather than a halo star.
    https://en.wikipedia.org/wiki/Arcturus

  6. #6
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    Also, a technical detail should be mentioned-- "proper motion" refers to apparent motion across the sky, which is why the above list is given in units of angle per year. It can also be converted to velocity if we know the distance, but then it gets called "tangential velocity." So tangential velocity is proper motion times distance, in appropriate units, and this means proper motions should be largest for nearby stars, while tangential velocities should be largest for faraway stars.

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    Quote Originally Posted by Hornblower View Post
    Those tangential speeds are primarily the result of eccentric orbits, including that of the Sun.

    Another bright star with an extremely high relative velocity is Arcturus. It is in a steeply inclined orbit and is plunging through the thin disk near its descending node.
    Eccentric orbits cause peculiar velocities in two dimensions: radial (stars between apoapse and periapse, moving inwards or outwards in the disc) and tangential (stars overtaking Sun if they are near periapse, or overtaken by Sun if near apoapse). Inclined orbits cause peculiar velocities in third dimension (across the disc). So most dimensions (two out of three) are affected by eccentric orbits... but are the scatters of peculiar velocities in the three dimensions also different, or are they equipartitioned?

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    Quote Originally Posted by Ken G View Post
    Also, a technical detail should be mentioned-- "proper motion" refers to apparent motion across the sky, which is why the above list is given in units of angle per year. It can also be converted to velocity if we know the distance, but then it gets called "tangential velocity." So tangential velocity is proper motion times distance, in appropriate units, and this means proper motions should be largest for nearby stars, while tangential velocities should be largest for faraway stars.
    Yes. Also, it is worth noting that alpha Centauri is coming towards the Sun, thus the tangential component of its actual direction will be less than it would be if it were fully tangential. I think the approach angle is something close to 45 deg. A star coming directly for us will have no proper motion, and would likely get some notoriety.

    But the relative speeds through space will affect what is observed, which gets to the OP. This is dramatic when comparing Barnard star to alpha Cen.; Barnard is haulin'! It is coming 5x faster towards the Sun than aCen and it's tangential apparent motion is ~ 2.8x that of alpha Centauri, thus Barnard's tangential speed is about 3.7x that observed for alpha Centauri (6 lyrs vs. 4.5 lyrs).
    Last edited by George; 2018-Dec-02 at 06:17 PM.
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  9. #9
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    Because Alpha Centauri belongs to the thin disk population, as the Sun does, their velocity relative to each other is not that great. The stars with the greatest proper motions are either halo stars, such as Kapteyn's Star and Groombridge 1830, or thick disk stars, such as Barnard's Star and Lacaille 9352.

  10. #10
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    Here's a simulation I put together showing Alpha Centauri's proper motion. It's fun to watch Proxima move with it. You can set the simulation to see any star's proper motion.
    http://orbitsimulator.com/gravitySim...0ff,0,0,0,-800

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