Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Retrograde star orbits in the Milky Way

  1. Dec 11, 2016 #1
    Are there any stars in the solar neighborhood that orbit the galactic center in the opposite direction?
  2. jcsd
  3. Dec 11, 2016 #2
    Not sure if this counts, it's a fun video with some corny music:

  4. Dec 11, 2016 #3
    None known, and no reason (that I know of) to expect that there might be.
  5. Dec 12, 2016 #4
    What about stars with retrograde rotation (like Venus and Uranus)?
  6. Dec 12, 2016 #5


    User Avatar
    Science Advisor
    Gold Member

    These are 'wandering stars', which in greek is asteria planetes, from which the English word 'planets' derives.
    That is, these are not actual stars. :)

    Furthermore, retrograde rotation is not the same as retrograde orbit. The difference is the same as between a day and a year.

    So, perhaps you can tell us exactly what you're looking for - it might be easier to get you an answer.
  7. Dec 12, 2016 #6
    All planets orbit the Sun in the same direction,
    Note that is the SUN, and has nothing to do with the galactic center.
    Sometimes a planet as seen from Earth appears to reverse then go forward again.
    However it isn't really doing that, this is an illusion due to the different speed of planet orbits.
    Rather like if you have a fast car and overtake a slower one, it look similar to as if the slow car moved backwards.
  8. Dec 12, 2016 #7
    Sorry for the confusion. I understand the difference between retrograde orbit and retrograde rotation. After my original question was answered that there are no known stars in the Milky Way with a retrograde orbit, I then asked if any stars have a retrograde rotation. "Like Venus and Uranus" was intended to solely point out their retrograde rotation with regard to our sun. I wasn't implying that these are stars.
    Does the black hole at the galactic center spin in a certain direction that we can detect?
  9. Dec 12, 2016 #8
    Ah, as for actual stars rather than planets.
    While they all appear to be rotating around the galactic center in the same direction, (more or less),
    that direction is is not correlated with their spin around their own axis.
    As far as I know the spin on their own axis, as far as can be determined. which can be hard, it can be just about anything.
    Certainly it is not correlated with the spin of our Sun on it's axis, or with the orbits of solar planets.
  10. Dec 12, 2016 #9
    No, people answered you question: Are there any nearby stars in retrograde orbit?

    The answer to that depends a lot if your definition of nearby (I would consider within 10 ly, 100 ly, 1000 ly, 10000 ly all to be resonable definitions of nearby depending on you purpose). There are however billions of stars in retrograde galactic orbits in the milky way, most of them are in the halo, in-falling star streams from dwarf galaxies being absorbed or in the galactic bulge. They are mostly tens or hundreds of thousand light years away from Earth but at just 12 ly one star that is hard to exclude is the M1 sub-dwarf Halo star known as Kapteyn's Star (A&A paper).
  11. Dec 12, 2016 #10
    Interesting! From Wikipedia's article Retrograde and prograde motion:

  12. Dec 13, 2016 #11
    So, stars with retrograde orbit exist, and the nearest of them is Kapteyn's star.
    No objects qualifying as planets orbit Sun in a retrograde direction. However, a number of bodies do orbit Sun in retrograde directions, but are classified as minor planets or comets rather than planets. Most conspicuously Halley Comet.
  13. Dec 15, 2016 #12
    A star that is tidally locked with the galactic center is also possible I suppose.
  14. Dec 16, 2016 #13
    I'm merely speculating here, but I guess a galactic collision between a big galaxy like ours and a much smaller galaxy (so that the big galaxy does not lose it's individuality) could produce retrograde motions of stars.
  15. Dec 16, 2016 #14
    No, I don't think so and I have never heard about it before. How would it even work?
  16. Dec 16, 2016 #15
    I guess this means, could a star be tidally locked with Sag A*, the SMBH embedded in the core.
    If it is possible at all it would have to be very close to the SMBH.
    Stars further out would be overwhelming more subject to gravitational influence of nearby stars in the own region.
  17. Dec 17, 2016 #16


    User Avatar
    2017 Award

    Staff: Mentor

    While technically correct, it hides the causal order. The Milky Way started with gas clouds with a random total angular momentum, which then assembled to a disk over time based on collisions between gas clouds. Those clouds then formed the stars in the disk with prograde motion. Stars with retrograde motion were captured from outside or got their motion from violent events in binary systems (e. g. their partner star exploded in a supernova) or similar events.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted