I read in papers that the Milky Way contains some retrograde stars (retrograde to the Milky Way rotation). Does anybody know where I can find a list of them, possibly including data on their distance from the Milky way center? many thanks


The Milky Way's outer halo has many globular clusters with a retrograde orbit (about 40% of all clusters in Milky Way). One of the more prominent example include Kapteyn's star which is highly retrograde due to it being ripped from a dwarf galaxy and merging with the Milky Way.

However, the structure of the halo is a topic of an ongoing debate. Several studies have claimed that the halo consists of two distinct components. The "inner" halo consists of more metal-rich, prograde stars and "outer" halo consisting of metal-poor, retrograde stars. These findings have been challenged many times due to the argument on the topic of "duality of motion".

More readings here:

  1. Carollo et al. 2007
  2. Kravtsov 2001
  3. Kordopatis et al. 2020
  • $\begingroup$ very interesting thanks. Can I ask what you mean with duality of motion? $\endgroup$ – magnolia1 Apr 18 at 8:31
  • $\begingroup$ also, is it at least true that the inner part contains prograde objects and the outer part retrograde? Or are they mixed? many thanks $\endgroup$ – magnolia1 Apr 18 at 8:40
  • 1
    $\begingroup$ @magnolia1 duality of motion means some object moving/rotation in one direction and others in opposite directions simultaneously within a same reference frame. Some studies have noted the separation of halo into two distinct components where the clusters show contrasting behaviour but it hasn't been proved yet. So, it is a subject of ongoing debate. $\endgroup$ – Nilay Ghosh Apr 18 at 8:59

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