I recently noticed that none of the planets in our solar system orbit the Sun in a retrograde direction. This is likely because they formed moving in the same direction as the planetary disk did in respect to the Sun.

Are there stars that move retrograde to the Milky Way galaxy's rotation? If so, where are they concentrated, and why?


About two-thirds of the Milky Way’s estimated 100 billion stars reside in the disk, and these all move in direct orbits. Most of the remaining third are in the football-shaped central bulge, and while their orbits are more varied, on average they too are direct. A few percent of our galaxy’s stars occupy a vast, spherical halo. These travel on randomly distributed orbits — accordingly, half are direct and half retrograde.

Source: Do astronomers have any idea what percentage of our galaxy’s stars move in retrograde orbits?

And while formation of disk galaxies is a complex process, which is not yet fully understood, the reason that almost all stars move in the same (direct) orbits is the conservation of angular momentum when gravitational force pulls gas clouds to come together. It is the same mechanism that is responsible for the planets to orbit the star in the same direction.

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    $\begingroup$ what about why? I'll accept that when you answer that :) $\endgroup$ – Max0815 Mar 6 '19 at 18:41
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    $\begingroup$ fair enough :) Added a paragraph to the answer about conservation of angular momentum being the reason for almost all stars to move in the same (direct) orbits. $\endgroup$ – Oleg Muir Lou Goff Mar 7 '19 at 3:13
  • $\begingroup$ sorry for the late response xD My inbox alert is broken so I didn't receive an alert that somebody resonded! Thank you! $\endgroup$ – Max0815 Mar 7 '19 at 23:09

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