Editing Stellar association (section)

===R associations===
Associations of stars that illuminate reflection nebulae are called ''R associations'', a name suggested by Sidney van den Bergh after he discovered that the stars in these nebulae had a non-uniform distribution.<ref name="ra1" /> These young stellar groupings contain main sequence stars that are not sufficiently massive to disperse the interstellar clouds in which they formed.<ref name="Mon R2">{{cite journal
 |author1=Herbst, W. |author2=Racine, R.
 | title=R associations. V. MON R2.
 | journal=Astronomical Journal | year=1976 | volume=81
 | pages=840
 | doi=10.1086/111963| bibcode=1976AJ.....81..840H| doi-access=free}}</ref> This allows the properties of the surrounding dark cloud to be examined by astronomers. Because R-associations are more plentiful than OB associations, they can be used to trace out the structure of the galactic spiral arms.<ref>{{cite journal
 | first=W. | last=Herbst
 | title=R-associations III. Local optical spiral structure
 | journal=Astronomical Journal | year=1975 | volume=80
 | pages=503
 | doi=10.1086/111771 | bibcode=1975AJ.....80..503H}}</ref> An example of an R-association is [[Monoceros R2]], located 830 ± 50 [[parsec]]s from the Sun.<ref name="Mon R2" />