If the cluster lies within a [[nebula|nebulous structure]] it's classification is appended with an 'n'. Under this classification method for example, the Pleiades is classified as '''I3rn''' (Strong concentration towards the center, richly populated with stars that have a wide range in brightness, with nebulous material present). <ref>http://adsabs.harvard.edu/cgi-bin/bib_query?1930LicOB.420..154T</ref> | If the cluster lies within a [[nebula|nebulous structure]] it's classification is appended with an 'n'. Under this classification method for example, the Pleiades is classified as '''I3rn''' (Strong concentration towards the center, richly populated with stars that have a wide range in brightness, with nebulous material present). <ref>http://adsabs.harvard.edu/cgi-bin/bib_query?1930LicOB.420..154T</ref> |