Messier 21
Messier 21 or M21 , also designated NGC 6531 , is an open cluster of stars in the north-east of Sagittarius , angularly close to the Messier objects M20 to M25 , save for M24 . 
<triplet> Messier 21 <subj> open cluster <obj> instance of <subj> Sagittarius <obj> constellation