M21

From South Dublin Astronomical Society

M21
Type	Open cluster
NGC	NGC 6531
Constellation	Sagittarius
Right Ascension	18h 04.6m
Declination	-22° 30’
Magnitude	6.5
Size	13 arc min.
Image:M21 messier image.jpg

A trio of Messier objects all fit in the same binocular field but we’ll treat M20 and M21 as a “dual” object for the purposes of this highlight. The pair are slightly over 2½ º southwest of the magnitude 3·8 star Mu Sagittarii. M20, the Trifid Nebula, appears as a small ghostly glow centred on a seventh magnitude star that marks the one of the “fins” of a northeast pointing arrow-shaped group. It’s a lovely sight but steadily mounted binoculars will be needed to gradually tease out the dim blur that is the gas cloud. The star that excites the nebula is ADS 10991, a triple system of extremely hot suns. M21, a sparse open cluster, is a little easier and can be seen sprinkled around the star marking the tip of the aforementioned “arrow” pattern. Varying estimates exist for the distance to the cluster though it is believed to be about 4,250 light years away.