Messier 23 (NGC 6494): Open Cluster in a Sagittarius Starfield

As you explore the lower southern skies near Sagittarius and the heart of our galaxy, you’ll discover breathtaking regions teeming with star clusters and nebulae. Unfortunately, those of us in the mid to higher latitudes may have to navigate around neighborhood trees to see all these wonderful objects. The good news is there are a fair number of nice clusters and nebulae in the upper reaches of Sagittarius and Scorpio, which gives us a better shot at seeing these southern gems.

The constellation Sagittarius is home to Messier 23 (M23), a spacious and radiant open star cluster. This cluster is one of several within the summer Milky Way and can be observed through binoculars in dark sky conditions or a small telescope from the suburbs.

M23 is about 2,150 light-years from Earth and has an apparent magnitude of +5.5. It contains around 150 stars, reaching a diameter of almost 20 light-years across. If the cluster were to shine as brightly as a streetlamp, it would fill an area of sky the size of. full moon!

Charles Messier first recorded M23 on June 20, and 20 years later, in 1784, William Herschel described it as a “cluster of beautiful scattered, large stars, nearly of equal magnitudes.”¹

My Observations

Tonight is one of the best backyard observation sessions I’ve ever had. The skies are clear; the moon is down. The night is settled, restful, and full. I decided to set the scope up further north in the yard, allowing me to see the lower south.

I decided to stay out a little longer for M23 to break the tree line, and I’m so glad I did. M23 is a fantastic, wide star cluster. It fills my 25 eyepiece well. The brightest stars appear to make a “V” shape with another bright line down the middle of the “V.” One could also say it appears like three strands of lights across the field.

I’m going to see if I can catch Messier 24 nearby at this midnight hour before everything falls beneath the Douglas Firs.

Key Stats

Constellation	Sagittarius
Best Viewing	Summer
Visual Magnitude	+5.50
Absolute Magnitude	-3.49
Distance from Earth	26,000 ly
Diameter	2000 ly
Apparent Size	29 arcmin
My Viewing Grade	A+
Designations	M 23, NGC 6494

Historical Observations

June 20, 1764, observation by Charles Messier

“A star cluster, between the end of the bow of Sagittarius & the right foot of Ophiuchus, very near to 65 Ophiuchi, according to Flamsteed. The stars of this cluster are very close to one another. Its position was determined from Mu Sagittarii. (diam. 15′)” ¹

Sources

Banner photo of M23 adapted from Sergio Eguivar and licensed under the Creative Commons Attribution-Share Alike 4.0 International license. Source here.

McArthur, G., & Frommert, H. (n.d.-b). Messier 23. http://www.messier.seds.org/Mdes/dm023.html

Sketches by Wayne McGraw