Messier 7 – Ptolemy’s Cluster

Observation Notes:

A beautiful, wide open cluster near the stinger of Scorpius. While the bulk of the cluster filled the 1.4° view, the more concentrated portion spanned about 15-20′. This concentration was bounded by a square of brighter stars. The SW corner of that square appeared to be marked by a reddish star, while a row of 6 stars spanned the E edge like a spine. As low as this was in the haze of the southern sky, it was still a striking view.


This cluster was recently named after Ptolemy who described it in 130 AD as a “nebula following the sting of Scorpius”. M7 is an open cluster consisting of 80 stars brighter than 10th magnitude. It is 800 to 1000 light years distant, and spans 20-25 light years across. The age of the cluster is estimated at 220 million years. The whole group is approaching us at 14 km/sec. The brightest star is a yellow giant. (Thus demonstrating my current ignorance in estimating the color of stars through the eyepiece–it looked red to me, at least in comparison to the neighboring stars.)

Subject M7/NGC 6475
Classification Open Cluster
Position Scorpius [RA: 17:53.9 / Dec: -34:49]*
Size* 80′
Brightness* 3.3
Date/Time 10/3/04 – 8:00 PM
Observing Loc. Flagstaff, AZ – Home
Instrument Orion SVP 6LT Reflector (150 mm dia./1200 mm F/L)
Eyepieces/Mag. 32 mm (37X)
Seeing 5/10
Transparency Mag 4.5

* Based on published data.