M6 (Butterfly Cluster) Complete Guide — Open Cluster in Scorpius

Overview

The Butterfly Cluster (M6) is an open cluster in Scorpius, catalogued as NGC 6405. At magnitude 4.2 and about 1,600 light-years away, it is a naked-eye object from dark sites and one of the most visually appealing summer clusters — its stars arranged in a rough butterfly or moth pattern that becomes obvious in binoculars.

M6 sits just a few degrees from Ptolemy's Cluster (M7) in the rich southern Milky Way, making the two a natural pairing for a single summer evening sweep through Scorpius.

Basic Data

Physical Characteristics

M6 lies about 1,600 light-years away and shines at magnitude 4.2. It contains roughly 80 confirmed members, predominantly hot blue-white B-type stars, with one prominent exception — BM Scorpii, an orange supergiant variable that stands out visually as the cluster's brightest and most colorful member. The cluster's age is estimated at around 100 million years.

Observing Guide

Finding It

M6 sits in Scorpius and transits around 8–10 PM in summer. The cluster lies roughly 5 degrees north-northeast of the Scorpion's stinger stars (Shaula and Lesath). From a dark site, the naked eye detects a bright concentration there even before binoculars are raised. M6 is slightly higher in the sky than M7 and tends to be a degree or two easier to reach from mid-northern latitudes.

Observing Tips

Open clusters show their best at low magnification. Binoculars at 7x or 10x reveal the butterfly wing pattern clearly. At the Eyepiece, the orange-red BM Scorpii is instantly recognizable against the blue-white field stars — one of the most striking color contrasts in any Messier cluster. Avoid high magnification, which fragments the pattern.

Recommended Equipment

Naked eye or binoculars are the optimal instruments. A wide-field telescope at 50–80mm delivers a rich, fully resolved cluster view. This is one of the Messier objects that gets worse as magnification increases, not better.

Astrophotography Tips

Wide-field images of M6 against the summer Milky Way show it in its best context. A camera lens or short refractor with a tracking mount and modest exposures captures the cluster's orange standout star and surrounding blue members. Including M7 in the same or neighboring frame tells a broader story of the southern Scorpius sky.

Summary

The Butterfly Cluster in Scorpius is a showcase summer cluster that needs nothing more than binoculars to appreciate fully. Its distinctive star pattern, the vivid orange of BM Scorpii, and its proximity to M7 make it one of the most satisfying naked-eye objects in the summer sky — a natural reward for any observer pointed toward the southern Milky Way.