M7 Star Cluster

M7 is visible to the naked eye, and in ancient times was known as Ptolemy's Cluster. This star cluster was listed in the Almagest in 130 BC. M7 is a great binocular object, and stands out well against a Milky Way background of numerous faint stars. In this image, the dark nebula B287 is on the immediate right of M7. Further to the right, dark nebula B286 also shows up well and provides a sharp contrast with the 6th magnitude star SAO 209404 at the western edge of that dark nebula. (North is to the left in this image.) Dark nebula B283 is to the left of M7 in this image. NGC 6453 is a tiny dim star cluster that appears as an unresolved nebulous ball in most amateur telescopes. NGC 6444 is an unremarkable star cluster with little concentration. Mouse over this image and annotations will appear that point out each of these objects.

Charles Messier observed M7 on May 23, 1764, adding it as the seventh object in Messier's Catalog. M7 is the southern-most object in Messier's Catalog, and at times escaped notice because it is located so far south. Rated at magnitude 3.3, M7 is the third brightest object on the Messier List, exceeded only by M45 (magnitude 1.5) and M44 (magnitude 3.1). The constellation Scorpius has four objects listed in the Messier Catalog. In addition to M7, M4, M6 and M80 are also located in Scorpius.

M7 has a number of spectral type A to F stars that are still on the main sequence burning hydrogen in their cores. However, M7 is a bit unusual in that it has two blue straggler stars of spectral types B6 and B8. One of them is HD 162586, located at the top left of M7 as shown in this image. Blue straggler stars more commonly occur in globular clusters, instead of open star clusters like M7. (The other blue straggler is designated V957 Sco in the General Catalog of Variable Stars.)

This RGB CCD image was taken from my backyard in Scottsdale, Arizona with a Takahashi Epsilon 180ED telescope using an SBIG STL-11000M CCD operating at f2.8.

M7 (NGC 6475)
Constellation: Scorpius
RA: 17h 53m 46s Dec: -34d 47' 00" (J2000)
July 3, 2010
Image by Sid Leach
Scottsdale, Arizona

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