Object Name: Messier 7
Alternative Designations: M7, NGC 6475, Lac II.14, The Scorpion’s Tail, Ptolemy’s Cluster
Object Type: Type “E” Open Star Cluster
Right Ascension: 17 : 53.9 (h:m)
Declination: -34 : 49 (deg:m)
Distance: 0.8 (kly)
Visual Brightness: 3.3 (mag)
Apparent Dimension: 80.0 (arc min)
Locating Messier 7: One of the easiest ways to find the “Ptolemy’s Cluster” is to recognize the two familiar constellation asterisms of Scorpius and Sagittarius. The bright star the represents the ‘stinger’ on the tail of the Scorpion is Lambda. Aim your binoculars three fingerwidths east. Under dark skies it will show as a conspicuous patch in the sky, but do not confuse it with its dimmer, northwestern neighbor, M6. In binoculars, Messier 7’s stars will appear of varied brightness with no particular pattern and will occupy about 1/3 the field of view in average binoculars. Easily seen in the finderscope, use lowest magnification when observing with any telescope because of Messier 7’s large apparent size. Because it is so bright, this open cluster is a great object on a moonlit night and larger telescopes can fully resolve its members.
What You Are Looking At: This bright collection of about 80 mixed magnitude stars is estimated to be about 800-1000 light years away from Earth. Moving along through space in an area spanning about 18-25 light years across, this group of stars were all born about the same time some 220 million years ago – yet they have evolved differently. Approaching us at a speed of about 14 kilometers per second, the brightest star you see is a yellow giant of spectral type G8. Messier 7 also contains four magnetic Ap/Bp stars: HD 162305, HD 162576, HD162725, and HD 320764
History: This great open star cluster is most often credited to Ptolemy, who listed in in his ‘Almagest’ as Object Number 567 in 130 AD. From his notes he describes it as “A nebulous cluster following the sting of Scorpius.” It was also independently recovered by Ulegh Begh and listed as 564 in his catalogs, Edmond Halley listed it as No. 29 in his catalog of southern stars of 1678 – as well as Ha II.2 by Hodierna and Lac II.14 by Lacaille. However, we know it best by its catalog given by Charles Messier when he discovered it for himself on the night of May 23, 1764: “I have determined in the same night [May 23 to 24, 1764] the position of another star cluster which is more considerable and of a larger extension: its diameter could occupy 30 arc minutes. This star cluster also appears at simple view [to the unaided eye] like a considerable nebulosity: but when examining it with a refractor, the nebulosity disappears, and one perceives nothing but a cluster of small stars, among which there is one which has more light: this cluster is little distant from the preceding; it is between the bow of Sagittarius and the tail of Scorpius. I observed in the Meridian the passage of the middle of this cluster, and compared it to the star Epsilon Sagittarii for determining its position: its right ascension was 264d 30′ 24″, & its declination 34d 40′ 34″ south.”
It’s truly a shame the Messier didn’t understand about spectroscopic binaries with red-giant primaries, searching for links between magnetic fields and stellar evolution, element abundances in the metal-rich open clusters or finding benchmark brown dwarfs to probe the substellar initial mass function as a function of time – but when we look at Messier 7? We do!
B&W image thanks to Palomar Observatory, courtesy of Caltech and color image thanks to N.A.Sharp, REU program/NOAO/AURA/NSF.