Degree of difficulty 1 (of 5)

Minimum aperture Naked eye

Designation NGC 6475

Type Open cluster

Class I3m

Distance 980 ly (K2005) 980 ly (proper motion, 2002) 910 ly (Hipparcos, 1999)

Size 23 ly

Constellation Scorpius

R.A. 17h 53.9min

Decl. –34° 49′

Magnitude 3.3

Surface brightness –

Apparent diameter 80′

Discoverer Ptolemy, 130 BC

History M 7 was already known to Ptolemy as a nebulous patch following the sting of the scorpion when he compiled his star catalog, the Almagest, in 130 BC. 1000 years later, the Persian scholar Al Sufiassigned it a brightness of magnitude 4.5.

Because of its very southerly position, M 7 remained unnoticed by many European observers. Hodierna counted 30 stars in 1654, and he was probably the first to recognize M 7's true nature as a star cluster. Lacaille observed it in 1752 from South Africa and wrote: “Group of 15 to 20 stars, very close to each other, in the form of a square.” He registered M 7 as his number 14 in the category of star clusters.

Charles Messier made his observation of M 7 on the 23rd of May 1764, and noted: “Cluster of stars more considerable than the previous [M 6]; this cluster appears to the naked eye like a nebula; 30' diameter.” Of the later observers, John Herschel gave M 7 some attention and characterized it as “cluster; very bright; pretty rich; little compressed; stars from 7th to 12th magnitude.”

Astrophysics With a total magnitude of 3.3, M 7 is one of the very brightest Messier objects; it takes third place closely behind M 44. At just under 1000 light-years distance, this open cluster gains much of its apparent brightness from its proximity. It still belongs to the local spiral arm, located on its inner side with respect to the galactic center. The rich star fields in the background, however, are made by the galactic bulge, almost 30,000 light-years away.