Messier 7

Right Ascension	17 : 53.9 (h:m)
Declination	-34 : 49 (deg:m)
Distance	0.98 (kly)
Visual Brightness	3.3 (mag)
Apparent Dimension	80.0 (arc min)

Known to Ptolemy 130 AD.

Messier 7 (M7, NGC 6475) is a large and brilliant group, easily detected with the naked eye. As Burnham describes it, "the cluster is seen projected on a background of numerous faint and distant Milky Way stars."

This splendid cluster was known to Ptolemy, who mentioned it about 130 AD and described it as the "nebula following the sting of Scorpius." The description may also include M6, but this is uncertain. Because of this presumable discovery, the present author [hf] has proposed the name "Ptolemy's Cluster" for M7 some years ago, a proposition which has found some acceptance meanwhile.

Consequently, M7 was observed by consecutive observers following Ptolemy, notably including Al Sufi around 964 AD, and Ulugh Begh (15th century). Al Sufi, in his drawing of Scorpius, has depicted the "nebulous" star reasonably close to the position of M7, which supports the identification.

M7 was observed by Hodierna before 1654, who counted 30 stars. Edmond Halley listed it as No. 29 in his catalog of southern stars of 1678, and Nicholas Louis de Lacaille added it to his catalog of southern objects as Lac II.14. Charles Messier included it as No. 7 in his catalog on May 23, 1764.

M7 consists of about 80 stars brighter mag 10 in a field of about 1.3 degrees apparent diameter which at its distance of perhaps 800 light years corresponds to a linear extension of 18 or 20 light years. It was classified as of Trumpler type I,3,m or I,3,r. This group is approaching us at 14 km/sec. The brightest star is a yellow giant (spectral type gG8, mag 5.6), the hottest main sequence star is of spectral type B6 (mag 5.89). M7's age was estimated at 220 million years, both according to the Sky Catalog 2000 and the new calculation of the Geneva Group of G. Meynet; Wu et.al. (2009) have it at about 300 Myr. Recent work suggests a slightly larger distance of about 1000 light-years, which would increase the size to 25 light-years, but not affect the age significantly.

Ake Wallenquist found that this is one of the clusters with the highest degree of concentration toward the center. Modern sources agree on M7's integrated apparent visual brightness at i magnitude 3.3, while older estimates, mostly from northern observers, had this southern cluster significantly underestimated at mag 4.1 to 5.0.

According to Wu et.al. (1009), the orbit of this cluster within the Milky Way galaxy is of little excentricity (0.02), typical for open clusters, with its distance changing between 24,800 and 26,100 light-years from the galactic center, and orbital period 218.6 million years. It is also oscillating around the galactic plane with maximum "z" distance of (only) about 130 ly and a period 31.7 Myr.

NGC Online data for M7

References:

• Zhen-Yu Wu, Xu Zhou, Yun Ma, and Cui-Hua Du, 2009. The orbits of open clusters in the Galaxy. Monthly Notices of the Royal Astronomical Society, Vol. 399, pp. 2146-2164 (November 2009) [ADS: 2009MNRAS.399.2146W]. Vizier Catalog: [ADS: 2011yCat..73992146W]

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Hartmut Frommert
Christine Kronberg
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Last Modification: August 14, 2023