M7, also known as Ptolemy’s Cluster, is a bright open cluster in Scorpius constellation. It lies at an approximate distance of 980 light-years from Earth and has the designation NGC 6475 in the New General Catalogue. It is also known as Collinder 354 or Lac II.14. With a visual magnitude of 3.3. The cluster is the southernmost Messier object in the sky at right ascension 17h 53m 51.2s and declination -34°47’34”, which makes it a challenging object for those in northern latitudes, as Scorpius constellation never rises very high above the horizon. The best time of year to observe M7 is in the summer months. M 7 can be found 4.75 degrees northeast of the star Lambda Scorpii, also known as Shaula. Shaula marks the scorpion’s stinger and is the second brightest star in Scorpius constellation, fainter only than the red supergiant Antares. Ptolemy’s Cluster lies only five degrees southeast of the Butterfly Cluster (Messier 6).
Messier 7 is one of the most prominent open clusters in the sky, known since antiquity. It was named Ptolemy’s Cluster because it was first recorded by the Greek astronomer and mathematician Claudius Ptolemy in the 2nd century. Ptolemy listed the cluster in his Almagest as Object Number 567 and described it as a “nebula following the sting of Scorpius” in 130 AD. Italian astronomer Giovanni Batista Hodierna counted 30 stars in the cluster before 1654. In 1678, English astronomer Edmond Halley included the cluster in his catalogue of southern stars as No. 29. French astronomer Nicolas Louis de Lacaille observed the cluster on June 15, 1752, and listed it as Lac. II.14 in his catalogue of southern objects. Charles Messier included the cluster as the seventh entry in his catalogue on May 23, 1764. In the 19th century, M7 was accurately described as “coarsely scattered clusters of stars” by the English astronomer John Herschel.
Messier 7 contains about 80 stars between magnitudes 6 and 10, all within an area corresponding to a linear diameter of 25 light-years. The tidal radius of M7 spans 40.1 light-years. The stars within this area can’t be pulled away from the cluster by the gravitational influence of the Milky Way. M 7 is believed to be about 220 million years old and has a mass about 735 times that of the Sun. It is approaching us at a speed of 14 km/s. The brightest star in M7 is a yellow G8-type giant with an apparent magnitude of 5.6. The stars in M7 were all formed at roughly the same time in the same large cosmic cloud. Groups of stars in open clusters, which are all approximately the same age and have similar chemical composition, are invaluable to scientists as they provide insight into stellar evolution and structure. The brightest members of the cluster – up to 10 per cent of M7’s population – will eventually end their lives in violent supernova explosions, while the remaining fainter stars will gradually drift apart until they no longer form a cluster. Messier 7 also contains four magnetic Ap/Bp stars: HD 162305, HD 162576, HD162725, and HD 320764.
Credits: Messier Objects, Universe Today, Wikipedia.