Photo Credit: Dieter Willasch from Hakos Farm, Nambia: CLICK photo for larger size and see below for links and information.

If you have a clear view of the constellation Scorpius you’ve likely seen this open star cluster or at least some of its stars simply by looking up without any aid from a telescope or binoculars. This cluster is about 1000 light years away, 200 million years old and contains about 100 stars with a total size of about 25 light years in diameter. It was known about since 130 A.D. when it was noted by the Greek astronomer, astrologer, mathematician & geographer, Cladius Ptolemy who described it as the “Nebula following the sting of Scorpius.”

M7 was also observed by Italian astronomer Giovanni Hodierna who counted 30 stars sometime around 1654. In Edmond Halley’s 1628 catalog of southern stars he had the cluster listed as No. 29. In Nicholas Louis de Lacaille’s catalog of southern objects he listed it as Lac II.14. So you can see, this very visible grouping of stars was well viewed and documented before Charles Messier added it as No. 7 to his list of non-comets on May 23, 1764.

The brightest star in the cluster is a yellow giant that shines at an apparent magnitude of 5.8 and the entire structure is approaching us at about 8.5 miles per second (14 km/sec. Something else that grabs your attention in this photo is the massive star field in the far distant background near the galactic center as well as thick dust bands which may or may not be related to the formation of the cluster itself.

FYI: An open star cluster is a loose grouping of upwards of 1000 stars formed from the same molecular cloud. This commonality helps in the study of stellar evolution just as the more ancient and longer lived globular star clusters do but on a different time scale. Though they are held together by mutual gravitational attraction they are often stretched and pulled apart through the millennia through interactions with other objects in the Milky Way and in time, through the radiation pressure from each other. Some famous open star clusters include M45 the Pleiades & the Hyades in the constellation Taurus, the Big Dipper asterism in Ursa Major and M11 the Wild Duck Cluster in the constellation Scutum.

NAME: Messier 7, M7, NGC 6475 and sometimes the Ptolemy Cluster.

WHAT IS IT?: Open star cluster.

HOW BIG IS IT?: Approximately 25 light years across, 80’ arcminutes on the night sky (about twice the Moon’s apparent diameter) and contains approximately 1000 stars.

HOW FAR AWAY IS IT?: Approximately 1000 light years away.

APPARENT MAGNITUDE?: Brightest star is approximately 5.8 or +5.8 while the cluster itself is 3.3 or +3.3.

WHERE IS IT? (General): Constellation Scorpius the scorpion.

WHERE IS IT? (Exact RA/DEC J2000): RA 17h 53m 51.2s / DEC –34° 47′ 34″.

Dieter Willasch page for this photo:

NASA Astronomy Picture Of the DAY (APOD) page for this object:

SEDS page on M7:

Image | This entry was posted in Astronomy (Learning), Astrophotography (DSOs), Images, People, Star Clusters (Globular-Open) and tagged , , , , , , , , , , . Bookmark the permalink.


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