Our summer season sky is bespangled with a whole bunch of glowing open star clusters. One of many best is M7 in Scorpius. It lies simply 5° east-northeast of the Scorpion’s stinger stars: Shaula (Lambda [λ] Scorpii) and Lesath (Upsilon [υ] Scorpii). Beneath darkish skies, M7 could be seen by the bare eye as a small patch of sunshine towards the glow of the encircling Milky Way. Relying on the place you reside, you will have a very good view to the south to identify it, because it lies practically 35° south of the celestial equator. That makes it the southernmost member of the Messier catalog.
Ptolemy will need to have loved a very good view of M7 in A.D. 130 as a result of he subsequently famous the cluster as a cloudy patch in his monumental tome Almagest. That’s why, even immediately, M7 continues to be generally known as Ptolemy’s Cluster. Had Ptolemy had binoculars, nevertheless, he would have instantly seen the cloudy patch resolve right into a hanging array of stars spanning greater than 30′. Astronomers estimate the cluster is a few 980 light-years away and stretches about 25 light-years throughout. Between 80 and 100 stars name M7 dwelling, though a plethora of area stars within the surrounding area make the precise quantity laborious to pin down.
As a result of M7 covers a large swath of sky, binoculars and wide-field scopes are finest for appreciating the thing’s magnificence. Greater than 30 of its stars are brighter than tenth magnitude and visual in 50mm binoculars. Via 10x50s, the brighter members seem to drift in entrance of a area strewn with fainter factors, creating a fake 3D impact. A number of of these cluster stars present delicate hues of yellow and blue.
All of M7’s stars have been born from an interstellar cloud of fuel and dust about 200 million years in the past. And that very same cloud additionally gave beginning to the open cluster M6, discovered 4° to the northwest. Whereas their stars share a typical ancestry, they’re, at finest, distant kin, as M6 is half once more the space.
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