M7 (NGC 6475)
Distance: 780 Light Years

Right Ascension: 17 : 53.9 (hours : minutes)
Declination: -34 : 49 (degrees : minutes)

M7 is a bright and populous cluster with its brightest blue members projected on a crowded background of dense star fields and dark dust clouds of the Milky Way. M7 is an important astronomical object being the closest prototypical middle aged cluster at an age of about 220 million years. Its age places it midway between a prototypical younger cluster like M45 with an age of about 100 million years, and a prototypical old cluster such as the Hyades, with an age of about 700 million years. The cluster contains about 100 stars and extends over 20 light years of space projected on 1.3 degrees of sky.


Stars change their properties as they age. Astronomers have noticed that lower mass stars tend to show a gradual slowing in their rotation rates and a general depletion in their lithium abundances as they age. Studies of lithium abundance in stars has importance as Lithium survives only in the outermost layers of stars due to its low burning temperature. Lithium is a good tracer of mixing mechanisms within stellar interiors that occur during the stages of stellar evolution. Sunlike stars undergo little lithium loss during their pre-main sequence phase but begin to deplete lithium relatively quickly after entering the main sequence. The lithium depletion stops by the time the star reach the age of the Hyades (700 million years). Lithium loss is related to several factors including the rotation rates of stars, their temperature, mass, age and composition. Rapid rotation tends to improve mixing and slow the transport of lithium into deeper layers where it is destroyed. Stars generally slow their rotation rates as they age causing an increased rate of lithium loss with age. Lithium burning will increase with higher temperatures and mass. The stars of NGC 6475 show rotation rates and lithium levels that are between the extremes of cluster age such as M45 and the Hyades.