M7 and M6. Photo: Akira Fuji.
Ah, the Ptolemy Cluster!
It's too far south for me, but it looks great in photographs. Seemingly suspended above billions of yellow grains of sand, yellow stars in the bulge of the Milky Way, M7 looks like collection of blue sapphires surrounding one brilliant orange spessartite garnet
. By contrast, the "sister cluster" of M7, M6
, the Butterfly Cluster, is spreading its wings against a lot of dark dust.
It's interesting to compare M7 with the Pleiades
. There are interesting similarities, but the Pleiades are a little brighter and bluer. There are several B-type stars in M7, but most of them are spectral type B8 and B9, whereas the brightest stars in the Pleiades are about spectral class B6 and B7. But there is a star in M7, HD 162374, which is classified as a B6 star and is described as a blue straggler by Bright Star Catalog. Fascinating! Blue stragglers are thought to form when a main sequence star gains extra mass somehow and remains on the main sequence, or remains higher up on the main sequence, than the other stars of the same age and mass. I guess the blue star of Algol
can be considered a blue straggler.
The bright stars of the Pleiades are brighter than the bright stars of M7, about 300-400 times solar or so versus about 150-300 times solar for M7. But the difference isn't great. The biggest difference is the presence of red giants in M7. Of course, there is a prominent red giant in M6, too.
When Alcyone, brightest star of the Pleiades, turns into a red giant, then the Pleiades might look somewhat like M7 or M6. Particularly since the Pleiades has most likely shed its lovely blue reflection nebula.