M7 – The Ptolemy Cluster

What is Ptolemy Cluster

Just southwest of its predecessor M6 we find the very conspicuous open cluster M7. This cluster is easily seen by the naked eye. With a good, dark sky you can find a nebula and maybe even several stars. Its name comes from the Greek-Roman astronomer Ptolemy, who first discovered it in the second century. It is sometimes called the “Tail of the Scorpion”.

Ptolemy Cluster


It stands right the sting of the Zodiac Constellation.

• Scorpio

• Other designations: NGC 6475

• Position: right ascension 17h53m51s; declination -34d47m36s

• Type: Open cluster

• Strong concentration towards the center, variation in brightness, more than 100 stars

• Magnitude: 3.3

• Diameter: 20 light years

• Distance: 800 light years

The M6 or M7 was first discovered by Ptolemy (90-168). In the year 130 he documented it in his Almagest, considered the most important book on the universe, as a nebula on the tail of the Scorpion. Over the years it has been seen and documented by many astronomers (including Edmond Halley, discoverer of the famous Comet Halley); In 1764, Messier placed it at number 7 in his catalog. He described it as “A group of stars, more prominent than the previous. With the naked eye this group on a nebula…there is some distance to the foregoing, sandwiched between the bow of Sagittarius and the tail of the Scorpion.”

Older but closer

M7 is a beautiful open cluster because it seems to be embedded in the Milky Way. In many ways it appears to be an M6, but it is 800 light years away and a lot closer to being over 220 million years old. The larger and hotter stars of M7 have grown into giants. The brightest star of M7 is a yellow giant star of type G8III. It has about 120 stars. (About 80 can be seen with a telescope.) Furthermore; it is more compact than M6, M7 which have the same mass (more than 700 times the mass of the Sun) as the stars located in a space that is 20% smaller. In many respects, M7 a typical open cluster. But there are a four stars of a different type that are still not fully understood; the so-called Bp / Ap star.

Magnetic mystery

Stars heavier and hotter than the sun, (types B and A), turn rapidly on their axis like Vega, and have weaker magnetic fields. The BP and Ap-stars are an exception. These rotate slowly and have extremely strong magnetic fields. High concentrations of heavy metals such as Chromium, Manganese and even Mercury can also be compared at this star. The explanation for these concentrations is as follows: because there is relatively little movement in the star the gases continue to move throughout its interior and around heavier floating metals. Why these stars are so magnetic is one of the greatest mysteries of modern astronomy. Several theories have been proposed, but they all stumble over the fact that only a small percentage of B and A stars exhibit these properties. Whoever manages to crack this issue will have great scientific fame; maybe even a Nobel Prize.