#messier 7

Addition to brief information from AI

In the middle northern latitudes, the Ptolemy cluster is not observable. There, it either does not rise or does not rise to a height sufficient for confident visibility. And only starting from the latitude of the subtropics can it be observed with light optics, and even further south - in the tropical zone of the northern hemisphere - with the naked eye.

Claudius Ptolemy conducted his observations from Egyptian Alexandria. And more northern astronomers of antiquity did not notice this cluster. Because its integral brightness - 3m - decreased significantly as it approached the horizon due to atmospheric absorption.

A small remark regarding the number of stars in this cluster

There are about a hundred identified participants. Why do astronomers talk about several hundred stars possibly included in the cluster?

Because for the stable existence of a cluster of 25 light years (moreover, the radius of gravitational dominance for this cluster is estimated at 40 light years), its mass must be at least 800 solar masses. On average, each visible star must have a mass of about 10 solar masses, and this is a fairly large mass - the mass of a supergiant star. But such are not observed in the cluster - there are mainly stars of medium masses and luminosities. Consequently, a significant part of the cluster is represented by dwarf stars, like the Sun or less massive.

By the way, if we place our Sun at a distance of 1000 light years (approximately this distance is removed from us by the Ptolemy cluster), then without taking into account the absorption of light by the interstellar medium, it would have only 13 stellar magnitude, and such stars - background and not only - in the visible limits of this cluster are a great number. But the amount of light absorption in the direction of the center of the Galaxy (and this is the direction we are looking when observing the Ptolemy cluster) is significant. And the main part of the stellar population of the cluster is most likely weaker than the 15th stellar magnitude. All objects of such brightness have not yet been studied by astronomers.

But astronomers have studied the distribution of stars by mass in open clusters, from which we can draw a simple conclusion: If we observe a certain number of bright and clearly visible stars in a particular open cluster, then most likely the total number of all stars in the cluster is at least 10 times greater.

New image from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile, shows the bright star cluster Messier 7, aka NGC 6475