Discovery of M55
Globular cluster Messier 55 was discovered in 1752 by French astronomer Nicolas-Louis de La Caille during his expedition to South Africa, more than 25 years before Messier. Although the primary goal of the expedition was to determine the distance of celestial bodies in Solar System using the parallax method, he also discovered several deep-sky objects, including the globular cluster visible in this photo which Messier could not observe until 1778.
As their name suggests, globular clusters are group of stars occupying a spherical part of space. They usually orbit around the core of their galaxy, in the galactic halo. There have been about 160 globular cluster discovered in the Milky Way, the galaxy we are living in, but globular clusters occur in other galaxies as well. In Messier 87, for example there are more than ten thousand globular clusters identified.
Globular clusters are ancient formations, containing hundreds of thousands of old stars, which were born in the first star forming waves of the galaxies. Unfortunately there is no generally accepted theory about their formation and evolution yet. Most of the stars of a globular cluster were formed at the same time, meaning they are in the same stage of stellar evolution. Metallicity of stars in globular clusters are low. (Astronomers refer to every element heavier than helium as metals.) The circumstances for star formation are not present in globular clusters, therefore no new stars are being born in them.
American astronomer Allan Sandage found some strange members of globular clusters that were not like the other stars in the cluster: they were more luminous and much hotter than what stellar evolution theories would suggest based on the age of the cluster. Because light of hotter stars are bluer, these stars appear bluer than the rest of the stars in the cluster. That's the reason for the catchy name these stars were given: the blue stragglers. Blue stragglers can easily be spotted in the image above too.
There is no solid theory explaining the existence of blue stragglers in globular clusters either. According to some early ideas blue stragglers weren't real members of globular clusters, just field stars visible in front of them. This theory was disproved quickly when reliable distance measurement methods showed that blue stragglers exist inside globular clusters too. Another theory says that blue stragglers were formed independently, outside of the clusters and were captured by the gravity of the cluster. The most probable explanation though is that blue stragglers were formed by interactions of nearby cluster members. Either by two or more colliding stars, or mass transfer between stars orbiting each other. This theory is supported by the fact that globular clusters are very dense formations with very small distance between their members, therefore gravitational interaction between the stars in them could happen more frequently than between ordinary stars in a galaxy.
Messier 55, the globular cluster shown in the picture is not the densest of its kind. On the Shapley-Sawyer scale that goes from I. (densest) to XII. (sparsest), it is classified as XI. It is located in the halo of the Milky Way, about 17600 light-years from us in the direction of Sagittarius. The mass of the cluster is about 270000 solar masses, confined in a 50 light-years diameter spherical space.