Today begins a marathon that will lead us to discover, week after week, the various objects in the stellar catalog of Charles Messier. The famous astronomer published the first version of his catalog, the first containing astronomical objects different from the stars, in 1771. In the following years new revisions followed, bringing the total of objects from the 45 of the first edition to the 110 of the last. The astronomical objects of Messier’s catalog are of different nature: galaxies, nebulae, globular clusters, open star clusters, and many of them lend themselves well to observation with amateur instruments even of reduced performances. In the previous months we had already seen some objects from Messier’s catalog, so I will not repeat myself and invite you to read the articles dedicated to the Crab Nebula (M1), the Pleiades (M45) and the globular cluster of Hercules (M13).
The M2 globular cluster is located in the constellation Aquarius, about 37,500 light years from Earth, and has a diameter of 175 light years. M2 is located near the celestial equator, and is therefore visible to almost all the peoples of the Earth. You can already observe it with binoculars, near the star Sadalmelik. With a 150-200 mm telescope you get a much better view, while with telescopes over 250 mm the component stars of the cluster are resolved.
The globular clusters are agglomerates of stars of typically spheroidal shape, maintained by the strong gravity present within it. They are in orbit around the spiral galaxies, for example around the Milky Way there are over 150, and they are formed by very ancient stars of population II, poor in metals. The latter, in fact, usually originate during the explosion of a supernova and therefore are present in greater quantities in the stars of the next generation, stars of population I, of which our Sun is an example.
Returning to our globular cluster, M2 was initially observed by Giovanni Domenico Maraldi in 1746 and, as often happened at the time, was rediscovered in a completely independent way by Charles Messier in 1760, who later included it in his catalog. Messier wrote, about this cluster:
Nebula without stars in the head of the Aquarius, the center is bright, and the light that surrounds it is round; it resembles the beautiful nebula that lies between the head and the arch of Sagittarius, and is seen very well in a two-foot telescope, placed on the parallel of (alpha) Aquarii.
The nucleus of the cluster is very concentrated, in about 3.7 light years, and the entire cluster with its gravity can reach the stars up to about 233 light years. The brightest stars are red and yellow giants and, by studying the HR cluster diagram, it was possible to determine an age of about 13 billion years.