m2 – globular cluster
Lupe Rioné, Amateur astronomer (Spain)
Constellation of Aquarius
Globular clusters: Globular clusters are concentrations of thousands to millions of stars joined gravitationally. They all orbit in the halo of a galaxy, although some of them can be located near the center of the Milky Way.
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History and discovery
M2 is a globular star cluster and the second object in the Messier catalog. Also called NGC 7089, it was discovered in 1746 by French astronomer Jean-Dominique Maraldi, while he was observing a comet. Because many astronomers were “comet hunters” at the time, a wide variety of other celestial bodies such as galaxies, nebulae, and star clusters were discovered. At the time, Maraldi described M2 as a very round nebula.
14 years later, in 1760, Charles Messier observed it on his own. Studying it again in 1764, he described it in his catalog as a nebula, because its structure was not easily distinguishable. However, in 1783 William Herschel detailed M2 much more precisely, and in a very approximate way to how we know it today. Herschel wrote: “The scattered stars were seen with a well-defined focus, in which it seems that the condensed central light is due to the multitude of stars that appeared at various distances behind and close to each other”.
This spectacular composite image of visible and infrared light taken by the Hubble Space Telescope shows the dense core of M2. Credit: Hubble
M2 – Globular Star Cluster. Credits NASA
Location and observation
M2 is located in the constellation of Aquarius, forming a triangle with the stars Alpha and Beta of that constellation. Its localization is easy and can be distinguished as the end of a row of stars. The cluster can be seen from both the northern and southern hemispheres with an apparent magnitude of 6.5. However, without excellent visibility conditions, it is not easy to observe with the naked eye. With the help of a small telescope or binoculars, M2 would be seen as a single object, while with a powerful telescope we can see the stars that make up this beautiful cluster.
Widefield photo where we can see the beautiful M2 Cluster . Credit: Oservatori de la Pobla Torna (OPT)
M2 star cluster. Credit: iniciacion.com
M2 is one of the most exciting objects to observe. Because globular clusters are ancient structures, these celestial bodies provide professional astronomers with clues about galactic evolution. Located 37,000 light-years away in the galactic halo, and with about 150,000 stars, its Halton Arp color-magnitude diagram (1962) estimated the age of its stars to be between 11 and 13 billion years. That is, they are very old stars that were formed during the dark age of the universe. The structure of the cluster is spherical with a dense, compact core about 3.7 light-years in diameter, while the entire cluster extends about 175 light-years. In the density scale, M2 is the second largest with 12° degrees of density.
The brightest stars in M2 are red and yellow giants, with a magnitude of 16.1, while the stars in its horizontal branch have a brightness of 13.6. The cluster has about 21 variable stars, the first two discovered by Bailey in 1895. Most of them belong to the variable star type RR Lyrae (named after its prototype which is RR de la Lyra). They are variable stars of spectral types A and F. A particularity of this cluster is that it contains a variable star of type RV Tauri (named after RV tau, the variable star in the constellation of Taurus), a type of supergiant variable star. These two types of variable stars show changes in luminosity linked to radial pulsations on their surface, which are accompanied by changes in the spectral type. For this reason, when these stars are at their maximum brightness they can give a different appearance to the cluster. Really fascinating.
M2 – Globular star cluster. Credit: NASA
M2 can be located in the constellation of Aquarius
Somewhere, something incredible is waiting to be known