Globular clusters are dynamically stable objects, due to their large total mass. Their name relates to their mostly spherical form. About 150 of these objects are known in the Galaxy, which they surround in a widespread halo. However, that number is dwarfed by the quantity of globular clusters surrounding other galaxies, e.g., 16,000 around the giant galaxy M 87. Looking at the distribution of our globular clusters in the sky, there is a remarkable concentration towards the galactic center - the inner halo. These globular clusters are frequently found in, or crossing, our galactic disk while orbiting the galactic center.
The nearest globular clusters have distances comparable to those of open clusters. M 4 at about 5600 light-years, for example, is nearer to us than M 11. Towards the galactic center, however, we not only see globular clusters in front of it (like M
22), but also next to it (like M 28) or on its far side (M 72). Towards M 75 at a distance of 78,000 light-years, our view crosses two-thirds of the galactic diameter.
A special case among the globular clusters is M 54. It is not a galactic object but belongs to the small dwarf galaxy SagDEG. Hence, at a distance of 85,000 light-years, this is the most distant globular cluster of the Messier catalog.
The physical size and the mass of globular clusters differ greatly. The smallest example is M 71: with 40,000 solar masses and fewer than 100,000 stars, it is not much larger than the largest open clusters. At the other extreme are M 19 and M 54, each with several million stars and a mass of 1.5 million solar masses, approximately that of a dwarf galaxy.
Photographs, in particular, give the impression of very high star densities in globular clusters. In reality, however, there are between 10 and 1000 stars per cubic light-year at their centers - which means that the average distance between stars is still 0.1 to 0.5 light-years. Only
The distribution of open clusters and galactic nebulae of the Messier catalog on the galactic plane, within a radius of 10,000 light-years around the Sun. The nearest spiral arms are indicated.
when a globular cluster has undergone a core collapse (by dynamical instability and transfer of kinetic energy out of the core region) are super-densities of up to 100,000 stars per cubic light-year reached. A visual measure of the degree of concentration is the classification of globular clusters into class I (extremely compact) to XII (very loose), analogous to the "Trumpler classes" for open clusters. While this scheme has no relevance to astrophysical research any more, it is a very useful one for the visual amateur observer. The most concentrated globular cluster in the Messier catalog is M 2 (II), the loosest examples are M 55 and M 71 (XI).
Galactic globular clusters have ages of about 10 to 13 thousand million years - about 100 times older than most open clusters. Having lost their interstellar matter, globular clusters cannot form new stars. Hence, all their member stars are very old, which is why they show the elemental composition of earlier stages of the Universe. Elements heavier than hydrogen and helium can form only by the central nuclear processes of stars and supernova explosions, and they are dispersed iy into the interstellar medium by supernovae and stellar winds to feed new star formation. Hence, globular clusters give testimony of the early Universe when heavy elements were much less abundant, and that distinguishes them from all other stellar clusters.
Low abundance of heavy elements and low stellar mass bring about a special group of variables, typical for globular clusters: the pulsating RR Lyrae stars. Lacking a static equilibrium between radiation pressure and gravity, these stars oscillate over the course of a few hours and show brightness variations of about 0.5 to 2 magnitudes. In addition, there are other types of variables in globular clusters. The record holder is M 3 with 274 known variable stars, while in M 10 only four variables have been discovered so far.
Luminous stars of spectral type O and B are not found in globular clusters, because these stars "die" very young. Oddly enough, a few blue stars are found in globular clusters: "blue stragglers." Hence, these must have been formed recently -supposedly by the merging of close, old binary stars. Some globular clusters even host pulsars. The record holder in the Messier catalog is M 62 with six of them.
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