Star clusters

Star clusters are, as the name implies, a substantial set of several stars relatively close together. Stars are commonly formed in large groups, from molecular clouds (rich in hydrogen). But how to explain “isolated” stars (that is, stars outside groups/star clusters)? One explanation is due to the gravitational interaction between approaching stars, with a possible gain of sufficient kinetic energy to “catapult” the star outside the cluster. In addition, star clusters are contained in galaxies and therefore orbit their center of mass. It is possible that, at a certain point in their trajectory/orbit, the outermost stars of the cluster will detach from it, due to a greater gravitational attraction provided by the galaxy itself. In other words, the galaxy exerts a greater gravitational pull on the stars than the cluster does, making the most distant stars detach. When this happens, the stars are said to have "evaporated". In general, there are two types of star clusters, open and globular. The open clusters typically have 100 to 1,000 stars, with ages ranging from 1x10 ^ 7 to 1x10 ^ 9 years. Globular clusters typically range from 10,000 stars to 10,000,000, aged from 1x10 ^ 10 to 1.3x10 ^ 10 years; its composition is marked by a high number of dwarf stars and a distribution of stars in a spherical pattern, with the highest density being close to the center of these star clusters. More massive clusters (with more stars) are rarer than less massive ones. It is possible to estimate the age of star clusters from the observation of the spectrum emitted by the stars contained within them; the presence of several red giants (which have a more intense brightness) indicates older clusters. Reference material: “The Evolving Universe” (astronomy course at Caltech); https://www.cfa.harvard.edu/research/oir/open-clusters 50 Astronomy Ideas You Really Need to Know (Giles Sparrow) Photo 1: globular cluster

Photo 2: open cluster (The Pleiades, specifically)