Star clusters are groups of stars that gravity holds together. We distinguish two different types of star clusters: globular clusters are close groups of hundreds of thousands of very old stars, while open clusters contain less than a few hundred stars and are often very young.
A globular cluster is a globular cluster of stars that orbits a galaxy like a satellite. Globular clusters are strongly gravitationally bound, giving them a spherical shape and a relatively very dense core. They contain hundreds of thousands to millions of stars. They concentrate around the core of the galaxy, forming a galactic halo.
Globular clusters have a spherical shape with lengt of 50-400 light-years. In the center of the cluster, the density of stars is up to several thousand times higher than in the vicinity of the Sun. For example, the 47 Tucanae cluster has a core density of about 1,000 stars per cubic light-year. Star clusters do not decay over time. The stars that make up a globular cluster are, with a few exceptions, very old. Today, there are 135 known globular clusters in the Galaxy, and the total number is estimated at 200. The globular clusters visible in the northern sky include, for example, M13 or the Great Cluster in Hercules. Our nearest globular cluster is M4 in the constellation Scorpio.
Open clusters are groups of stars whose number does not exceed several thousand stars. They are usually much younger than globular clusters, often containing the remnants of their parent nebula. They are found in the disks of spiral galaxies, or in irregular galaxies. Their gravitational bonds are relatively weak and the star clusters disintegrate into individual stars over time. The most famous open star cluster, easily visible to the naked eye, is called the Pleiades in the constellation Taurus. Other open clusters visible to the naked eye under suitable atmospheric conditions are Chi and H Persei in the constellation Perseus or M7 in the constellation Scorpio.
According to the color diagrams, there are 3 groups of open star clusters:
In the first group, there are star clusters whose all stars are on the main sequence (for example, the Pleiades). The stars of such open star clusters are often white, or even bluish in color, which may be due to the presence of reflective nebulae as well as the young age of the stars themselves.
In the case of second-class star clusters, most of the stars are on the main sequence, but a small part has already reached the giant branch (eg the Jaslička Star Cluster).
Yellow and red giants are mostly obsolete in third-class star clusters.
There is also a so-called a motion star cluster, which is a special type of open star cluster that is so close to Earth that it occupies a larger area in the sky and its stars move toward a common point in the sky (the vertex). Their apparent motion towards the vertex is caused by the perspective convergence of the parallel orbits of the stars of the motion star cluster. This fact is used to accurately determine the distance of the motion star clusters (distances in space). The most famous motion star clusters are the Hyades (distance 145 ly), Scorpio-Centaurus (72 ly) and Ursa Maior (or Collinder 285, distance 330 ly), which contains about 100 stars scattered throughout the sky; its center is formed by the bright stars of the Big Dipper.
The study of star clusters is very important in various fields of astronomy. Because all the stars in the cluster formed at about the same time, the different properties of these stars are caused only by the different amounts of matter that make them up, and therefore theories of stellar evolution rely on observations of open and globular clusters.
Star clusters are also a crucial step in determining the distance scale in space. Several of the nearest star clusters are close enough that their distances can be measured using parallax. For these star clusters, a Hertzsprung-Russell diagram can be compiled, which has absolutely known values on the luminosity axis. After compiling similar diagrams for star clusters whose distance is unknown, the position of the main sequence can be compared with the position of the first cluster, so that the distance can be determined.