A cluster is an area of greatly increased density of stars compared to the surrounding region of a galaxy. How much its concentration exceeds the star background, but it can be very different. The stars of a cluster are usually together in the sense that they are formed together. A distinction Open clusters - which are relatively young and are in the spiral arms - of the globular clusters that surround the old structure to the galaxy in a halo.
Star clusters are important for the study of stellar evolution, since all stars have a similar age. Moreover, they are in almost the same distance, making the interpretation of measurements is safer.
Open clusters represent rather loose collections of stars that have emerged from large molecular clouds (gas and dust) together. Nevertheless, they dissolve mostly with the time on it because their proper motions are somewhat different. Therefore, many open clusters " astronomically young " objects from one age to a few hundred million years. You still have a lot of young, massive stars that shine by their high temperature, especially in the blue and white light. One of the most famous open star clusters are the Pleiades.
In the much richer globular cluster star one goes often from a common star formation from; remain gravitationally bound to each other the stars. Globular clusters surrounding the galaxy at a greater distance and are much older than the open clusters, they are part of the halo. The age of many globular clusters is of the order of 10 billion years (the age of the universe is estimated to be 13.7 billion years ). The brightest observable with the naked eye globular cluster Omega Centauri and is located in the southern hemisphere of the sky.
A third group are called stellar associations, also moving clusters or stellar streams. They are related to the open cluster and have mostly young, hot stars that seem to lie move together towards a vanishing point (vertex ). They include the Hyades, whose stellar distribution has contributed decisively to the development of the Hertzsprung- Russell diagram.
Super star clusters
The only discovered lately class of super star clusters is between the open and globular clusters. There are areas with extremely high rates of star formation, which leads to very compact, massive and relatively long-lived star clusters. They show a large number of bright main sequence stars are surrounded by ionized H II regions. Typical is the emergence in active starburst galaxies.