Star clusters vary in their structure and are classified into open and globular clusters. The first catalogue of star clusters was compiled in the 18th century by the French astronomer Charles Messier.
The Pleiades are among the best known open clusters. They are located in the constellation of Taurus and are commonly known as the "Seven Sisters", although six, nine or even ten stars may be observed there, depending on visibility. The Pleiades belong to the group of loose clusters and are about 410 light-years from our solar system. The individual stars, about 130 in total, are about one light-year apart from each other.
The Hyades is another star cluster that can be seen with the naked eye. Containing around 350 stars, it lies about 150 light-years away from us. It is also a loose cluster, which means that all of its stars are moving independently and away from each other.
Open star clusters dissolve over time. They are groups of up to several hundred new stars that are still located at where they were born.
globular_star_clusters are much larger conglomerations of stars. They can consist of several hundred thousand up to one million stars. There is a higher density of stars at the center than at the periphery. Stars in a globular cluster are generally very old, dating back to the birth of the galaxy. Their life span is approximately 10 billion years. Most of them are red giants, but there are also some variable stars.
The brightest of the globular clusters are visible to the naked eye as luminous, hazy patches. Alpha Centauri and NGC104 in the constellation of 47 Tucanae, both in the southern sky and therefore not visible from Europe, belong to this category. The Small Magellanic Cloud also lies in the southern hemisphere.
