Lupe Rioné, Amateur astronomer (Spain)
Our star, the Sun, was formed about 4.57 billion years ago. It belongs to the spectral class G2, known as a yellow dwarf, a medium-sized star. At the center of the solar system, the Sun keeps numerous planets, comets, and meteors orbiting around it thanks to its gravitational force. Similarly, there are numerous solar systems that house planets. Likewise, stars can group into small or immense clusters of up to millions of stars. These groups are called open or globular star clusters, according to their characteristics.
Also called galactic clusters, because they are found within the galaxy, open clusters are groups of only tens or hundreds of young stars formed from the same nebula. They have a random, usually asymmetric, structure, and their stars are gravitationally bound to each other. Using a moderate telescope, many of the cluster-forming stars may be individually observed, such as the case of the well-known Pleiades or M45, one of the most amazing star clusters in the sky.
Pleiades or M45. Credit: Jaume Zapata
Globular clusters are concentrations of thousands to millions of stars that are gravitationally bound. These objects are usually found in the galactic halo orbiting the host galaxy, but they can also be found in the center of the Milky Way.
Unlike open clusters, globular clusters are home to ancient stars whose ages can reach 13 billion years. In other words, they contain stars that were formed in the early universe. Therefore, they offer information about galactic and stellar evolution. Most of their stars are low-mass red dwarfs and medium-mass yellow stars, and it is estimated that they are regions where no star formation takes place. The density of these clusters is responsible for their spherical and compact structure. There are about 150 known globular clusters in our Milky Way, but our neighbor, the Andromeda Galaxy, is estimated to contain about 500.
M13 – Globular cluster. Credit: NASA.
Somewhere, something incredible is waiting to be known