An open cluster refers to a group of stars that are relatively close together and are held together by their mutual gravitational attraction. Full definition
Wide - field imaging surveys on moderate - size telescopes can now efficiently derive rotation periods for hundreds to thousands of open cluster members, providing unprecedented sample sizes which are ripe for exploration. (arxiv.org)
It became possible in the 1990s to discover very young open clusters that previously had been entirely hidden in deep, dusty regions. (britannica.com)
He considered it to be one of the older open clusters. (maa.clell.de)