Star clusters, also called star clouds, a clusters of stars that are very close together. To the naked eye, they may appear is a irregular, bright clouds. But through telescopes, they are easily visible, that is the stars in the cluster. Open clusters are exactly that, open. The stars are spread out, so you can see some space between them.
Closed clusters are closed.
This is why, cluster appears as a ball. With a telescope, the outer stars are well visible. A star is a luminous and gaseous body that makes energy by nuclear fusion reactions, which occur in the core. All stars must exceed .05 solar masses in order to have the correct amount of central pressure and temperatures within the core. Only a few stars are known to have a mass greater than 50 times of our sun.