The clusters appear to be grouped physically, but they are not. Some of the stars in a constellation can be millions of miles apart. Some are closer to the earth than others, they just appear to be beside each other to us. Constellations are used by astronomers to help them separate stars from each other. The size of the stars depend on their brightness.
Bigger stars are brighter than smaller stars.
There are 88 recognizable constellations, some of which date back as far as 5000 years ago. Most constellations were created based on Greek mythology and outline the shape of someone or something from the myths. Constellations fill the night sky and on a very dark night, a person can see between 1000 and 1500 stars. Some of the star groups that are common to us are not really constellations. For instance, most people know what the Big Dipper looks like, but they do not realize that it is not a constellation.