Reflection nebulae don't glow by their own power, but we see them because light from nearby stars are reflecting off them. There are also what's known as a Planetary nebulae. These are hollow balls of gas caused by an exploding star. Often, the star can still be seen at the center of the nebula. Star Clusters Star clusters are groups of stars, usually ranging from a few hundred to a few thousand, that are moving together through space.
Open clusters are loose-knit, randomly sprinkled groups of stars.
One of the finest examples of an open cluster is the Pleiades, or Seven Sisters. With the unaided eye, many observers can make out six or seven stars. With binoculars or telescopes, this number jumps to dozens. Globular clusters, on the other hand, form tightly-packed spheres of stars. Under low power, they look like a fuzzy ball, bright in the center and fading into the background at the edges.