globular cluster
Globular clusters are gravitationally bound groups of many thousand to millions of stars. They are made up of mostly older stars. They can almost be classified as a galaxy, but they are randomly distributed throughout the universe light years across.
open cluster
Most open clusters are just a hundred young stars, that are not bound toghter with gravity. It will break up in a shout time . A open cluster is about 50 light years across.
emission nebulae
Emission nebulae are clouds of high temperature gas. The atoms in the cloud are energized by ultraviolet light from a nearby star and emit radiation as they fall back into lower energy states. Usualy emission nebulae are signs of forming stars.
reflection nebulae
Another name for a reflection nebulae is diffuse nebulae. It is simply just a cloud of dust that reflects light from nearby stars. It is aso a sign of stars forming.
dark nebulae
Dark nebulaes are almost physically the same as reflection nebulaes, but instead of reflecting light they are blocking light. A typical sark nebulae is a few hundred light years across.
planetary nebulae
Planetary nebulae are shells of gas left from dyeing stars. Our great sun will one day become a planetary nebulae, but only in about 5 billion years. The name planetary doesn't mean that it has to deal with planets. It came around because, when you look at one though small telescopes it would simply look like a small planet. It is about one light year across.
