As particles in one part of the cloud become gravitationally attracted to one another, that part of the nebula begins to collapse in on itself. And sometimes we can only see the silhouette of part of a nebula as it blocks out the light from other parts of the nebula or from distant star fields. This is called a dark or absorption nebula .
Galaxies are very large and distinct communities of stars.
This is where star formation is primarily found. It was generally believed that most of the galaxies we see were nebulae, until Hubble proved, in the 1920s, that the Great Andromeda Galaxy, M31, lay far out side of our own galaxy. Lord Rosse was the first to notice the spiral pattern in a galaxy when he viewed the Whirlpool Galaxy, M51, in 1845 through his gigantic reflecting telescope in Ireland. There are about 100 stellar clusters in our galaxy that formed at the same time as our galaxy. Globular clusters appear to contain the oldest stars in the universe.