A star cluster is different from a stellar nursery, as was pictured in yesterday's JSVB post. However, they are related. The mature masses in a star cluster have stayed aggregated, either because of mutual gravitational attraction, or simply because the cluster is too young to have drifted apart. I've illustrated a series of so-called Bok Bodies, which are dense black clouds you sometimes see in photographs of nebulae. They are among the densest, darkest, and coolest features in the galaxy, making them very mysterious since they seldom emit anything we can measure from Earth. Scientists believe Bok Bodies are associated with the birth of stars.
Star clusters are important since they provide a measurable scale with which we can compare the rest of the visible Universe. By comparing our own Sun with similar stars in nearby clusters, we can deduce both the age of the Universe and the size of our galaxy within it.
If you need to know, our galaxy is approximately 110,000 light years across, so it's roughly 50,000 light years from where you live to the center of the Galaxy. That, and the entire Universe seems to be 13 billion years old.
Thank you local star clusters, for all of that!