Globular clusters and open clusters are collections of stars bound together by gravity.

Open clusters are most common in the spiral arms of the Milky Way. The most spectacular cluster is M45, also known as the Pleiades or the Seven Sisters. A pair of binoculars reveals a swarm of magnificent stars, glowing with an intense blue light.

Globular clusters are the older, bigger brothers of open clusters. These tightly packed balls contain up to a million stars and were formed more than ten billion years ago, in the early days of our universe. Globular clusters cannot be found in the disk of the Milky Way, but do form a spherical halo surrounding the disk. M5 is the best globular cluster you can see from the Northern Hemisphere. Nestled beneath the feet of Virgo, this 13-billion-year-old cluster is so tightly packed that you will probably find it impossible to differentiate the individual stars in its core, even with a good telescope.