What is the difference between a galaxy and a star cluster?
These have vastly different scales. A galaxy can be 100,000 light-years (or more) across, and contain anything from 100 million up to a trillion () stars. The Milky Way and Andromeda are examples of galaxies. Star clusters come in two types: open clusters are the kind we’ve been discussing in the context of main-sequence fitting. These are small groupings of stars that are born at about the same time out of the same cloud of dust and gas. They are much smaller-scale objects than galaxies, typically a few tens of light-years across, and contain up to a few thousand stars. They exist within galaxies. Two examples of nearby open star clusters are the Hyades and the Pleiades. Another kind of star cluster is a globular cluster: this is a spherical clump of old stars, containing up to 10,000 stars within a hundred light-years or so.
