Although there are billions of galaxies out there, we live in the Galaxy. When we capitalize the word, it refers to the Milky Way Galaxy. That's a leftover from the days when no one knew that there were other galaxies. But what is a galaxy and how is it different from a star cluster?
Both galaxies and star clusters are collections of stars held together by their mutual gravitational force. The greater the mass, the stronger the attractive force. However there are more differences than similarities.
Star clusters come in two varieties: open and globular. Open clusters are irregular in shape and globular clusters are spherical. Galaxies are usually elliptical or spiral in shape, but there are irregular ones. [You can find out more about galaxy types by clicking on the link "What Is a Galaxy?" at the end of this article.]
Galaxies contain stars across a wide range of age and composition. But all star clusters have one thing in common. The stars in a given cluster formed from the same molecular cloud and so are the same age and similar in composition.
In the Milky Way, the stars of the globular clusters were some of the first stars in the Galaxy. They're over ten billion years old, more than twice the age of the Sun. There are about 150 of them. Open clusters, on the other hand, are rarely more than a few hundred million years old. In fact, they're still forming in regions with plenty of gas and dust for new stars. Although astronomers know of over two thousand open clusters, there are probably over ten times that many.
Globular clusters have low levels of elements heavier than hydrogen and helium. Since these elements are made in stars and in supernova explosions, nebulae are enriched by previous generations of stars. This means that old globular clusters have low proportions of these heavier elements, but young open clusters tend to have a high proportion of them.
Many open clusters have fewer than a hundred stars and the largest ones rarely contain more than a thousand stars. This means that the gravitational attraction of the stars is comparatively weak, so these clusters tend to break up over time.
The globular clusters are spherical. They have diameters of 100 - 300 light years, and contain from tens of thousands to over a million stars. For example, the Hercules Globular Cluster has about 300,000 stars.
To me, 300,000 stars – and certainly a million stars – sounds like a lot. Nevertheless galaxies are much bigger. Even the smaller dwarf galaxies are likely to have a few tens of millions of stars. The largest dwarfs may have a few billion.
If a dwarf galaxy can have over a billion stars, how big is a full-sized galaxy? The Milky Way Galaxy, for example, has about 300 billion stars and its disk is over 100,000 light years in diameter. Our neighbor, the Andromeda Galaxy, has three times as many stars and a disk about twice the diameter of the Milky Way's. They are both large galaxies, but not monster galaxies. All the galaxies we know are dwarfed by IC 1101, a supergiant elliptical galaxy six million light years in diameter.
Star clusters orbit the center of a galaxy just as the other stars do. Whatever size a cluster may be, it's part of an even bigger galaxy. The galaxy is not only bigger, but in addition to all its visible matter, has even larger amounts of dark matter. Dark matter has a gravitational effect, but can't be seen in any kind of light.
Open cluster or galactic cluster?
Galactic cluster is just an older name for an open cluster. These clusters are located in the galactic disk since this is where star formation is occurring. The term "galactic" distinguished them from the globular clusters which are in the bulge or out in the halo of the Galaxy. This diagram shows the structure of the Milky Way and you can see that the halo is outside the disk, which is in the galactic plane.
We now prefer the term open cluster, both as a contrast to the shape of the globular clusters and to avoid confusion with galaxy clusters. Galaxy clusters are entirely different objects – they are clusters of galaxies.