Constellations, asterisms, galaxies, and star clusters all contain stars, but they’re quite different entities. It can be confusing if you don't know.
Constellation or asterism?
There are different ways that we understand constellations. Commonly, people think of them as the traditional patterns of stars that are often shown as “stick figures” and may represent classical myths.
The star patterns in constellations are human inventions based on what we see from Earth. The stars aren’t actually in groups. Very rarely are stars in a constellation close together. For example, of the five brightest stars in Cygnus (the swan), the nearest one to us, Epsilon Cygni, is 72 light years away. The most distant one is over twenty times farther away than that. (A light year is around ten trillion kilometers or six trillion miles.) If we were in another part of the Galaxy, the stars of Cygnus would form a different pattern.
The celestial sphere
When we see objects in the night sky, it’s as if they’re projected onto a sphere surrounding the Earth. This is the celestial sphere. To specify locations in the sky, we use a grid system which is like the terrestrial coordinate system. The celestial equator is a projection of the terrestrial equator and there are also celestial poles and the equivalent of lines of latitude and longitude. [Click to see diagram of the celestial sphere.]
The astronomical definition of a constellation is both broader and yet more precise than the popular one.
The International Astronomical Union (IAU) agreed on a list of 88 constellations, and in 1928 also agreed on their boundaries. The entire celestial sphere is covered by 88 constellations with no overlap. An astronomical constellation includes traditional constellations, but it is not so much a pattern of stars, as an area of sky. You might think of a constellation, in this sense, as analogous to a country on the Earth’s surface. If we say that a comet is appearing in Leo or that Saturn is in Virgo, this means that people can see these objects within the boundaries of that constellation.
“Asterism” is an unfamiliar word to most people, though you’ll probably recognise the “aster” part of it, which comes from the Greek for “star”. The best-known asterism is the Big Dipper (the Plough, in Britain), which is part of the constellation Ursa Major. Another is the Summer Triangle, which consists of the brightest stars of the constellations Cygnus, Lyra, and Aquila.
Very simply, an asterism is a recognizable pattern of stars that isn’t a constellation, but contains a part of one or more constellations. Click to see a comparison between a constellation and an asterism.
Galaxy or galaxy?
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 we didn’t know there were other galaxies.
Galaxy or star cluster?
A galaxy is an enormous collection of stars of different ages, the gas and dust between the stars, and dark matter. It’s all held together by gravitational attraction. Click to see a diagram of the structure of the Milky Way, an example of a spiral galaxy. You can also find out more about galaxies by following the link at the end of this article to “What Is a Galaxy”.
A star cluster is also a collection of stars bound together by gravity. However it’s smaller, less complex, and consists of stars formed from one giant gas cloud. This means cluster stars are similar in age. There are two kinds of cluster: globular clusters and open clusters.
Globular clusters are big, though considerably smaller than galaxies which may have billions of stars. A globular cluster contains between tens of thousands and hundreds of thousands of stars, their mutual gravitational attraction pulling them into a globular shape. They are almost as old as the Galaxy and there are about 150 of them orbiting the Galactic Center.
Open clusters are much more common than globular clusters. We know of well over a thousand of them in the Milky Way, and there are probably many more. They’re also smaller than globular clusters, containing between hundreds and thousands of stars. This makes the gravitational attraction weaker, so the clusters have irregular shapes and are subject to disruption over time. The two brightest open clusters in our night sky are the Pleiades and the Hyades, both in Taurus.
Click to see a comparison between globular clusters and open clusters.
Galactic cluster or open cluster?
They’re the same thing, but open clusters used to be called galactic clusters. Galactic clusters shouldn't be confused with galaxy clusters which are clusters of galaxies.