When is a grouping of stars not a galaxy? When it is a star cluster. Alas, if only it were that simple, as both star clusters and galaxies contain stars held together by gravity, and as scientists have discovered over the years it’s not always easy to know where to draw the line. As a general rule, though, when astronomers talk about star clusters they are simply referring to groups of stars which grew up together and have essentially the same origins. These are then divided into either globular clusters, which contain hundreds of thousands of old stars tightly grouped together, or open clusters, which usually contain a few hundred loosely clustered younger stars.
There are around 200 billion stars in the Milky Way Galaxy (where we live), and of all those stars there are about 160 tight little groupings that we call globular clusters that hang around in our neck-of-the-woods. Not to say they’re nearby, but they orbit the galactic center at our range in the spiral arms, in a halo around the entire thing. In fact the nearest one called Messier 4 is located about 7,000 light years distant.
These ball-shaped collections of stars have a certain glow about them, literally. They can consist of as few as ten thousand stars, or as many as a million. Sometimes they are so densely packed in the center that they cannot be differentiated. These ancient objects have been around for 10-13 billion years and the Universe is only about 13.8 billion years old. Because of this most of the stars in a globular cluster are Red Giants.
They’re certainly not unique to our galaxy. They’ve been observed in thousands of galaxies in every direction we look. One, the Omega Centauri globular cluster, is known to have an intermediately-sized black hole at its center, so they’re not the exclusive domains of galaxies alone!
Open Clusters or galactic cluster, are much more common. They seem to appear in a ratio of ten times the number of globular clusters though there is no current reason to suspect they are associated. You will probably recognize the name Pleiades (photo), also known as The Seven Sisters, which has a membership of about 500 stars and is about 100 million years old. Open Clusters are composed of just a few thousand stars at most and can have less than one hundred. They also have common roots because they condensed out of the same primordial cloud, are approximately the same age, and have the same metallicity (which means they are made up of essentially the same proportion of elements that are not hydrogen or helium).
Their brightest stars tend to be 150,000 times as bright as the Sun which is why such a tiny collection of stars can outshine a globular cluster with thousands or tens of thousands more stars. On the whole, they’re not as durable as their older cousins, the globular clusters. Open clusters only last a few hundred million years with the eldest managing a few billion years. They tend to disassociate their component stars as they encounter others or pass through gas clouds. These escapees become field stars in the galactic disk following their own path. All stars are thought to be escapees from open clusters dropping them off as they interact with other objects nearby.
Most of the easy ones have been discovered, but if you want to try, you could put your name on a new one. There should be about 1,500 open clusters, but we’ve only discovered around 1,100 so far, and probably another dozen or so of globular clusters in our galaxy. Go on.. Go make a discovery!