Star clusters

Stars are not usually born one at a time. A single cloud of gas makes many stars together, and for a while gravity holds them in a group. A group like this is called a star cluster.

You can see one without a telescope. On a dark night in December, look for a tiny, tight bunch of stars in the constellation Taurus. These are the Pleiades, also called the Seven Sisters. The eye picks out a handful of bright ones. There are really more than a thousand.

Clusters like the Pleiades are loose. Over time their stars wander off and mix in with all the others. The Sun may have started in a cluster like this and left it long ago.

There is a second kind, far bigger. A globular cluster is a ball of many thousands of stars, sometimes millions, packed so close that in a telescope the middle is one solid blaze. These hold together, and they contain some of the oldest stars there are.

A star cluster is a group of stars that formed from the same cloud and are held together by their own gravity. A cluster can have as few as ten stars or as many as millions.

Three kinds.

	Embedded cluster	Open cluster	Globular cluster
Stars	New and still forming	A few dozen to a few thousand	Several thousand to millions
Shape	Hidden inside its cloud	Loose and irregular	A ball, crowded at the middle
Age of the stars	The youngest	Usually young	Among the oldest known
Where in a galaxy	In clouds of gas and dust	Mostly in the spiral arms	In the outskirts
How long it lasts	2 to 7 million years in this stage	Its stars slowly scatter	Holds together
In the Milky Way		More than a thousand	Over 150

From cloud to cluster. A cluster begins buried in the cloud that is making it. Dust hides it from ordinary telescopes, and it is seen in infrared, which gets through. The four heavy stars at the heart of the Orion Nebula are part of one. A large share of all stars are thought to begin this way. After a few million years the leftover gas and dust blow away or go into planets, and the stars stand in the open.

Open clusters. What is left is an open cluster: stars far enough apart to be picked out one by one. As the cluster goes round the galaxy, the pull of things it passes tugs stars loose, and in time it comes apart. That fits with open clusters being mostly young. The Sun may have begun in one and now travels alone.

The Pleiades are the best known. The cluster is roughly 445 light years away, though that figure is not agreed by everyone, and it holds over a thousand stars. A handful of bright ones give it its shape in the sky. Galileo was the first to look at it through a telescope.

Globular clusters. A globular cluster is a different thing: far more stars, far closer together, pulled into a ball by their combined gravity. That strong grip is why globular clusters do not break up. M13, in the constellation Hercules, is one that binoculars will show. It is 25,000 light years away and holds over 100,000 stars. Near its middle the stars are about a hundred times more crowded than they are round the Sun, close enough that two can sometimes collide and make a new star.

Old stars are mostly reddish, the hot blue ones having burnt out long ago, so a globular cluster looks redder than an open one. Our neighbour galaxy Andromeda has over 400 of them.

Why clusters are useful. The stars of a cluster were born together from one cloud, and they are all at very nearly the same distance from us. So when one star in a cluster looks brighter than another, it really is brighter, and not just nearer. That makes a cluster a fair test of how stars of different masses live. It is how the faint white dwarfs in the cluster M4 could be picked out and compared in 1995, more than 75 of them in one picture.

Still being worked out. For a long time every star in a cluster was taken to be the same age. Hubble found clusters that hold several generations of stars. How a cluster comes to make stars more than once is still being studied.

What the 3D view shows. Among the stars of the night sky, the bright stars of the Pleiades are drawn at their measured places in the direction of Taurus. The catalogue used here puts them between about 380 and 405 light years out, nearer than NASA’s 445, which shows how unsettled that distance has been. Move the view round and the constellations change shape, but the Pleiades stay bunched, because they really are close together in space. The fainter members are below what the catalogue here holds. Globular clusters are too far away to appear in this view.