Thursday, 25 September 2014

What are star clusters?

What are star clusters? Star clusters are large groups of stars held together by some force, usually the gravitational attraction of all the stars in the cluster.

You may be familiar with two types of star cluster that can be readily observed with the naked eye or a pair of binoculars: open clusters and globular clusters, though there are also other types of cluster such as embedded clusters, young massive clusters, and even slight variants such as OB associations and T associations.

Open clusters may be the most well-known type of cluster because this category includes famous naked-eye examples such as the Pleiades and Hyades clusters. Open clusters are small groups of stars, commonly from a few hundred to a few tens of thousands of members (though there is no fixed upper or lower limit), with ages anywhere from a few tens of Myr (mega-year or 1 million years) up to billions of years (or Gyrs). Open clusters are common in our galaxy, with over a thousand already known, and many are close enough for us to study in detail.

The Pleiades open cluster (Credit: Alson Wong)

Globular clusters on the other hand are very different to open clusters. For a start they are much more massive (in mass as well as in size), containing anything from hundreds of thousands to millions of stars, and appearing considerably more rounder or 'globular' than open clusters. Globular clusters are also much older than open clusters, with typical ages of 12-13 Gyr, similar to age of our galaxy. Furthermore, while open clusters are typically found in the disk of our galaxy (where the majority of current star formation takes place), globular clusters are found in the halo of our galaxy (where star formation might have primarily taken place when our galaxy first formed).

There are about 150 known globular clusters in our galaxy and many more have been found in other galaxies. Some globular clusters are bright and close enough to observe with a pair of binoculars, such as Omega Centauri, the largest known globular cluster in our galaxy.

The Omega Centauri globular cluster (Credit: University of Manchester)

Both open and globular clusters are considered by astronomers to be 'long lived', that is they are thought to be gravitationally stable on long time scales. Clearly globular clusters must be stable for billions of years because they have ages of 12-13 Gyr, though for open clusters the current picture isn't entirely clear. Open clusters have ages up to billions of years, but the majority appear to be quite young with ages of tens or hundreds of Myr, perhaps suggesting that there is a process that disrupts open clusters as they age (I'll come back to this in a future post).

There are also varieties of star cluster that are much younger than typical or open or globular clusters. These are usually divided into two categories: embedded clusters and young massive clusters. The latter appear very similar to the most massive open clusters (containing thousands to tens of thousands of stars) although they are much younger, typically only a few to ten million years old, and therefore contain many massive, but short-lived, stars that are not seen in the older open and globular clusters. The image below shows Westerlund 1, one of the most massive young clusters in our galaxy.

The Westerlund 1 young massive cluster (Credit: Wikimedia commons)

And finally we come to embedded clusters. Like young massive clusters these are also young, with typical ages of up to a few Myrs. Because they are so young these clusters are still embedded within (or on the edge of) the molecular cloud that the stars formed from (I'll discuss how stars form out of clouds of molecular gas in a future post). This leads to an important difference between embedded clusters and other varieties of star cluster. Because the cluster is embedded within a cloud of gas, this gas adds mass to the cluster that can help keep the cluster gravitationally bound (perhaps to the point that the cluster might not be gravitationally bound if it were not embedded within the cloud).

Because embedded clusters are found within molecular clouds, and because the light from all the stars can illuminate and ionise the cloud of gas in the cloud, we often find embedded clusters within some of the famous nebulae in the sky that were discovered in the 17th and 18th centuries.

For example one of the most famous nebulae is the Orion Nebula, which can be seen with the naked eye in the constellation of Orion (surprise!). At the centre of the nebula and illuminating the gas for all to see is a relatively massive cluster of young stars known as the Orion Nebula Cluster. Because of its size and proximity the Orion Nebula Cluster has been very well studied and is one of the most important embedded clusters in all of astronomy.

The Orion Nebula Cluster (Credit: NASA)

Next time you're outside on a clear night, try and find some of these star clusters. You'll need binoculars to see a globular cluster, but open clusters such as the Pleiades or the Hyades are visible to the naked eye, and the even the nebula surrounding the Orion Nebula Cluster can be seen on a clear night!

In a future post I'll discuss the other types of stellar groupings such as OB associations, but for now that covers the main varieties of star cluster. 

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