Open Clusters

An open cluster (also sometimes referred to as a galactic cluster) can be thought of as a loosely gravitationally bound collection of tens or hundreds of stars, many of which are young, bright, blue stars. These stars were formed at the same time (give or take a few thousands of years!) from the same initial cloud of gas (mostly hydrogen) and dust. There are approximately 1500 or so of these open clusters in our Galaxy and we know that they also exist in nearby galaxies such as the Large and Small Magellanic Clouds (LMC and SMC). The stars in an open cluster are therefore relatively close to each other which makes them different to constellations (such as Orion and Ursa Major) since constellations are group of stars that only appear to be close to each other but which are in fact at different distances from us.

When they are young (a few million or tens of millions of years old), these clusters can contain some very massive and bright stars (perhaps as massive as 200 times the mass of our Sun) with spectral types O or B. The youngest open clusters (less than 10 million years old) often contain the remnants of the gas cloud from which they were formed. This gas is now visible as 'cloudiness' within many astronomical images and is known as nebulosity.

Stars in open clusters have proved very useful to astronomers since they were all formed from the same giant cloud (so they have the same chemical composition) and are all at approximately the same distance from us. It is therefore safe to assume that any differences between the stars in an open cluster are really caused by their different masses. These differences manifest themselves in terms of brightness, surface temperature and the star's life-time and make them perfect targets for studies such as those that produce colour magnitude diagrams from photometry.

Stars obey Wien's Law - the more massive stars are usually very blue (and therefore hot - perhaps around 30,000 K), intermediate mass stars (like the Sun) are yellow (cooler - approximately 6,000 K), and the very lowest mass stars are red (cool - around 3,000 K).

Many of these open clusters are included in the Messier catalogue. This is a list of around deep sky objects, mostly visible from the northern hemisphere. Among the most striking objects in this list are M25, M44 (the Beehive), M45 (the Pleiades), M67 and NGC290 (see Figure 1). 

Figure 1: A Hubble Space Telescope image of the open cluster, NGC290.
Credit: ESA, NASA, E.Olszewski

Further information on open clusters can be found at the WEBDA and Dias databases.

While we can describe open clusters subjectively, science teaches us that more thorough analysis can allow us to understand more about these objects. With this in mind, a classification system for open clusters was designed in the 1930s by the Swiss astronomer, Robert Trumpler. The Trumpler system classifies a cluster based on three properties

  • a Roman numeral from I-IV denoting concentration (I = strongly concentrated, IV = loosely concentrated)
  • a number from 1 to 3 indicating the range in stellar brightness (1 = small, 3 = large)
  • the letter p, m or r to indicate whether the cluster is poor, medium or rich in stars

An additional 'n' is given if the cluster shows signs of nebulosity.

Examples using this system include the Pleiades which are I3rn (strongly concentrated with a large range in brightness, richly populated and containing nebulosity), while the nearby Hyades are classified as II3m (more dispersed with fewer stars and no nebulosity). 

Read more about spectral types.

Find out about colour magnitude diagrams and photometry.

Which of the following best describes an open cluster?

A collection of tens to hundreds of old stars
No, it is generally younger stars that are found in open clusters
A collection of hundreds to thousands of young stars
No, open clusters do not normally have this many stars
A collection of tens to hundreds of young stars
Yes, this is correct
A collection of hundreds to thousands of old stars
No, open clusters generally have less stars and the stars are younger

How many clusters are there in our Galaxy (open and globular)?

Around 150 of each type
No, there are more open clusters than this
Around 1500 of each type
No, there are fewer open clusters than this
150 open and 1500 globular
No, these values are the wrong way around
150 globular and 1500 open
Yes, this is correct

What does the Trumpler System classify?

Globular clusters
No, globular clusters are classified by the Harris classification system
Spectral types of stars
No, spectral types are denoted by the Morgan-Keenan system
Open clusters
Yes, this is correct
White dwarfs
No, white dwarfs are normally classified by the material they are made of (e.g. helium,carbon, oxygen, etc)