open cluster - Historical observations, Formation, Morphology and classification, Numbers and distribution, Stellar composition, Eventual fate, Studying stellar evolution

A family of a few dozen to a few hundred relatively young stars (typically no more than a few hundred million years old) formed simultaneously and still physically close together; also called a galactic cluster. There are c.1000 in our Galaxy. Pleiades and Hyades are examples visible to the naked eye.

An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. Open clusters are found only in spiral and irregular galaxies, in which active star formation is occurring.

Young open clusters may still be contained within the molecular cloud from which they formed, illuminating it to create an H II region. Because the stars are all of very similar age and chemical composition, the effects of other more subtle variables on the properties of stars are much more easily studied than they are for isolated stars.

Historical observations

The most prominent open clusters such as the Pleiades have been known and recognised as groups of stars since antiquity. Telescopic observations revealed two distinct types of clusters, one of which contained thousands of stars in a regular spherical distribution and was found preferentially towards the centre of the Milky Way, and the other of which consisted of a generally sparser population of stars in a more irregular shape and found all over the sky.

It was realised early on that the stars in the open clusters were physically related. As astrometry became more accurate, cluster stars were found to share a common proper motion through space, while spectroscopic measurements revealed common radial velocities, thus showing that the clusters consist of stars born at the same time and bound together as a group.

While open clusters and globular clusters form two fairly distinct groups, there may not be a great deal of difference in appearance between a very sparse globular cluster and a very rich open cluster. Some astronomers believe the two types of star clusters form via the same basic mechanism, with the difference being that the conditions which allowed the formation of the very rich globular clusters containing hundreds of thousands of stars no longer prevail in our galaxy.

Formation

All stars are originally formed in multiple systems, because only a cloud of gas containing many times the mass of the Sun will be heavy enough to collapse under its own gravity, but such a heavy cloud cannot collapse into a single star .

The formation of an open cluster begins with the collapse of part of a giant molecular cloud, a cold dense cloud of gas containing up to many thousands of times the mass of the Sun. Many factors may trigger the collapse of a giant molecular cloud (or part of it) and a burst of star formation which will result in an open cluster, including shock waves from a nearby supernova and gravitational interactions. Once a giant molecular cloud begins to collapse, star formation proceeds via successive fragmentations of the cloud into smaller and smaller clumps, resulting eventually in the formation of up to several thousand stars.

Once star formation has begun, the hottest and most massive stars (known as OB stars) will emit copious amounts of ultraviolet radiation. After a few tens of millions of years, the cluster will be stripped of gas and no further star formation will take place. Typically, less than 10% of the gas originally in the cluster will form into stars before it is dissipated . In the Large Magellanic Cloud, both Hodge 301 and R136 are forming from the gases of the Tarantula Nebula, while in our own galaxy, tracing back the motion through space of the Hyades and Praesepe, two prominent nearby open clusters, suggests that they formed in the same cloud about 600 million years ago .

Morphology and classification

Open clusters range from very sparse clusters with only a few members to large agglomerations containing thousands of stars. Typical star densities in the centre of a cluster are about 1.5 stars per cubic light year (the stellar density near the sun is about 0.003 star per cubic light year) . The Trumpler scheme gives a cluster a three part designation, with a Roman numeral from I-IV indicating its concentration and detachment from the surrounding star field (from strongly to weakly concentrated), an Arabic numeral from 1 to 3 indicating the range in brightness of members (from small to large range), and p, m or r to indication whether the cluster is poor, medium or rich in stars.

Numbers and distribution

There are over 1,000 known open clusters in our galaxy, but the true total may be up to ten times higher than that . In spiral galaxies, open clusters are invariably found in the spiral arms where gas densities are highest and so most star formation occurs, and clusters usually disperse before they have had time to travel beyond their spiral arm. Open clusters are not seen in elliptical galaxies: star formation ceased many millions of years ago in ellipticals, and so the open clusters which were originally present have long since dispersed.

In our galaxy, the distribution of clusters depends on age, with older clusters being preferentially found at greater distances from the galactic centre. Tidal forces are stronger nearer the centre of the galaxy, increasing the rate of disruption of clusters, and also the giant molecular clouds which cause the disruption of clusters are concentrated towards the inner regions of the galaxy, so clusters in the inner regions of the galaxy tend to get dispersed at a younger age than their counterparts in the outer regions .

Stellar composition

Because open clusters tend to be dispersed before most of their stars reach the end of their lives, the light from them tends to be dominated by the young, hot blue stars. The older open clusters tend to contain more yellow stars.

Some open clusters contain hot blue stars which seem to be much younger than the rest of the cluster. These blue stragglers are also observed in globular clusters, and in the very dense cores of globulars they are believed to arise when stars collide, forming a much hotter, more massive star. However, the stellar density in open clusters is much lower than that in globular clusters, and stellar collisions cannot explain the numbers of blue stragglers observed. While most clusters become dispersed before a large proportion of their members have reached the white dwarf stage, the number of white dwarfs in open clusters is still generally much lower than would be expected, given the age of the cluster and the expected initial mass distribution of the stars.

Eventual fate

Many open clusters are inherently unstable, with a small enough mass that the escape velocity of the system is lower than the average velocity of the constituent stars. In many cases, the stripping away of the gas from which the cluster formed by the radiation pressure of the hot young stars reduces the cluster mass enough to allow rapid dispersal. Internally, close encounters between members of the cluster will often result in the velocity of one being increased to beyond the escape velocity of the cluster, which results in the gradual 'evaporation' of cluster members. Eventually, the cluster becomes a stream of stars, not close enough to be a cluster but all related and moving in similar directions at similar speeds.

After a cluster has become gravitationally unbound, many of its constituent stars will still be moving through space on similar trajectories, in what is known as a stellar association, moving cluster or moving group. Several of the brightest stars in the 'Plough' of Ursa Major are former members of an open cluster which now form such an association, in this case, the Ursa Major moving group.

Studying stellar evolution

When a Hertzsprung-Russell diagram is plotted for an open cluster, most stars lie on the main sequence. The most massive stars have begun to evolve away from the main sequence and are becoming red giants, the position of the turn-off from the main sequence can be used to estimate the age of the cluster.

Because the stars in an open cluster are all at roughly the same distance from Earth, and were born at roughly the same time from the same raw material, the differences in apparent brightness among cluster members is due only to their mass. This makes open clusters very useful in the study of stellar evolution, because when comparing one star to another, many of the variable parameters are fixed.

The study of the abundances of lithium and beryllium in open cluster stars can give important clues about the evolution of stars and their interior structures. By studying their abundances in open cluster stars, variables such as age and chemical composition are fixed.

Open clusters and the astronomical distance scale

Determining the distances to astronomical objects is crucial to understanding them, but the vast majority of objects are too far away for their distances to be directly determined. First, the parallax (the small change in apparent position over the course of a year caused by the Earth moving from one side of its orbit around the Sun to the other) of stars in close open clusters can be measured, like other individual stars. The radial velocity of cluster members can be determined from Doppler shift measurements of their spectra, and once the radial velocity, proper motion and angular distance from the cluster to its vanishing point are known, simple trigonometry will reveal the distance to the cluster.

Once the distances to nearby clusters have been established, further techniques can extend the distance scale to more distant clusters. By matching the main sequence on the Hertzsprung-Russell diagram for a cluster at a known distance with that of a more distant cluster, the distance to the more distant cluster can be estimated. The nearest open cluster is the Hyades: the stellar association consisting of most of the Plough stars is at about half the distance of the Hyades, but is a stellar association rather than an open cluster as the stars are not gravitationally bound to each other.

Accurate knowledge of open cluster distances is vital for calibrating the period-luminosity relationship shown by variable stars such as cepheid and RR Lyrae stars, which allows them to be used as standard candles.