The colour-magnitude diagram below for M55 illustrates this point.Center for Astrophysics | Harvard & Smithsonian astronomers study star clusters in a variety of ways: They also have no high-mass stars left on the main sequence. The higher up the main sequence, the more massive the star is.Īs globular clusters are generally much older than open clusters their colour-magnitude diagrams show more evolved stars. The zero-age main sequence ( ZAMS) is the main sequence of all the stars when they initially form in a cluster. When a star first achieves core hydrogen fusion and appears on the main sequence it is said to be zero-age. The further down the main sequence the turnoff point is, the older the cluster. Of key importance is the turnoff point on the diagram where the cluster turns-off the main sequence. M67, a very old open cluster has no star hotter than +0.4 colour index left on the main sequence. The more massive cluster members have already evolved off the main sequence to the giant branches. The Pleiades, being slightly older, has no stars hotter than colour index 0 (A0 spectral class) left on the main sequence. A cluster such as h + χ Persei is so young that most of its stars are still on the main sequence - they have not yet turned-off. The "years" here refers to the age of the cluster. If you study this diagram closely you will notice a new scale on the right-hand vertical axis. Do you notice any differences between the two images?
On the left we have h + χ Persei, a double open cluster in which the two clusters, 2,200 parsecs distant are only separated by about 30 parsecs. The images below show some open clusters. By doing this for several open clusters we find an interesting result. Using spectroscopic parallax they can then calibrate the diagram to obtain values for absolute magnitude, M or M V. This is simply an HR diagram that plots apparent magnitude, usually V (or m V) on the vertical axis against colour index, B - V on the horizontal. The stars that appear brightest within a cluster are intrinsically more luminous than fainter members.Īstronomers use this fact to obtain a colour-magnitude diagram for a cluster. If we take photometric readings for the cluster stars, the apparent magnitude of each thus also allows us to infer the relative absolute luminosities of the cluster members. Even though a cluster may be a few parsecs across this size is insignificant compared with the much greater distance of the cluster from Earth. Another important point is that all stars within a cluster are effectively at the same distance form an observer on Earth. They therefore share the same initial metallicity so any effect of this on stellar evolution is effectively the same for the members of the cluster. Stars in an open cluster have a common origin from a given nebula. Let us look firstly at open clusters to understand why this is so. Star clusters are particularly important because they allow astronomers to check models of stellar evolution and the ages of stars. Prominent examples include 47 Tuc, M4 and Omega Centauri although there is some debate as to whether this may in fact be a captured dwarf spheroidal galaxy. Our Milky Way has about 200 globular clusters. The stars within them are not dispersed out of the cluster. Unlike open clusters, globular clusters normally remain gravitationally coherent throughout their lives. The stars are about 10 billion years old so many are red giants or white dwarfs.Īs with open clusters, stars in globular clusters probably had a common origin. The globular cluster 47 Tuc from the ground-based AAT and the HST. Clusters typically contain a few hundred stars though this can vary from as low as a few dozen up to a few thousand. Stars in an open cluster have a common origin - they formed from the same initial giant molecular cloud. They are sometimes called galactic clusters due to their location on the dusty spiral arms on the plane of spiral galaxies.
Some examples such as the Hyades and Pleiades are so close that the individual stars can be clearly distinguished by the naked eye.
Open clusters are so-named due to the fact that the individual component stars are easily resolved through a telescope. Cluster Ages and Zero-Age Main Sequence.The two basic categories of stellar clusters are open clusters, also known as galactic clusters, and globular clusters. They are are particularly useful to astronomers as they provide a way to study and model stellar evolution and ages. A star cluster is a group of stars that share a common origin and are gravitationally bound for some length of time.
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