Globular Clusters

Type of physical science: Astronomy; Astrophysics

Field of study: Galaxies

Globular clusters are a spherically symmetrical compact systems containing several tens of thousands of stars that share a common origin. They are important to astronomical research because they contain a mix of variously sized stars that all lie at approximately the same distance from Earth and, for the most part, all evolved from the same cloud of gas at the same time.

Overview

Globular clusters are spherically symmetrical compact stars located in the halo of the galaxy. They include some of the oldest stars in a galaxy, with low metal content, and those in the Milky Way galaxy contain more than a million stars with typical radii of 1 to 10 parsecs. An example of a globular cluster is the Great Cluster in the constellation Hercules.

When the Milky Way galaxy was first condensed from a huge cloud of gas, it was roughly spherical. As the collapse proceeded, the rotation of the cloud began to take effect, eventually causing it to become a flattened disk rather than a ball. In the earliest stages of the collapse, however, stars formed in some locally denser sections. These stars—the oldest in the galaxy—occupy a more nearly spherical distribution known as the galactic spheroid or halo.

These globular clusters are usually made up of red giants, and they typically have diameters of 40 to 50 light-years. A light-year is the distance that light travels in a year: 10 trillion kilometers or 102,000 astronomical kilometers. A globular cluster is an impressive sight. Looking through large telescopes, astronomers have discovered that globular clusters consist of many thousands of stars and are concentrated so tightly that they cannot be fully separated by any ground-based telescope. Globular clusters are generally better studied from the Southern Hemisphere, except for M13 in Hercules.

Globular clusters have an enormous range in chemical composition. Few are close enough for analysis of their individual stars. The brightest and nearest stars are only of the tenth magnitude. Globular clusters are so populous and their total magnitude is so bright that collective spectra have been recorded for most of them. The interpretation of such spectra presents special problems because they are made of contributions of many stars that differ in luminosity and temperature. These stars have a very large range in metal content, far larger than anything that has been found among open clusters. Globular clusters are rich sources of variable stars whose periods range from about an hour and a half to little more than a day. The period of a variable star is the interval in light variations from maximum-to-minimum-to-maximum amount of light received from the star. Variable stars were first detected in 1895 by Solon Irving Bailey on photographs of certain globular clusters. The brightest of the globular clusters—Omega Centauri (NGC 5139)—lies in the Southern Hemisphere and resembles a hazy star of about fourth magnitude. In reality, it is an immense ball-shaped swarm of at least several hundred thousand stars. Sir John Frederick Herschel (1792–1871), an English astronomer, described it as a "noble globular cluster, beyond all comparison the richest and largest object of the kind in the heavens."

Globular clusters are stellar population II systems, in that all the stars within them are relatively old, actually older than Earth's sun, and have a very low metal content. The distribution and other characteristics of globular clusters indicate that they were formed early in the life of the galaxy, probably around 12 billion years ago, before the main body of the galactic disk had evolved. Globular clusters can be seen around other galaxies; however, they are extremely faint spots of light and can be separated from foreground stars only by extremely close observations.

There are always more globular clusters around elliptical galaxies than spiral galaxies. The Milky Way is typical of spirals having fewer than 150 globular clusters. Some globular clusters occupy the flattened disk of the Milky Way. A problem with globular clusters lying near the plane is that they are difficult to detect and, as a consequence, a few remain to be discovered. Globular clusters move in orbits around the galactic center under the gravitational control of the whole system. They have highly inclined eccentric orbits, and many have large velocities. Globular clusters pass in and out of the plane of the Milky Way, suffering total disturbance as they pass through the most massive regions of the system.

Since most of the members in a globular cluster will have evolved away from the main sequence, the Hertzsprung-Russell (H-R) diagram for stars of a globular cluster will differ greatly from the conventional H-R diagram. The turnoff point, or the point on the main sequence where the giant branch originates in the H-R diagram, gives a measure of the age of a cluster.

Distances to globular clusters are usually calculated from the apparent magnitudes of the variable RR Lyrae stars in the globular cluster. RR Lyrae stars appear as a nearly horizontal grouping in the H-R diagram, known as a horizontal branch. The intrinsic brightness of the RR Lyrae stars in globular clusters can be determined and can be recognized in other locations. Because the stars of a globular cluster lie at the same distance from Earth, H-R diagrams can be plotted for them simply by measuring their color and their apparent magnitude; however, it is very important to consider that there is a constant difference between the apparent and absolute magnitude according to the cluster's distance. Apparent magnitude represents the relative brightness of a star or starlike object. A star of the first magnitude is said to be one hundred times brighter than one of the sixth magnitude. A difference of 1 in magnitude corresponds to a ratio in brightness of 2.512. On the other hand, absolute magnitude is the magnitude that a star would have if it were at a distance of 10 parsecs from Earth; it corresponds to a parallax of 0.1 second of arc. The H-R diagram clearly shows which stars have begun their transformation into red giants, which leads to a determination of the age of the cluster.

Applications

The most important feature of globular clusters is their value as a means of estimating distances. The presence of globular clusters in the halo of other galaxies presents one of the better ways to measure distances between galaxies. Globular clusters contain a particular type of variable—the RR Lyrae stars—that have brightnesses that can be determined and recognized in other galaxies; astronomers use this variable to measure intergalactic distances.

Globular clusters are important because they contain a mixture of stars of various sizes that all lie at the same distance from Earth and typically all evolved from the same cloud of gas at the same time (the largest clusters can include stars of varying ages, likely due to the effects of gravity). Therefore, they play a vital role in understanding the life history of stars. They have facilitated the development of the H-R diagram, particularly for red giant and main sequence stars. For example, since the stars of a globular cluster lie at the same distance from Earth, an H-R diagram can be plotted for them by measuring their color and their apparent magnitude. It is important to know that there is a constant difference between the apparent and the absolute magnitude according to the cluster's distance. That distance can be determined by comparison with a standard H-R diagram because the main sequence will appear in the correct place only if the distance is right.

There are virtually no main sequence stars in globular clusters that are brighter by about one magnitude than the sun. Main sequence theory premises that if stars near the sun are arranged in order of increasing luminosity, most of them form a sequence of increasing mass, surface temperature, and size. These stars are also referred to as dwarf stars. The giant sequence, when stars are more luminous and, therefore, larger than the dwarf stars of the same spectral type, is joined to the main sequence by an almost vertical bridge. There is sometimes a narrow branch of white and blue stars of about absolute magnitude zero. When color-magnitude arrays of globular clusters are compared, significant differences are noted: Spectra of the brightest stars in a globular cluster often show extremely weak lines of heavy elements, ranking from 0.1 to 0.01 percent; however, in open clusters, the strength of the lines of heavy elements is between 1 and 4 percent.

The spectra of the individual stars in globular clusters can be studied only with a powerful 508-centimeter telescope, for example. A careful study of the spectra of stars of a given intrinsic brightness in different globular clusters differs not only from those stars near the sun but also from one cluster to another. The differences occur in the sense that the metal-to-hydrogen ratio often was smaller in globular clusters than in the sun.

Stellar evolution can be explained from cluster color-magnitude diagrams, which are a plot of the magnitudes (apparent and absolute) of the stars in a cluster against their color indices.

For example, if one assumes that star clusters composed of differing masses were formed at about the same time, that their masses ranged from 10 solar masses to 0.1 solar mass (solar mass is 1.989 x 1030), and that they were shining on the main sequence, then their luminosities would be correlated with their masses; however, the more massive the star, the more rapidly it would liberate energy.

Each gram of the most massive stars would liberate energy about a hundred times as fast as a gram of the sun. Taking this a step further and assuming that the total amount of energy that can be squeezed out of each gram is the same for all matter everywhere, then the more massive stars will exhaust their fuel more quickly, leave the main sequence, and eventually disappear. Older clusters show this sequence: The brightest, most profligate stars have used up their energy resources and have disappeared, at least from the main sequence. Further, giants and supergiants represent stars that have evolved from the main sequence, and white dwarfs represent the final stage of evolution.

Another important role of globular clusters is that they are found in all parts of the sky, not merely within the disk of the Milky Way. The distribution as seen from Earth is not uniform, since there appears to be more in the region of the constellation Sagittarius than in the diametrically opposite direction.

Context

The ancient Greeks first noted the existence of clusters of any type when they described the Hyades, Pleiades, and Praesepe. These clusters are the most visible northern open clusters; however, it was not until the first telescopic surveys by Edmond Halley (1656–1742), a seventeenth century English astronomer, and Charles Messier (1730–1817), an eighteenth century French astronomer, that the existence of globular clusters were made known. It was believed initially that these objects were nebulous; however, the spectroscopic observations of Sir William Huggins (1824–1910), an English astronomer, and Pietro Angelo Secchi (1818–1878), an Italian astronomer, in the late nineteenth century showed how to distinguish nebulas from clusters.

Bailey and later Harlow Shapley (1885–1972), an American astronomer with Harvard University, noted that there is a distinct class of variable stars associated with the clusters. These RR Lyrae stars are a group of horizontal branch stars with periods of about one-half day that display a distinctive period-luminosity relation.

Globular clusters are found in all parts of the sky, are useful in estimating distances, and have value in determining the age of galaxies. They have played a role in the history of astronomy as a result of the fact that early attempts to determine the location of the sun within the galaxy were based on observation of the distributions in space of stars of similar brightness; however, the conclusions drawn were invalid because the absorption of light by interstellar dust had been overlooked. The first reliable results were obtained in 1917 from Shapley's studies of the globular clusters.

Globular clusters will continue to benefit astronomy in the future as they have in the past; as more powerful space-based telescopes are perfected and placed in orbit, new globular clusters will most certainly be identified. Their measurement and analysis will help reveal the history of the galaxy in which they are found. Such research will undoubtedly provide more insight into the origins of globular clusters themselves.

Principal terms

GALAXY: an aggregate of stars, dust, and gas with a more or less definite structure

HERTZSPRUNG-RUSSELL DIAGRAM: a graph upon which the luminosities of stars are plotted against their surface temperatures; every dot represents a star whose brightness and temperatures have been measured

HORIZONTAL BRANCH: a stage of helium core burning; in evolving clusters, this appears as a nearly horizontal grouping in the Hertzsprung-Russell diagram

LUMINOSITY: the intrinsic brightness or light output of a star as distinct from its apparent brightness

MESSIER NUMBER: the number of an object listed in the Messier Catalog of 103 nebulas and star clusters prepared by Charles Messier in 1784; an object is referred to as M followed by the catalog number, such as M13 in Hercules

NEW GENERAL CATALOGUE: galaxies, star clusters, and nebulas are frequently identified by their NGC numbers in the NEW GENERAL CATALOGUE OF NEBULAE AND CLUSTERS OF STARS issued by John Louis Emil Dreyer in 1888; for example, Omega Centauri is NGC 5139

PARSEC: the distance at which the stellar parallax is 1 second of arc, or approximately 3.26 light-years

RED GIANTS: stars that emit mostly reddish light and have huge surface areas

STELLAR POPULATIONS: a differentiation between stars of different age determined from their metal abundances and distribution

Bibliography

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