The Milky Way and Beyond (22 page)

Because of the high luminosity of their brightest stars, some open clusters have a total luminosity as bright as that of some globular clusters (absolute magnitude of −8), which contain thousands of times as many stars. In the centre of rich clusters, the stars may be only one light-year apart. The density can be 100 times that of the solar neighbourhood. In some, such as the Pleiades and the Orion clusters, nebulosity is a prominent feature, while others have none. In clusters younger than 25 million years, masses of neutral hydrogen extending over three times the optical diameter of the cluster have been detected with radio telescopes. Many of the OB clusters mentioned above contain globules—relatively small, apparently spherical regions of absorbing matter.

The most-numerous variables connected with young open clusters are the T Tauri type and related stars that occur by the hundreds in some nebulous regions of the sky. Conspicuously absent from open clusters is the type most common in globular clusters, the RR Lyrae stars. Other variables include eclipsing binary stars (both Algol type and contact binaries), flare stars, and spectrum variables, such as Pleione. The last-named star, one of the Pleiades, is known to cast off shells of matter from time to time, perhaps as a result of its high rotational speed (up to 322 km/sec [20 miles/sec]). About two dozen open clusters are known to contain Population I Cepheids, and since the distances of these clusters can be determined accurately, the absolute magnitudes of those Cepheids are well-determined. This has been of paramount importance in calibrating the period-luminosity relation for Cepheids, and thus in determining the distance scale of the universe.

The colour- or spectrum-magnitude diagram derived from the individual stars holds vital information. Colour-magnitude diagrams are available for about 200 clusters on the UBV photometric system, in which colour is measured from the amount of light radiated by the stars in the ultraviolet, blue, and visual (yellow) wavelength regions.
In young clusters, stars are found along the luminous bright blue branch, whereas in old clusters, beyond a turnoff only a magnitude or two brighter than the Sun, they are red giants and supergiants.

Distances can be determined by many methods—geometric, photometric, and spectroscopic—with corrections for interstellar absorption. For the very nearest clusters, direct (trigonometric) parallaxes may be obtained, and these are inversely proportional to the distance. Distances can be derived from proper motions, apparent magnitudes of the brightest stars, and spectroscopically from individual bright stars. Colour-magnitude diagrams, fitted to a standard plot of the main sequence, provide a common and reliable tool for determining distance. The nearest open cluster is the nucleus of the Ursa Major group at a distance of 65 light-years; the farthest clusters are thousands of light-years away.

Motions, including radial velocities and proper motion, have been measured for thousands of cluster stars. The radial velocities of open cluster stars are much smaller than those of globular clusters, averaging tens of kilometres per second, but their proper motions are larger. Open clusters share in the galactic rotation. Used with galactic rotation formulas, the radial velocities provide another means of distance determination.

A few clusters are known as moving clusters because the convergence of the proper motions of their individual stars toward a “convergent point” is pronounced. The apparent convergence is caused by perspective: the cluster members are really moving as a swarm in almost parallel directions and with about the same speeds. The Hyades is the most-prominent example of a moving cluster. (The Hyades stars are converging with a velocity of 45 km/sec (28 miles/sec) toward the point in the sky with position coordinates right ascension 94 arc degrees, declination +7.6 arc degrees.) The Ursa Major group, another moving cluster, occupies a volume of space containing the Sun, but the Sun is not a member. The cluster consists of a compact nucleus of 14 stars and an extended stream.

Stellar groups are composed of stars presumed to have been formed together in a batch, but the members are now too widely separated to be recognized as a cluster.

Of all the open clusters, the Pleiades is the best known and perhaps the most thoroughly studied. This cluster, with a diameter of 35 light-years at a distance of 380 light-years, is composed of about 500 stars and is 100 million years old. Near the Pleiades in the sky but not so conspicuous, the Hyades is the second nearest cluster at 150 light-years. Its stars are similar to those in the solar neighbourhood, and it is an older cluster (about 615 million years in age). Measurements of the Hyades long formed a basis for astronomical determinations of distance and age because its thoroughly studied main sequence was used as a standard. The higher-than-usual metal abundance in its
stars, however, complicated matters, and it is no longer favoured in this way. Coma Berenices, located 290 light-years away, is an example of a “poor” cluster, containing only about 40 stars. There are some extremely young open clusters. Of these, the one associated with the Orion Nebula, which is some 4 million years old, is the closest at a distance of 1,400 light-years. A still younger cluster is NGC 6611, some of the stars in which formed only a few hundred thousand years ago. At the other end of the scale, some open clusters have ages approaching those of the globular clusters. M67 in the constellation Cancer is 4.5 billion years old, and NGC 188 in Cepheus is 6.5 billion years of age. The oldest known open cluster, Collinder 261 in the southern constellation of Musca, is 8.9 billion years old.

Though several globular clusters, such as Omega Centauri and Messier 13 in the constellation Hercules, are visible to the unaided eye as hazy patches of light, attention was paid to them only after the invention of the telescope. The first record of a globular cluster, in the constellation Sagittarius, dates to 1665 (it was later named Messier 22); the next, Omega Centauri, was recorded in 1677 by the English astronomer and mathematician Edmund Halley.

Investigations of globular and open clusters greatly aided the understanding of the Milky Way Galaxy. In 1917, from a study of the distances and distributions of globular clusters, the American astronomer Harlow Shapley, then of the Mount Wilson Observatory in California, determined that its galactic centre lies in the Sagittarius region. In 1930, from measurements of angular sizes and distribution of open clusters, Robert J. Trumpler of Lick Observatory in California showed that light is absorbed as it travels through many parts of space.

More than 150 globular clusters were known in the Milky Way Galaxy by the early years of the 21st century. Most are widely scattered in galactic latitude, but about a third of them are concentrated around the galactic centre, as satellite systems in the rich Sagittarius-Scorpius star fields. Individual cluster masses include up to one million suns, and their linear diameters can be several hundred light-years; their apparent diameters range from one degree for Omega Centauri down to knots of one minute of arc. In a cluster such as M3, 90 percent of the light is contained within a diameter of 100 light-years, but star counts and the study of RR Lyrae member stars (whose intrinsic brightness varies regularly within well-known limits) include a larger one of 325 light-years. The clusters differ markedly in the degree to which stars are concentrated at their centres. Most of them appear circular and are probably spherical, but a few (e.g., Omega Centauri) are noticeably elliptical. The most elliptical cluster is M19, its major axis being about double its minor axis.

Globular clusters are composed of Population II objects (i.e., old stars). The brightest stars are the red giants, bright red stars with an absolute magnitude of −2, about 600 times the Sun's brightness or luminosity. In relatively few globular clusters have stars as intrinsically faint as the Sun been measured, and in no such clusters have the faintest stars yet been recorded. The luminosity function for M3 shows that 90 percent of the visual light comes from stars at least twice as bright as the Sun, but more than 90 percent of the cluster mass is made up of fainter stars. The density near the centres of globular clusters is roughly two stars per cubic light-year, compared with one star per 300 cubic light-years in the solar neighbourhood. Studies of globular clusters have shown a difference in spectral properties from stars in the solar neighbourhood—a difference that proved to be due to a deficiency of metals in the clusters, which have been classified on the basis of increasing metal abundance. Globular cluster stars are between 2 and 300 times poorer in metals than stars like the Sun, with the metal abundance being higher for clusters near the galactic centre than for those in the halo (the outermost reaches of the Galaxy extending far above and below its plane). The amounts of other elements, such as helium, may also differ from cluster to cluster. The hydrogen in cluster stars is thought to amount to 70–75 percent by mass, helium 25–30 percent, and the heavier elements 0.01–0.1 percent. Radio astronomical studies have set a low upper limit on the amount of neutral hydrogen in globular clusters. Dark lanes of nebulous matter are puzzling features in some of these clusters. Though it is difficult to explain the presence of distinct, separate masses of unformed matter in old systems, the nebulosity cannot be foreground material between the cluster and the observer.

About 2,000 variable stars are known in the 100 or more globular clusters that have been examined. Of these, perhaps 90 percent are members of the class called RR Lyrae variables. Other variables that occur in globular clusters are Population II Cepheids, RV Tauri, and U Geminorum stars, as well as Mira stars, eclipsing binaries, and novae.

The colour of a star, as previously noted, has been found generally to correspond to its surface temperature, and in a somewhat similar way the type of spectrum shown by a star depends on the degree of excitation of the light-radiating atoms in it and therefore also on the temperature. All stars in a given globular cluster are, within a very small percentage of the total distance, at equal distances from the Earth so that the effect of distance on brightness is common to all. Colour-magnitude and spectrum-magnitude diagrams can thus be plotted for the stars of a cluster, and the position of the stars in the array, except for a factor that is the same for all stars, will be independent of distance.

In globular clusters all such arrays show a major grouping of stars along the
lower main sequence, with a giant branch containing more-luminous stars curving from there upward to the red and with a horizontal branch starting about halfway up the giant branch and extending toward the blue.

This basic picture was explained as owing to differences in the courses of evolutionary change that stars with similar compositions but different masses would follow after long intervals of time. The absolute magnitude at which the brighter main-sequence stars leave the main sequence (the turnoff point, or “knee”) is a measure of the age of the cluster, assuming that most of the stars formed at the same time. Globular clusters in the Milky Way Galaxy prove to be nearly as old as the universe, averaging perhaps 14 billion years in age and ranging between approximately 12 billion and 16 billion years, although these figures continue to be revised. RR Lyrae variables, when present, lie in a special region of the colour-magnitude diagram called the RR Lyrae gap, near the blue end of the horizontal branch in the diagram.

Two features of globular cluster colour-magnitude diagrams remain enigmatic. The first is the so-called “blue straggler” problem. Blue stragglers are stars located near the lower main sequence, although their temperature and mass indicate that they already should have evolved off the main sequence, like the great majority of other such stars in the cluster. A possible explanation is that a blue straggler is the coalescence of two lower mass stars in a “born-again” scenario that turned them into a single, more-massive, and seemingly younger star farther up the main sequence, although this does not fit all cases.

The other enigma is referred to as the “second parameter” problem. Apart from the obvious effect of age, the shape and extent of the various sequences in a globular cluster's colour-magnitude diagram are governed by the abundance of metals in the chemical makeup of the cluster's members. This is the “first parameter.” Nevertheless, there are cases in which two clusters, seemingly almost identical in age and metal abundance, show horizontal branches that are quite different—one may be short and stubby, and the other may extend far toward the blue. There is thus evidently another, as-yet-unidentified parameter involved. Stellar rotation has been mooted as a possible second parameter, but that now seems unlikely.

Integrated magnitudes (measurements of the total brightness of the cluster), cluster diameters, and the mean magnitude of the 25 brightest stars made possible the first distance determinations on the basis of the assumption that the apparent differences were due entirely to distance. The colour-magnitude diagram, or the apparent magnitudes of the RR Lyrae variables, however, leads to the best distance estimates. The correction factor for interstellar reddening, which is caused by the presence of intervening matter that absorbs and reddens stellar light, is substantial for many globular clusters but
small for those in high galactic latitudes, away from the plane of the Milky Way. Distances range from about 8,000 light-years for NGC 6397 to an intergalactic distance of 390,000 light-years for the cluster called AM-1.

The radial velocities (the speed at which objects approach or recede from an observer, taken as positive when the distance is increasing) measured by the Doppler effect have been determined from integrated spectra for some 138 globular clusters. The largest negative velocity is 384 km/sec (239 miles/sec) for NGC 7006, while the largest positive velocity is 494 km/sec (307 miles/sec) for NGC 3201. These velocities suggest that the globular clusters are moving around the galactic centre in highly elliptical orbits. The globular cluster system as a whole has a rotational velocity of about 180 km/s relative to the Sun, or 30 km/s on an absolute basis. For one cluster, Omega Centauri, motions of the individual stars around the massive centre have actually been observed and measured. Though proper motions of the clusters are very small, those for individual stars provide a useful criterion for cluster membership.