When presenting a general programme about star-clusters, in November 1961, it occurred to me that there was a good opportunity to clear up a minor problem - that of the number of stars in the Pleiades visible without a telescope. The cluster is known popularly as the 'Seven Sisters' but it was generally stated that eight or nine stars could generally be made out under good conditions by people with normal eyesight.
I therefore asked for the co-operation of those who watched the programme, and the results were very satisfactory. Hundreds of viewers co-operated, and the final conclusion was clear-cut: most people could indeed see seven stars. About 10 per cent recorded eight or nine; a few could make out only 5 or 6; the highest reliable figure was 10. When I had sorted out the obviously erroneous observations (including that of the viewer who recorded no less than 170 stars, having included not only the rest of the constellation Taurus, but also Orion!) it was apparent that the percentage of’ seven-star' results was well over 85. Therefore, the cluster's nickname of the Seven Sisters is more appropriate than had been previously thought.
Clusters of stars have been known from ancient times, since there are several clusters which rank as prominent naked-eye objects. They are of various kinds, but for most purposes may be divided into two main classes: open (or loose) and globular.
The open clusters are entirely different. There is no regularity in shape, and the concentration of stars is very much less, so that it is not always easy to tell what is a genuine cluster and what is not.
The Seven Sisters, or Pleiades, are particularly notable, and are well seen in the evening skies during winter, so that it seems worth discussing them in rather more detail.
The cluster is easily visible without optical aid under even moderate conditions, but city-dwellers will naturally be unable to see it to advantage. When the sky is illuminated by street-lamps and brilliant shop-window lights, and when the air contains smoke, relatively faint stars are obscured, and the leading member of the Pleiades - Alcyone - is only of the third magnitude. People who live in, say, central London, will be able to do no more than see the cluster as a dim, hazy patch. Matters are very different when the sky is dark and there are no artificial illuminations nearby. The Pleiades show up conspicuously, and several individual stars may be seen without optical aid.
As an extra means of identification, Orion may be used as a direction-finder. The three stars of the Hunter's Belt point upward to Aldebaran, the orange-red star known as 'the Eye of the Bull'; extending the line and bringing it slightly downward leads us to the Pleiades. There should be no difficulty; anyone who looks to the south on a clear winter's evening will be able to pick up the cluster at a glance. It is not surprising that the Pleiades have been described in many ancient legends. To the Red Indians, for instance, they were children who loved to wander among the stars, and huddled close together when they lost their way to make sure that they would not be separated.
The nickname of 'Seven Sisters' does not necessarily mean that only seven stars of the Pleiades are visible to the naked eye. In fact there has never been any full agreement on this point, and though the matter is of no real scientific importance it could well be cleared up. In the television programme of 22 November viewers were asked to wait for a clear night and then examine the cluster, noting the number of stars clearly visible without optical aid and appending a rough sketch - together with the observer's name and address, and a word or two about the conditions under which the observation was made (particularly with regard to the presence absence of street-lights). It is hoped that the response will settle a minor problem which is, nevertheless, not without interest.
The distance of the cluster is about 400 light-years, and the full diameter is about twenty light-years. Alcyone and the other chief stars are of spectrum type B, and so are bluish-white in colour, with high surface temperatures. Photographs taken with large telescopes reveal the presence of extensive nebulosity, and in fact we have here a splendid example of a reflection nebula.
It is interesting to compare the Pleiades with the second of the Taurus clusters, the Hyades, which surrounds Aldebaran. Both are open clusters, but they are not in the least alike. The Hyades stars are individually brighter, but are more scattered, so that the overall impression is not nearly so beautiful. The best views of the Hyades are obtained either with binoculars or else with very low telescopic powers. (The Pleiades, too, are excellently seen with binoculars; with moderate telescopic powers only a part of the cluster may be seen in the field, so that the general effect is less striking.)
There is also the point that the Hyades are overpowered by the brilliant orange light of Aldebaran. Actually Aldebaran is not a member of the cluster at all, and simply happens to lie in the same direction as seen from Earth. The Hyades stars lie at a distance of about 100 light-years, but Aldebaran is only 57 light-years from us. In fact, Aldebaran lies about half-way between the cluster and ourselves.
Line-of-sight effects are often confusing, and the principle may be extended to the constellation-patterns themselves. It is wrong to suppose that the stars of any particular constellation have any true connection with each other. Orion is a good example of what is meant. Most people can recognize its two first-magnitude stars, Betelgeux and Rigel, and the outline of the 'celestial hunter' is unmistakable. Yet Betelgeux, at 520 light-years, lies at about half the distance of Rigel (900 light-years) - so that Rigel is as far away from Betelgeux as we are. An even better example is provided by Centaurus, the Centaur, which is unfortunately too far south in the sky to be visible from Britain. The two brightest stars, Alpha and Beta Centauri, lie apparently side by side; yet Alpha is a mere 4.3 light-years away, and so is the closest of the brilliant naked-eye stars, while Beta is a highly luminous B-type giant at a distance of at least 300 light-years.
Aldebaran, then, has no connection with the Hyades cluster, and the true Hyades may be seen extending westward in a rough V-form. One of them, 6 Tauri, is a naked-eye double, and is a good test for the seeing conditions; if both components are clearly visible, the atmosphere must be at least reasonably transparent.
There is no reflection nebula in the Hyades cluster, and the stars themselves appear to be of different type. There are no very luminous B-type stars such as those of the Pleiades, and the leading members are red supergiants of later spectral type. This gives us a clue to the relative ages of the two groups.
It used to be thought that the Red Giants were young, the bluish B-type stars older, and the Red Dwarfs nearing the end of their careers. According to a theory put forward more than half a century ago by Lockyer, a star began as a large, cool red body, contracting and becoming more luminous until it reached the peak of its existence as a B-type giant and then fading away until it became a feeble Red Dwarf. Nowadays the contrary view is held. It seems that the Red Giants have used up much of their hydrogen 'fuel', and are at a relatively late stage in their evolution.
It follows, then, that in a young cluster the chief stars will not have reached the Red Giant stage; the leaders will be hot and bluish, and there will still be considerable quantities of interstellar material in the form of gas and dust. In an old cluster, very luminous B- and O-type stars will no longer exist, but Red Giants will occur; moreover there will be an absence of interstellar material. Everything indicates that the Pleiades cluster is 'young', the Hyades 'old'.
The Double Cluster in the constellation of Perseus
Other examples may be cited. The 'Sword-Handle double cluster in Perseus (H.VI. 33-34), visible to the naked eye in the region of Cassiopeia, contains many O-type stars which are using up their hydrogen at a prodigious rate and which may eventually develop into Red Giants. Since these O-stars run through their life-stories relatively quickly, we may be sure that the Sword- Handle is 'young'. On the other hand the much fainter cluster Messier 67, in Cancer, contains no known stars of type earlier than Ao, while dwarfs of later than type F5 are numerous. It is generally believed that Messier 67 is the 'oldest' open cluster so far studied.
Several more open clusters are visible to the naked eye. Praesepe, in Cancer - roughly between Castor and Pollux on the one side, and Regulus in Leo on the other - is nicknamed the 'Beehive', and is quite prominent in a clear sky, though moonlight is generally sufficient to obscure it. Another lovely cluster is Messier 35, in Gemini. Observers equipped with small telescopes will be able to find many more of them.
The stability of a cluster-system is of great interest, and is bound up with the general problem of the arrangement of stars in the Galaxy. Globular clusters, of course, are entirely stable, and will retain their identities indefinitely; but the much less densely populated and more scattered open clusters are a different proposition altogether.
The Galaxy itself is a flattened system, with a central nucleus (in the direction of the Sagittarius star-clouds) and spiral arms. The spiral form was established only comparatively recently, by the use of radio astronomy, but it was not unexpected, since many external galaxies are spiral - notably the Great Galaxy in Andromeda (Messier 31), which, at its distance of roughly 2,200,000 light-years, is the largest of our neighbours in the Local Group. The Sun lies some way from the galactic centre, as Shapley established forty years ago, following his studies of the distribution of globular clusters.
It has also been shown that the Galaxy is in a state of rotation round its nucleus. In the neighbourhood of the Sun, the revolution period is some 225,000,000 years; this figure is naturally uncertain to some degree, but is of the right order, and is often termed the 'cosmic year'. On Earth, the Carboniferous Period, when the coal measures were laid down and amphibians represented the highest form of terrestrial life, occurred only one cosmic year ago.
Open clusters share in the general rotation of the Galaxy, and tend to move in parallel paths, so maintaining the same distances from each other. Yet there are many perturbing influences which disrupt them. There is the tremendous force of the Galaxy itself, and there are also neighbouring stars to be taken into account; some of these, moving in different types of orbits, may pass through open clusters and produce marked effects.
It may be said that if the internal gravitation of a cluster exceeds the disruptive force of the Galaxy as a whole, then the cluster will be stable, and will retain its identity over a very long period. If however the internal gravitation is the weaker, then the cluster will soon be disrupted. With the Pleiades, it seems that the central regions of the cluster are just about stable, but the outer parts are not, and so the system has a limited 'life'. The looser Hyades are less stable still.
On the whole, therefore, it seems that few open clusters can persist for more than a few revolutions round the galactic nucleus; they will then have been so scattered by perturbing forces that the clusters will cease to be identifiable. Most of the open clusters which we now see are relatively 'young', but there are exceptions. Messier 67, referred to above as being the 'oldest' of the open clusters so far studied, lies over 1,000 light-years above the plane of the Milky Way, and so is not subjected to the large perturbations which disrupt clusters closer to the plane.
But though open clusters can persist for periods which are brief in comparison with the total age of the Galaxy, their disruption is a very slow process by our everyday standards, and systems such as the Pleiades and the Hyades appeared to our remote cave- dwelling ancestors precisely in the form in which we see them today. In binoculars or low-powered telescopes the Pleiades, in particular, are magnificent, and so to a lesser extent are the Hyades - despite the spurious inclusion of Aldebaran. Observers who can make use of more powerful telescopes will be able to explore the glories of systems such as the Sword-Handle in Perseus and the aptly named 'Wild Duck' cluster in Scutum, while those who live in the southern hemisphere can add the 'Jewel Box', * Crucis in the Southern Cross. Though the open clusters must be regarded as essentially temporary associations, they are of great importance to astronomers, and even the less serious observer will derive much enjoyment from looking at these superb groups of suns.
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