The Hyades and Pleiades in Taurus

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Nature Magazine, December 1947

Among the many objects of special interest now visible in the winter evening skies, one will have no difficulty in finding the two famous clusters in Taurus, known as The Hyades and The Pleiades, which have been celebrated in song and legend all over the world from the earliest days of recorded history.

The Hyades, in the form of a V, outline the forehead of The Bull. The group is easily recognized by this distinctive appearance, and because the brilliant, red, first magnitude star, Aldebaran, tops one branch of the V. It is supposed to represent the fiery red eye of Taurus. Aldebaran is not a true member of The Hyades star cluster since it does not share in the common velocity and direction of motion of the stars that belong to the cluster. It is one of the stars nearest to the earth, at a distance of about 54 light years, and is seen, projected against the more distant moving cluster composed of stars bound loosely together by their mutual gravitational attraction, and journeying onward through space like a great flock of birds flying toward a common destination.

The Hyades were known as The Rainy Hyades since the word Hyades was supposed to be derived from a Greek word meaning "to rain," and they were so referred to in the literature of all nations. Mariners in particular held them in disfavor because they were associated with the rainy seasons, and were supposed to bring storms and tempests and generally disagreeable weather.

The Pleiades, which lie about ten degrees to the northwest of The Hyades, are, for more than six months in the northern hemisphere, one of the most beautiful and easily identified objects visible in the night sky. They have always been regarded with high favor in all lands. They appear to have been widely associated with religious and other ceremonies, and there is probably no more popular group of stars in the heavens. They have been often referred to as The Seven Sisters, and the story of the Lost Pleiad and the "Seven Sisters Who Now Are Six" persisted for centuries. It is possible that at some time in the past one of these stars was more luminous than it is now, and that this led to the belief that it had become lost, when its brightness faded beyond visibility to the naked eye.

The Pleiades have been poetically likened to a swarm of golden bees clustered upon the mane of Taurus, although they are generally supposed to be located upon the shoulder. One with fair eyesight will readily see six stars in this cluster in the form of a small dipper. Sharper eyes, with exceptionally fine seeing conditions, have seen, without optical aid, as many as 11 or 12 of the brightest stars in the group. Actually there are known to be 250 stars in this cluster, having a common motion through space. It is suspected that the actual number will be found eventually to be close to 500 as more extensive studies of the group are made. Most of the brightest stars are enmeshed in nebulosity, which is believed to be cosmic dust shining by the reflected light of these stars associated with it.

These two famous star clusters are known as loose, or open, galactic clusters. They are members of our own galaxy, or Milky Way system of stars as distinguished from the great globular star clusters that are much more compact, and are composed of thousands or tens of thousands of stars, as compared with several hundred stars in the galactic clusters. The globular clusters are not members of our Galaxy but lie far exterior to it at distances ranging from 20,000 to 200,000 light years, and are found both above and below our great galactic system. Some of the galactic clusters, including both The Hyades and The Pleiades, are in our own immediate neighborhood. Our solar system is actually in the midst of, although it does not belong to, the great Ursa Major cluster, which includes five of the seven stars in the Big Dipper and, in addition, Sirius and nearby stars in Leo, Auriga, and Eridanus. Other well-known galactic star clusters are Praesepe, or The Bee-hive in Cancer, Coma Berenices, and the double cluster in Perseus.

There have been catalogued, in all, 249 galactic star clusters. The members of a cluster are loosely bound together by their mutual gravitational attractions. There are two general types of such clusters, and they are known as The Hyades type and The Pleiades type because these two particular clusters are so well known and so representative of the two types of clusters. The Pleiades, and star clusters of similar type, consist of bluish or white stars, known generally as helium or B type stars, and hydrogen, or A type, stars. They contain no yellow-red giants or super giants. They are in nearly every instance dwarf stars of an earlier type than our sun, which is also a dwarf star. Alcyone, brightest of the Pleiades, is 1400 times more luminous than our sun, and 15 times more massive, with a diameter about ten times that of the sun. Yet it would be classed as a dwarf. It is bluish-white in color and has a surface temperature of about 13,000 degrees Absolute, as compared with about 6000 degrees for our own sun. The faintest members of the Pleiades cluster, on the other hand, are 200 times fainter in luminosity than the sun. They have about one third the mass and only two-fifths of the diameter of the sun, according to Trumpler, who has made a special study of this cluster. The temperatures of these stars are low, only about 3500 degrees Absolute. The Pleiades are, then, early type dwarfs, bluish or white and, in general, smaller and less luminous than our own sun, and their most luminous members are associated with and embedded in nebulosity.

The Pleiades are at a distance of 500 light years. The cluster has a diameter of about 50 light years and is receding from the earth at the rate of 5 miles a second. In a galaxy that has an extent of 100,000 light years they are, if not our next door neighbors, at least members of our own community. It has been estimated that, in the more crowded central part of the Pleiades cluster, the stars are 100 times more numerous than in the average stellar region. Yet they cannot be considered greatly crowded together with an estimated average distance between neighboring stars of one or one and a half light years.

The Hyades cluster is even less compact than The Pleiades. Its distance from us is about 120 light years, and its diameter 35 light years. It is moving at the high velocity of 30 miles a second toward a point to the east of Betelgeuse in Orion. There are about 100 members of this cluster. Unlike the Pleiades group, there are red and yellow giant stars among The Hyades; but helium and hydrogen stars, which appear in great numbers in The Pleiades, are not to be found in The Hyades. There are, however, yellow dwarf stars, such as our sun, and also red stars in The Hyades cluster. No reason is evident for the division of galactic clusters into two such markedly different types of stars, that is the early type found in clusters such as The Pleiades, often associated with nebulosity, and the later type, chiefly yellow and reddish stars, appearing in clusters of the Hyades type. Nebulosity among the Pleiades shows an abundance of cosmic dust in that cluster.

In all galactic star clusters there are many stars, some of them brilliant stars, like Aldebaran, that are not true members of the cluster but temporarily within them. It is possible from a study of the "proper motions" of stars, or their motion across the line of sight, and their radial velocities, continued over a long period of time, to determine which are true members of the cluster and which stars have no connection with it physically. Eventually the statistics are compiled and one knows, as is the case with the two well-studied groups in Taurus, how far away the cluster is, how great is its extent, and whether it is spherical or flattened in shape, how many members there are in it, few or many, and how fast it is traveling through space and in what direction. As time passes on, and the observations made many years ago of the motions of the stars in the cluster are compared with those made in recent years, sometimes after a lapse of a hundred years, these statistics on the galactic star clusters become more complete and valuable, and also add to our information regarding the structure of the Galaxy itself.

During the first few days of December Mercury may be glimpsed low in the southeastern sky before sunrise. It will be too close to the sun to be seen later in the month. Venus is now in the evening sky, but it is not far above the southwestern horizon at sunset and will not be well placed for observation in the northern hemisphere.

Mars now rises before midnight, and by the middle of December will be seen a few degrees east of Regulus in Leo; and at the same time Saturn also will be found in Leo about an equal distance to the west of Regulus. It will rise shortly before Mars. At the beginning of the month the two planets are nearly equal in brightness, but Mars is now increasing quite rapidly in brightness and by the end of December will be decidedly brighter than Saturn. It will be interesting to note the relative change of these two planets in position and brightness during the month, and also their marked difference in color. Mars is, of course, reddish, and Saturn pale yellow, in color. Jupiter will be in conjunction with the sun on December 1, when it will pass to the morning sky. It will be too close to the sun to be observed this month.

On December 22 at 11:43 A.M. Eastern Standard Time the sun will be at the solstice and farthest south of the equator; winter will then commence in the northern hemisphere, summer in the southern.