IN STARRY SKIES

Evening Post, Volume CXI, Issue 31, 6 February 1931, Page 16

IN STARRY SKIES

THE HYADES

(By "Omega Ceutauri.") ■ Last week we gave a sketch map of the Hyades cluster, and described what we can see in it with the naked eye and with field glasses. , To-day we show a photograph of the same region taken by Professor J. E. Keeler with tho six-inch Willard lens and Crocker telescope at tho Lick Observatory. The exposure lasted two hours. Tho original photograph is full of, fine detail, which unfortunately is unsuited i for newspaper reproduction. The whole background is dotted over with innumerable tiuy points of light. Most of these are the images of galactic stars which do uot belong to the cluster. These show excellently a remarkable peculiarity of distribution that is noticeable in most long-exposure photographs of the galaxy. The stars do not appear to be scattered at random .They seem to form part of an organised structure. . What the real arrangement is, it is very hard to imagine for in the photograph wo have only tho projection of a three dimensional structure on a plane surface. But the appearance resembles the arrangement of the cells in a section of a plant seen through the microscope. Nearly everywhere the faint stars that form the background appear to be arranged in ovals,, the stars forming curved lines in which they are closely spaced, the

parts within the ovals'being comparatively dark and empty. The expJanaition'of this appearance is a problem that awaits solution. But projected on this delicate stellar background are some brighter points arranged roughly !in the form of capital V turned over on its side. The brightest star of all, Aldebaran, though forming an essential part of the letter V, does not belong to the cluster. Most of- the others are not accidentally, grouped,' but form a definite stellar-family. This is proved by their'motions iv space. When their proper motions are plotted on a large scale it is found that forty or more are directed towards a single point lin the sky about .six degrees east of Betelgeuse. .If lines are drawn so that each represents the apparent movement that would take place iv 20,000 years, it is evident that they are by no means all exactly equal. But this inequality is at once accounted for by perspective. The cluster extends over about fifteen degrees. It is approximately globular, and its diameter is about 32 light years. The centre of the group is about 130 light years from us, so some of the stars must be about a quarter as far off again as others, and the nearer ones. must have apparently greater displacements. Now the speeds, though appearing to be directed towards a single point, are almost certainly parallel or very slightly diverging. This conclusion is confirmed by a study of their radial motions. It is found that the common resultant velocity is about 28i miles a second. Individual variations from this common speed must be very slight, or ere now the cluster would have been dispersed. Tho stars of the.cluster are mostly giants. Though appearing to bo between the third and seventh magnitude, they* are iil-reality suns much bi"ighter tha|i ours. The faintest, indeed, are about five times and the brightest at least' a hundred times as brilliant as the sun. Then combined light i's equal to that of a

thousand suns. But this refers only •to the brighter stars of the cluster. There are no doubt many fainter ones. It is estimated that 80 stars above the tenth magnitude belong to the group, as do also an unknown number oi fainter ones. But the fact that any particular star appears within the, boundaries of the cluster does not prove that it really belongs to it, Aldebaran, as we have seen, is less than half as far away from us as the real cluster stars, aild 'moves in a different direction. Many others must happen to be in the same Hue, though much nearer or much further off, and possibly some interlopers may be actually within the bounds of the cluster. The test of relationship 'is the motion through space. It is interesting to notice how the actual speed is estimated. In this case all the proper motions appear to converge towards a point about 30 degrees from the apparent position of" the centre of the cluster. The transverse speed must therefor-? be about half the actual speed. Tht radial speed in the line of sight forms the third side of a right-angled triangle, of which the transverse speed is one side and the actual speed of the hypotenuse. If the length of any side of this triangle can be found, the others are at once determined, since we know two angles. Now the radial speed can be measured directly with the spectroscope. This was done in the case of half a .dozen of the stars of the Hyados, -and all the results were concordant. It is thus fo"n<l t'ha* «*<» system was nearest to the sun about 800,000 years ago. At present its distance from the solar system.is increasing by over 24} miles a second. In 65 million years, Eddington tells us, it will have shrunk to an

appareat diameter of only 20 minutes. The individual stars which now range from the 3rd to the 7th magnitude Trill then be between the 9th and the 12th. The Pleiades and the Hyades we have taken as typical examples of what are known as open clusters.' Of these about two hundred are known. Although very L.nall beside the globular clusters such as Omega Centauri, 47 Toneani, or M.13 in Hercules, they are built on an impressive scale. Their diameters mostly lie between 10 and ii light years, but Messier 37 is much larger. The Hyades and the Pleiades are the nearest to us. Four others are less than a -thousand light years away. Many others are five or six, and some as much as ten, times as far away. Some are known to contain several thousand stars. In the Northern Hemisphere the Pleiades, the Hyades, and nimbosus Orion have long been associated with rain and storms. The epithet "watery" is often applied specially to the Hyades. In Greek mythology the Hyades were tho daughters of Atlas and half-sisters of the Pleiades. One story says that they were changed into stars by their grief at tho death of their brother Hyas. Mr. Elsdon Best gives three names .used by the Maori for this group. These are Mata, Mata-kaheru, and Te Kokota. Taumata-kuku was the Maori name for Aldebaran. It was probably the Hyades to which Job referred by the name of Mazzaroth when he said: "Caust thou bind the sweet influences of the Pleiades, or loose the bauds of Orion? Canst thou bring forth Mazzaroth in his season, or canst thou guide Arcturus with his sons?" Homer speaks of the same constellations, and us that Vulcan traced on his shield all the stars that wreath the sky: the Pleiades, the Hyades, the brilliant Orion, 'the Bear, and the Chariot, which revolves round the Pole.