The running bull

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On summer nights the sky is dominated by the bright constellation of Orion, the Hunter. Traditionally, he is seen wielding a club with his right hand and holding a lion’s skin with his left as he faces the charging bull, Taurus.

Marking the bull’s head, which lies a hand’s breadth west of Orion, are the Hyades, a V-shaped group which is also distinguished by the bright orange star, Aldebaran, the bull’s left eye. The great horns loom threateningly over Orion’s head, and the massive hump of the shoulders is marked by the Pleiades, the Seven Sisters.

For southern observers this dramatic tableau is upside down, and so the figures are far from obvi­ous. To us, Taurus appears as the two bright clusters, the Pleiades and the Hyades, and Orion is reduced to his belt and sword, commonly seen as the humble Pot.

Whether they depict military hardware or domestic kitchenware, the constellations with which the sky has been patterned are mere artifacts of our position in the galaxy. Were we able to change our position by a few hundred light years then the night sky would be totally unfamiliar, for the nearer stars would fall into quite different patterns.

It was not until 1838 that the first measurement of the distance of a star from Earth was able to be made. In that year Friedrich Bessel measured the distance of 61 Cygni using the technique of trigonometric parallax. Since then, refinements in instrumentation and the introduction of new techniques have enabled us to model the galaxy in three dimensions, and to locate the position of our solar system within the observable universe.

Five thousand years ago this model was unguessed at by the peoples of the emerging civilisations of the fertile crescent, the Indus valley, China and the Americas. For them the idea of a simple celestial sphere was unavoidable, the stars fixed in their positions upon it and their relationship to the all-important cycle of the seasons self-evident. Thus a rich fabric of associations and mythology was woven, and each centre of civilisa­tion patterned the sky with constellations of varying significance.

Fixed though the stars may appear to be, never­theless the overall picture relative to the seasons gradually changes owing to the fact that the axis of rotation of the Earth is not fixed in space but, like a child’s gyroscope, wanders in a circle, taking about 25,800 years for each cycle. In 3000Bc, Aldebaran, the bull’s bloodshot left eye, marked the sun’s position at the spring equinox, and was called by the Babylonians I-ku-u, the leading star of stars. The name we use today is a corruption of the Arabic Al Dabaran, the Follower (of the Pleiades), since by 1000AD the position of the spring equinox had moved into the constellation Aries and so was no longer seen as associated with the bull’s eye.

Amongst the constella­tions and other lesser groupings of stars recog­nised by the ancients, the Pleiades and the Hyades are unique in being composed of physically associated stars rather than chance conjunctions of stars at widely different distances from us.

The Pleiades, M45, often called the Seven Sisters (for all that a keen eye under ideal conditions can see 11), is composed of hot young blue stars. Invisible to the naked eye but showing on photo­graphs are luminous wisps of dust swathing the stars. Striated like some celestial cirrostratus, this nebulos­ity appears to map the magnetic fields around the various stars.

The dramatic vee of the Hyades is actually a composite pattern. The majority of the stars belong to a galactic or open cluster about 130 light years away. However, Aldebaran, a Taurii, the lucida or brightest star of the constellation, is not of the company, being an orange giant only 65 light years distant. About three times the mass of the sun, it is bloated by the onset of old age to about 40 times the sun’s diameter.

Although apparently similar to nearby Betelgeuse, a Orionis, Aldebaran is but a baby in terms of mass, and so is denied the terminal splendour of a supernova explosion, but will die of hiccups. With the exhaus­tion of its hydrogen, the core will contract and reach a temperature of about 100,000,000 Kelvin, initiating helium fusion. The sudden onset of this process is termed the helium flash, and it leads to a carbon core sur­rounded by a dense envelope of helium undergoing fusion. Being an inherently unstable phase, the temperature of this composite core fluctuates and the now extended atmosphere, mainly the remnants of the original hydrogen, is blown off into space as a series of concentric shells.

When the fusible fuels are exhausted and the atmosphere has dissipated into space, all that remains will be a white dwarf, a sphere of superdense material composed of carbon nuclei and a degenerate electron gas—a swarm of electrons no longer bound to the nuclei of atoms. With a tempera­ture of about 12,000 Kelvin this object will shine whitely, but, being small, will be so dim as to be scarcely visible from Earth.

Long before the eye of the bull is closed, the Hyades will have parted company from Aldebaran, for most of the stars forming the vee are in relatively rapid motion towards a vanishing point which is a little to the east of Betelgeuse. Measure­ments by H.G. van Bueren in 1952 showed that this common motion is shared by 132 stars scattered over an area of sky about 15° in diameter. More recent studies have greatly increased the membership of this cluster, which has a space motion of 26 km/s.

The clear inference of this common motion is that these stars are all members of the same “family”—that is, that they were all formed in the same volume of space from a gas/dust cloud whose motion they perforce imitate. Being formed in the same parent cloud,these stars will all be effectively of the same age. The scatter of “birth dates” may be several million years, but stellar life spans are typically  measured in billions of years, so a million years is neither here nor there. Since stars formed from the same parent material will have the same chemical  composi­tion, their develop­ment as they age and the differences which they  exhibit one from another will be a function of their masses. The greater the mass of a star, the shorter its life, for high mass means greater internal pressures and hence greater core temperatures where the fusion of hydrogen to helium is occurring.

Since stars live so very much longer than we or even our civilisations do, we cannot hope to disen­tangle their life histories by just observing them as individuals. However, the study of a same age population with its members evolving at different rates provides us with clues we need.

Being so close, we have been able to measure the along and across line-of­sight velocities of the individual members of the cluster, and also to deter­mine the convergent, or vanishing, point accu­rately. From this data, individual stellar distances have been calculated, and from these the absolute magnitude of each star, i.e. its brightness at the standard distance of 10 parsecs (32.6 light years).

Spectroscopic examina­tion, such as is performed at Canterbury University’s Mount John Observatory, reveals both the spectral class and surface tempera­ture of a star, and from these data both the size and the mass of a star can be determined.

Because they are, astronomically speaking, so accessible, the Hyades have been one of the key targets for astrophysicists searching for clues to the life cycles of the stars. As their analysis unfolds, we pass from myth to mass, from the intriguing pattern of the celestial wallpaper to the absorbing quest for a coherent understanding of the workings of our galaxy and its constituents, and thence outward to the limits of observability.