What a star!
Once again the Sky Serpent has been defeated and forced to disgorge the Sun almost as soon as it had swallowed it.
Thirty-three hundred years ago, the Eighteenth Dynasty Pharaoh Akhenaten (“he who is beneficial to Aten”) made a shortlived attempt to replace the gods of Egypt with a monotheistic religion centred on the Aten, the divine personification of the Sun’s disk, of whom pharaoh was the earthly embodiment. Pictorially, the Aten was represented by a golden disk from which sprang a multitude of arms radiating towards the Earth, each bearing the open hand of donation.
The movement and its monuments barely survived Akhenaten’s death. During the reign of his young son Tutankhamon, there was an orthodox counter-revolution, and the accustomed theocracy was restored. The capital was moved back to the old site at Memphis, and Akhenaten’s new city of Akhetaten was stripped. The neglected bureaucracy was set in order, the many gods restored to their temples, and their attendant priests re-endowed. An interesting experiment was over.
Throughout history, societies have recognised the importance of the Sun, and, whether as a god or not, raised to it some of the most impressive monuments ever built. The position on the horizon of sun rise and set marked the march of the year and the cycle of the seasons—essential information for both hunters and agriculturalists. Thus, the occasional disappearance of the Sun and the replacement of daylight by the night of mid-eclipse provoked strong reactions, even though the Babylonians had determined the Saros, the 18 year 11 day cycle of eclipses, by about 1500BC.
This recurrent pattern depends upon the fact that the plane of the Moon’s orbit about the Earth is not only inclined at about 28° to that of the equator, but also that it rotates once every nineteen years, after which time the relative positions of the Sun, Moon, and Earth are again the same. Saros enables eclipses to be predicted, although it does not help in determining the path of totality on the surface of the Earth.
But understanding detracts not at all from the impact of the sudden appearance of the Sun’s corona at totality. All but the most disciplined astronomers find their plans awry as they are distracted by the event, and there is an overwhelming temptation to drop everything just to savour the experience as it is.
Thus it was that tens of thousands of people descended upon Hawaii for the total eclipse of July 11 this year. Wind patterns were analysed, along with the figures for cloud cover and precipitation; local microclimates were studied and sites chosen accordingly, while the souvenir industry switched to shift work and hired extra deposit boxes.
Alas, come the dawn unseasonable cloud shrouded the island, and, apart from some fortuitous gaps, the event was hidden from all but the lucky few who had just minutes of less than perfect viewing. Even the professional observers sequestered atop Mauna Kea at 4200m were deprived of a really clear view, as high cloud attributed to the dust from the Mount Pinatubo eruption marred the sky.
All this activity was not just some atavistic rite. For dozens of amateur and professional astronomers this was an all too rare opportunity to probe the Sun’s baffling complexity.
By the end of the 1950s we thought that we had pretty well disentangled its mysteries, and seemed able to give a comprehensive account of the mid-life of an unexceptional G2-type star, in the steady light of which life had evolved on Earth. We looked forward to another four or five billion years of impeccable solar performance. But as more refined techniques of measurement were developed and the spectral range of detectors extended, so the rather bland picture was ever more extensively modified. We found ourselves having to model a complex and variable star, the surface of which is in turmoil, the interior of which pulsates with many modes of vibration and whose outpouring of plasma, with its associated magnetic field, extends as far beyond Pluto as that planet is from the Sun.
To the naked eye, the Sun presents itself as an orange/yellow circular disk on which the occasional spot may be seen. Little or nothing hints at either its size or the ferocity of the events involved in its existence. Basically, it is a sphere of gas, about 25% hydrogen and 75% helium—the two lightest of all the elements—totalling nearly 300,000 times the weight of the Earth and Moon combined, and held together by the force of gravity. Although this is the weakest of the fundamental forces, and insignificant at the subatomic level, it is the force which arbitrates the fate of bulk mass, be it the fall of dust or the path of the solar system around the centre of the Galaxy.
During the Sun’s genesis, as the solar primordial cloud contracted, the temperature and pressure at the centre rose to such an extent that nuclear fusion of the protons, the core of a hydrogen atom, commenced a process known as the proton-proton cycle, in which four protons fuse to yield a helium nucleus together with a neutrino and high energy photons. On Earth we have duplicated this process in the hydrogen or fusion bomb.
That the solar hydrogen bomb does not blow itself apart is due to the force of gravity. As the radiation pressure developed in the core tends to expand it, which slows down the rate of fusion and hence of energy production, so does gravity pull everything towards the centre. Thus the forces of contraction and expansion achieve a nice balance, and the Sun’s size is remarkably stable.
This was not always so, and will probably be true for only another 5,000,000,000 years, by which time the major portion of the available hydrogen will have been consumed in fusion reactions, and a series of changes will take place, with gravitational contraction of the core and the initiation of helium fusion to form carbon at these increased temperatures and pressures.
These latter stages of the Sun’s life will result in dramatic changes as its outer layers expand and engulf the inner planets; thereafter the gravitational contraction is the only significant source of energy, and apart from short-lived outbursts it will cool and shrink until it becomes a White Dwarf and ultimately a cold cinder.
But all this lies far in the future; our current concern is to understand the Sun as it is. Though relatively stable when compared to the variable stars, nevertheless it exhibits a variety of recurrent and transitory phenomena which must be understood if we are to have an accurate picture of what is going on in the interior. Such knowledge may be of more significance to the future of the Earth than is currently credited. For example, the relationship between our climate and the sunspot cycle, which itself reflects the behaviour of the Sun’s periodically reversing magnetic field, is still the subject of debate.
Solar flares and prominences, which are coupled with magnetic storms on the Sun, result in charged particles—protons and electrons—bursting from the surface and arching up to 800,000km above the photosphere, with much of this material breaking free and radiating away in the inter-planetary magnetic field. Its arrival at the Earth creates geomagnetic storms signalled by intensification of the aurorae and disturbances of the Earth’s magnetic field, and can disrupt shortwave radio and telegraphic communication. In the case of major flares, disruptive induction effects on power lines generate currents sufficient to trip circuit breakers and cause widespread blackouts.
Launched on October 6, 1990, the spacecraft Ulysses will, after gravitational assistance from a Jupiter encounter arc high above the ecliptic (the plane in which the planets rotate about the Sun), pass over the Sun’s south pole in mid-1994, and then over its north pole 12 months later. For the first time we will get a view of these two crucial areas and a three-dimensional map of the composition of the solar wind and the corona.
Our understanding of the Sun is not only a key to our understanding of the stars in general, but also a matter of immediate concern to us on Earth. Not only are we becoming ever more dependent on complex networks of communications and power distribution (remember the effects of the Great Blackout on New York in 1965?), but also, as our overburdened planet struggles under the impact of its currently dominant species, it may well be that our survival will hinge on our understanding of the Sun and its complex interaction with the Earth, its weather engine and its magnetosphere.
Today we know that the Aten must be understood as well as praised if its many arms are to be truly beneficial.