IN STARRY SKIES

Evening Post, Volume CXV, Issue 111, 13 May 1933, Page 21

IN STARRY SKIES

THE COSMIC SPARK

(By "Omega Centauri.")

We have pictured, as the result of a partial stellar impact, two wounded stars flying away in different directions and spinning they'go, ■ but inoro or, less restrained in their efforts to:escape, not only by their mutual gravitation, but, in addition, by the extra pull of the "Third Body" midway between them. This Third Body, or Cosmic Spark, is for a time the most spectacular object in the heavens. It contains but a small fraction of the total mass, but, during its brief period of splendour, it throws its parent stars completely into the shade. Or more exactly, they are absolutely hidden in ■its blaze of light. Th<? spectrum of every Nova, during its time of maximum brightness, and for some weeks or months afterwards, is essentially the spectrum of the Third Body. Its characteristics were predicted and explained in 1878 ,by Professor A. W. Bickerton. The wounded stars are unable to make their influence felt, until their offspring, this astounding Cosmic spark, has scattered its substance and dissipated its energy far into surrounding space. We have already noted briefly the strange constitution of this whirling mass. Initially the heavier elements are chiefly at the two opposite ends of a diameter, leaving two regions of comparative weakness between, whilst the .. turbulent light atoms are temporarily imprisoned in tho centre. But nothing can long restrain these most energetic atoms. We have seen the way in which they steal more- energy from every heavier atom they encounter, and very soon they acquire such velocities that they irresistibly force their way out and form an atmosphere, in which they soon become roughly sorted into expanding and approximately spherical shells. But the original want of balance must be borne in mind. Gravitation tends to mould the whirling mass into a spherical form, but for some time the lighter gases find the two regions of weakness their readiest way of escape. The atomic gpoed of • every light gas is continually increasing whilst tho atoms of the heaviest gases move more and more slowly. If the different gases were intermingled long enough the final speeds of the atoms would be approximately inversely proportional to

their atomic ■weights. Hydrogen atoms would then on the average be moving about twice as fast as those of helium, four times, as fast as those of oxygen, nearly \ seven and a half times as fast as those of iron, and more than 14 times as fast as lead. In the case of a comparatively slight graze all the light. atoms will. inevitably acquire more than the critical velocity of escape,, If the mass of the Third Body is a small fraction of -that of the two stars, even the heaviest atoms may retain more than this critical velocity, and the body may, therefore,' be entirely dissipated. If a larger fraction is struck off the heavy atoms may be gathered again at the centre whilst the light ones form a.> more or less permanent surrounding shell. But what is the nature of the spectrum to be expected at each stage?' Directly after the collision the body is so condensed that we have a continuous spectrum. Atomic velocity and pressure from behind quickly force the outer layers to form a gaseous atmosphere. In this all kinds of atoms may be found. So very soon the spectrum is crossed by great numbers of narrow dark absorption lines. Since the gaseous atmosphere is rapidly expanding, the gas directly between the nucleus and our eyes is coming towards us. So these absorption lines are displaced towards the violet. As the atmosphere expands it gives an increasing share of the light. As it is spreading in all directions it adds light bands to the spectrum almost in their normal positions. Each bright band is then fringed by a dark line on its violet side, and most of these lines will be widened because the atomic speeds arc increasing. The hydrogen, for instance, that has just escaped from the nucleus is moving faster than any that was set free earlier. As the atmosphere fills a larger and larger) sphere the fraction of it directly in front of the nucleus grows continually smaller. But now a very strange effect is noticed. The body as it spins brings J the regions of weakness opposite to us again and again. '' Each . time this occurs during the equalisation of temperature a fresh jet of light gas is sent in our direction. In each • now jet the speeds are greater than any be-

fore. Absorption takes place at the same time in the new jet and in its predecessors. Thus tho dark lilies on the violet side of the bright bands may become doubled or trebled. The first few may be wide enough and near enough together to blend into a single dark band. But as each is getting narrower at a certain stage, the doubling or trebling becomes clearly apparent. But after a time the portion of the shell, between the nucleus and the eye becomes such a small fraction of the sphere that the dark lines become too narrow to be detected. The spectrum is now one consisting .of bright bands or lines on a fainter continuous background. The glowing atmosphere may become so brilliant as almost to hide- the' light of the nucleus. The spectrum - then consists almost entirely of bright lines and bands. The subsequent history depends chiefly on the fraction struck from each star. If this is very small, the "Third Body" will be completely dissipated. In this case any light that remains will be due to the wounded, stars.. Part of this is the same as their light before the encounter. But now there is on each star a lake of fire, with an atmosphere of glowing gas above it. A new rotation has been given to each which struggles with the old. The fiery lakes may alternately be hidden from us or come into view. The two stars thus form for a time a pair of variables, and the lakes of fire and the -glowing atmospheres all tell their story in the spectrum. But the encounter may be more nearly direct. This modifies the future history chiefly in two ways. The attraction of the third body may prevent the'escape of the wounded stars and wed them into a binary. Or its mass, may be so great that many or most of its atoms fail to attain the critical velocity of escape. Then we find a very remarkable structure —a glowing rotating shell of gas with an extremely hot star in its centre. This is, in fact, a planetary nebula. The beautiful planetaries, of which wo "have shown, photographs, may be the result of terrific encounters in the past. The jNTovae pb^erved in historic tim.es, though.far the: ; most impressive outbursts-tliat are known; have not led to the formation •of such gigantic structures, but. those Novae which have been sufficiently observed, have Tea:ched-a planetary nebula stage. The Nova then loses its red colour, which was due chiefly to glowing hydrogen, and becomes greenish white. The green tint is due to^the same line which Sir William Huggins detected in 1864 in a small bright nebula, so ' confirming its gaseous character. The star in the centre, formed-chiefly ■ of-the heavier elements, is of the' class' O' or "Wolf■Eayet" type, which include the hottest known stars.,