IN STARRY SKIES

Evening Post, Volume CIV, Issue 68, 17 September 1927, Page 29

IN STARRY SKIES

(By "Ome^a Centai-ri.")

WHAT IS A NEW STAR?

THE IMAGINARY DISCUSSION

Dingle.— ln spite of all you have said I uphold Seeliger's theor^nf the origin of Novae. The hypothesis that -the entry of a dark star into a nebula is the cause of the outburst leads to a sufficiently plausible explanation of the appearance of a -Nova. But what appeals to me most is that this theory is not called upon to bring millions of supposititious bodies into existence. The vast extent of a nebula makes the chance of itseneounter with a star incalculably greater than the chance that two dark stars should collide.

Bickerton.—But whilst you shrink from imagining a sufficient number of dark stars, are you not making still greater demands on our credulity? I see you have modified the original theory. Former advocates explained the suddenness of the rise to maximum, brightness by supposing that the star passed completely through the nebula in that brief : time. Any subsequent brightenings were attributed to its meeting scattered wisps of nebulosity. This necessitated the supposition that every nebula, so encountered by a star, was of stellar thickness only, though laterally of vast nebular extent. Every nebula, according to this theory, must be shaped like an enormous sheet of extremely thin paper, and every star, destined to act. as a Nova, must plunge through it at right angles to the surface, or very nearly so, in order to traverse it sufficiently fast. The minor brightenings'always occur after the main outburst-, so the nobTilous wisps must always arrange themselves behind the.main nebula when a star approaches. The absurdity of such an idea is manifest. In avoiding this horn of the dilemma, do you not find yourself impaled on a sharper one? Instead of allowing Athe star to pass through, the nebula, yon call upon the nebula to stop the star. To be able to do this the nebula would have to be millions of times denser than is usually considered possible, and if the star were stopped the heat generated would be sufficient to keep' it shining for thousands of millions of years. And is the existence of dark stars supposititious? Eclipsing variables proclaim that some dark stars exist, whilst the chance that a dark star chosen at random may be revealed in. this way is infinitesimal. So* is also the chance that its presence may bo proved by its gravitational effect, as in the easo of the companion of Sirius. Again M stars are believed to'fado into invisibility ..at the end of their dwarf stage. As far as we know, they have been doing so during infinite ages of past time. "Where are these stars now? Surely those that have not been revivified must be dark stars still. The number of these dark stars should boar to the number of luminous ones the same ratio as the average length of the dark period bears to that of the luminous one. The rarer the process of revivification the more numerous must the dark stars bo. ■ . ■ i

D.—But even if dark- stars are as numerous as you think, each ■' one is small, whilst a nebula is immense. It is easier to hit a large target than to fina the centre of the bull's-eye. To mo the explanation of a Nova which Soeliger's theory gives is natural and complete in all its details. B. —And to me, instead of being natural and complete, the explanation appears contrary to the laws of Nature. How could a massive body like a star, endowed with terrific energy, be brought to rest in a day or two by a shoet of tenuous nebulosity. If so stopped, a miracle would be required to keep tho heat on the surface only. Further, if once heated to such vivid incandescence, as the destruction of a velocity of twenty miles a second must produco, how could it cool in less than thous-ands-of millions of years? According to your theory, its gravitating powor is unimpaired, and so it is unable to fade through dissipation into space in tho way-that a Third Body does.

D. —But we have still to apply tlio supremo test, that afforded.by the spectrum. . Here any theory must meet the greatest difficulties of all. I cannot say that the success of the hypothesis I uphold is altogether complete. But I affirm that the explanation which it offers is the best that has yet been given. Supposing a star to rush into a nebula—conceived in. the absence of definite knowledge, as a cloud of very fine particles embedded in a mass of gas —we may imagine the following series of events to occur. The original sudden heating of the stellar surface would naturally yield a continuoui spectrum, for although a large part of the surface would probably bo vaporised, the vapour would at first be very dense. The extreme brightness attained by the star would repel the small particle! of the nebula, which, b"oth by radiation from the star and by the friction arising from their rapid motion, would soon become heated enough to emit, and therefore to absorb light of their own. The rotreating nebula would therefore ho the expanding shell of gas. In this way we got the first absorption spectrum, gradually increasing in strength, and later, when the particles have rotroated far enough for their light to bo perceived independently, the broad, bright lines would appear.

B.— You ask us to believe that a star can bo stopped by a resistance which heats its surface only. This in itself appears absolutely impossible, owing to the equivalence of heat and work. Every particle of tho star's mass had its own kinetic energy, and that has been turned into thermal agitation. Then you say that as soon as tho surface is brilliantly heated the nebula is driven away by radiation pressure. How can it then make any further opposition to the motion of the star? Again, as long as the particles you speak of arc solid they can only obstruct the light generally, and cannot exercise any selective absorption. Let us compare the definiteness of tho partial impact theory with the vague surmises of your one. If two stars liko our sun are drawn together by gravitation we can say definitely with what velocity they collide. It is easy to : .ake allowance for possible original proper motions. We therefore know the initial kinetic energy per unit mass, or, as I prefer to call it, tho initial kinetol. Knowing this, we can predict how the third body will behave, and can point out how this behaviour will vary for different depths of graze. The process of equalisation of temperature, leading to selective molecular escape, must ensure that later emissions of light gases have higher velocities than earlier ones. The energy liberated in a grazing collision is so tremendous that ionisation is much more readily explained than by any mere heating of the surface.

D.—l acknowledge that it is hardly possible to predict what typo of spectrum the retreating nebulae would-give. Considering the extremely low pressuresthat must exist in nebulae, however, it is not a matter of surprise that we obtain enhanced lines.

B.—That is a .delightful admission You ascribo absolutely contradictory properties to your nebula. It has to exert such tremendous pressure as to stop a star as suddenly as if it had stellar density itself, and at the samp time the pressure has to Tjo so ex-

tremcly low that enhanced- linec are to be expected in its spectrum.

D.—But notice how Seeliger's theory explains the backward drift of the first absorption lines. During the onwar/1 motion of the nebula, radiation pressure on it muat decrease fairly rapidly on account of its increasing distance from the luminous star and of the waning brightness of the star itself.' The particles, therefore, subject also to the resistance of the more outlying nebular regions into which they are moving, must lose speed. So, of course, we find th'a first absorption spactrum moving slowly towards the red until it is lost in the broad, bright lines of the second spectrum. . . ! B.—l much prefer the explanation' I have given of this backward drift, but let me hear how yon explain the second spectrum. You say the star is now on the wane. It must therefore either have passed right through the nebula or have been stopped by it. The former is incredible if the nebula is of more than stellar dimensions. The latter implies that the whole mass is heated so highly that it must shine for ages as a permanent star. D.—lt is hardly possible to predict in detail what will happen, to a huge body- in an unknown state when, its surface is heated to an enormous temperature with extreme suddenness. Something must happen, however, and it is as likely a hypothesis as any other that the star will expand rapidly outwards. . ; i B. —No! Surely that is most unlikely. If the centre of the etar were heated, it would naturally tend to press the rest of the body outwards to make room for its expansion. But if the surface of a body only is heated there seems no reason whatsoever, for the expansion of any pait but the material of the heated surface itself. - ' D—But this assumption of rapid expansion is in one form or another, a part of erery existing explanation of the Nova spectrum. B-^-On the partial impact theory such an outrush is inevitable. In this one it only occurs because something must happen. In a third body. all elements are at first forced outwards together. Those that form the initial atmosphere give the first absorption spectrum. Later the light elements take energy from the heavier. Their velocities thus increase until they are so far above the critical velocity of escape that gravitation can produce no noticeable diminution in their speed. D—l do not attribute the first spec: trum to the star at all. It is due to the outrush of the nebula. The expanding shell of gas gives the second absorption and emission spectrum, which follows the same general course as the first in the circumstances of its appearance. B—But if the two spectra are utterly dissimilar in origin why do they follow the same general course? D: —It seems inevitable that the bombardment by the nebular particles must destroy the surface of discontinuity between the interior and the atmosphere of the star. The boundary could scarcely be in any but an extremely unstable state, and it is dimcult to imagine what it would do but expand outwards. B —This is all too vague. What is this surface of discontinuity that is destroyed? If tho star is entirely gaseous there is no such surface. If it is solid there will be discontinuity until all is vapourised. In any -case only that which is heated will expand, and even it only to tho extent allowed by gravitation. Tho expanding 'shell cannot drag the cold interior after it. D—But the outrush actually occurs. We cannot, indeed, predict* what spectrum should appear, but we can say that in all probability it will be of an earlier typo than the first. The temperature of the actual stellar. gases must bo considerably higher than that of a nebula heated by radiation and friction, and further the gases will expand into the almost perfect vacuum left by the flying nebula, so that their pressure will rapidly fall to a very low value. As a matter of fact, we get a B type spectrum—a stage earlier than the original one of type A. B—l am glad that you acknowledge that thi star-nebula theory cannot predict tho type of spectrum. Tho partial impact thoory can. But what an extraordinary thing your nebula must be, if it can stop a star in two days and not become heated in doing so. You say it is heated only by radiation and friction. How, again, can the nobula stop tho star if it retreats at high but diminishing speed, as soon as the stellar surface is heated, leaving as you say an almost perfect vacuum behind it. As long as radiation pressurell acts, if as it seems, you disregard gravitation altogether, the motion should bo accelerated, not retarded. Gravitation, however, must not be neglected. You remind me of the old problems about a fly of mass in crawling np the trunk of an elephant of negliijible weight. If tho star were like our sun. its critical velocity of escape would be about 400 miles per second. If U« velocity, says 20 miles second, were destroyed, the energy of atomic agitation so produced would be only onefour hundredth part of that necessary to enable the surface layer to escape. The expansion must therefore be very slight, and the inital displacement of the absorption lines should, on your hypothesis, quickly disappear. This is exactly the opposite of. what really happons* . *