IN STARRY SKIES.

Evening Post, Volume CIV, Issue 74, 24 September 1927, Page 28

IN STARRY SKIES.

WHAT IS A NEW STAR? DINGLE V. BICKERTON THE DISCUSSION ENDS : (By "Omega Centauri.") Dingle.—l acknowledge that Seeli- | ger's theory has not yet been quite successful in explaining the spectrum of a nova, but progress; is being made. With regard to the velocity of the second outrush, although its actual magnitude is beyond' calculation, ive can say that it must be'greater than that of the nebular particles. The new outflow encounters no resisting medium. Tho atoms producing the observed lines are lighter than those of the nebula. The radiation pressure at equal distances ia also greater in this second outrush, when the radiation is more intense.' There may also be electrical repulsion of the ionised atoms, of the kind imagined by Fowler to operate in the solar chromosphere. All these things added to the original impetus responible for the outward rush itself could hardly fail to give the second outburst a' higher velocity than the first. It is found, in ;fact, to be about twice as great, if the'displacements of the spectrum lines are due entirely to Doppler effects. ■-,■■

Bickerton.- —The velocity of the outward rush, from the third body due to a partial impact,..can.be calculated, and the results appear to agree with observed facts. Though you are unable to calculate.. the - velocity on. the star nebula hypothesis, it is quite possible for you to set an upper limit, which, however, does not agree with observation at all. Again, is it true, as you say; that the atoms producing the second set. of lines are lighter than those of the nebula? The successive expanding shells from a third body show diminishing atomic weights, on account of atom sorting leading to selective molecular escape. ■ But how do you det rmine the atomic weights of the nebula? The new spectrum^ you tell us in your book, appears two or three days after maximum; now you attribute the greater speed of the new outrush partly to more intense radiation pressure. W. H. Wright, in the journal of the Astronomical Society of the Pacific, makes the maximum of a nova thirteen magnitudes brighter than the sun. Its radiation is therefore one hundred and fifty thousand times as intense. Could this be due to mere surface heating? Every new star, as the result of its outburst, generates a .nebular which rapidly expands, and becomes the source of the bright emission bands. In the special case of Nova Perses there is evidence also of a pre-existing nebula. But there is no indication that the outburst even in that case was due to the nebula. In the case of Nova Aquilae the nebular disc, due to the outburst,' became visible in sis .months' time. The six stages enumerated by Wright are all suggestive of partial impact. 1. From birth to maximum the spectrum is continuous with a few faint and diffuse emission and absorption lines. This is the first outrush in which all elements take part. 2. The Alpha Cygni or enhanced metallic lino stage. The ionised atoms, which are helped in their cscapo by electrical repulsion, show their presence by characteristic lines in the spectrum. 3. The Orion stage marked, by helium, nitrogen, and oxygen. Atom sorting has begun, and these are exactly the elements to •be expected to show. prominently. 4. A stage characterised by. enhanced nitrogen. 5. The nebular stage. The gaseous shell is now becoming extremely tenuous. (6) .The Wolf-Eayet stage marked by the fading of some of the nebular lines., Tho light of the expanding shell fades ; owing to the infroquency of atomic encounters. The radiation from tho Wolf-Rayet star, formed of the heavier materials which have becif^unable to oscapo, then becomes noticeable in the fading light. But how do you account for the spectrum in the later stages? D.—What should happen after tho second outrush is pure conjecture. Tho star is getting fainter and tho outgoing nebular and stellar gases must be. getting cooler. If matter continues to.be poured out from the star, as observations seem to indicate, tho maintenance of the second absorption spectrum is naturaliy explained. .B. —-But why should matter continue to bo poured, out if, as you say, tho action between the star and tho nebula is over, and if it has resulted in the hoating of the surf ace only of tho star? If the star is getting fainter, and if the outgoing nebula and gases are getting continually cooler, how can the velocity of the outrush be maint'ainedf Surely, according to your theory, the displacement of the spectral lines should, at this stage, be diminishing rapidly. Observation shows on the other hand that these lines retain their, full width until they fade away. "Why ere you satisfied with these unsupported conjectures and unsuccessful hypotheses when, an explanationl is available which really meets the facts? Should you not first try whether you can find any flaw in this suggested explanation?

D.—But you must acknowledge that the apparent connection between Novae and Planetary Nebulae is highly Rignificant. After the first set, of absorption and emission lines, has faded away the spectrum of a Nova begins to show, nebular characteristics. It is hard to, avoid the suggestion that tho material source of the two spectra is the same, and that the difference in the appearance of the spectra arises from the_ difference between temperature radiation arid fluorescent or other radiation. If this is so, it must have a very important bearing on the nature of nebulae and the nebular radiations.

B.^—The connection between Novae and Planetary nebulae strongly supports the partial: impact theory. The latter suggests the origin of the central star and'of the expanding and rotating gaseous-shells. It predicted the shell within'::shell structure which is clearly shown in the shtless spectrograph o£ the Ring Nebula in ' Lyra taken by Dr. W. H. Wright at. Lick. It, explains the structureof .the lines in spectra of planetary nebular when photographed with a slit spectroscope. The opposite twists at the ends' of the lines are due to; rotation, the doubling in the centre is due to the hollowness of the .expanding shell. On your theory the concentric shells thus indicated are inexplicable. The vast nebula is supposed by you to have been there before. There is nothing to produce the definite forms which are so characteristic of these bodies. A star heated on its surface only would soon cool,, and could not continue to illuminate the gaseous shells even if they did exist. Even a slight stellar graze cannot lead to a permanent planetary nebula. The Third Body, in such a case, is quickly dissipated ; through ; apace. Its planetary nebular, stage is evanescent. But a deep graze of massive stars may produce a Third Body which has not sufficient energy to scatter its materials completely. The very lightest elements may leave it altogether. The heaviest may fall back to form the central "WolfEayet star, and the: intermediate ones may constitute the hollow structure, built of shell within shell. A still more massive Third Body may be able to retain even Hydrogen and Helium, which are seen : to form some of the shells in the Bing Nebula in Lyra. P.-— The disappearance of the nebular

lines and the development of "WolfBayet bands in the later spectrum of a Nova appears to me to be-associated with the weakening of the stellar light which is then no longer able to induce the peculiar glow in the surrounding Nebula. Why the star should assume Wolf-Eayet characteristics is inexplicable, but then Wolf-Rayet-radiation is inexplicable itself. But when a star, through encountering a nebula, loses its surface of discontinuity, I should expect a spectrum similar to that of a Wolf-Eayet star —namely, one with broad, bright radiations on a fainter continuous background. The nucleus of a planetary nebula is always a WolfKayet star, and whenever we find a Wolf-Kayet star the chances are that it will be involved in nebulosity. Wolf-Rayet stars and nebulae go together, and when we find, in a Nova, a nebular spectrum succeeded by a WolfEayet spectrum, we must, believe that a star and a nebula are both involved. In the case of Nova Persei there was evidence of this, almost amounting to proof. • B.—Stars • are indeed quite often involved in nebulae, and so a stellar collision may occur, within a nebula. In. the case of Nova . Persei apparently it did so. But even in this case a nebulous disc, produced by the collision, became visible long after the illuminating flash had disappeared. This was quite distinct from the original nebula. •■'.'., . - D.—l agree that both in the case of Nova Persei and that, of Nova Aquilae No. 3, the growing discs,, which appeared months after maximum, and which continued to enlarge for years, denoted a rapid actual outward movement of luminous matter. It is about the cause of the outrusn that we fail to agree. B. —But I. have stated exactly the points in which I consider your theory fails. You reject mine without giving your reasons. If you. can advance any arguments which tell as severely against the partial impact theory as those I have urged tell against 1 yours, I shall be forced to abandon it. Till than I-shall believe mine to be the most fruitful working hypothesis yet advanced to deal, with the problem.

D. —There are some stars which show strange exceptions to the ordinary phenomena of Novae. Will they fit into your theory?

B.—Collisions may occur between celestial bodies or systems of any kind. I do not suggest; for example, that a star never collides with a nebula. What I say is that the resulting changes in such a case are too slow for us to observe. The bodies or systems, when they meet may be in any stage of their respective life histories. The encounters may be of any type from- a distant passing to a head-on collision. There is thus plenty of opportunity for variation, and for the development of individual peculiarities. We notice typical Novae, because their changes are so rapid, and the rapidity of the changes suggests that they are due to encounters of comparatively dense bodies, moving at great speeds. A direct collision of such bodies would lead to great and sudden development of heat, but it could not produce the typical transient flash. The peculiar properties of the Third Body in a grazing collision provide the key to the solution of the problem.

D.—All . Typical Novae, except T. Coronae, whose parallaxes are known, were dwarfs before the outburst. How do you account for that?

B.—lt implies that a star usually has a long timo. to: wait for each revivification, so that it will generally have reached a late stage in its history .before it is reborn: It also means that the stars, whose clash produces a Nova, arc usually comparatively dense. This is an essential condition for a Very great and sudden change of luminosity. Collisions of giant stars are not nearly so spectacular. They meet when centres are comparatively far apart, and their velocities correspondingly 6low. One type of encounter which produces a rather long period variable is the plunge of a dense body into a giant.

D.—lt has been; estimated by Bailey that more than nine novae occur per year, brighter than the 9th apparent magnitude. At this rate in a few million years there should bo as many old Novae as they are stars in the sky. Lundmark suggests that, in the future, Novae may come to be regarded, not as accidents, but as representatives Of a ; stage through which-the majority of stars must pass. This hypothesis involves too many gratuitous assumptions. I cannot reconcilo it with the known facts., of stellar spectra. These indicate a courso of development in which there is no room for the catastrophic changes associated with tho career of a nova. ■■

B.—lam glad to find at last a point on which we can agree. We both believe a-.typical nova to be something quite different from an. ordinary star. A nova, in my opinion, never develops into an. ordinary star; a star, on the I other hand, in a fiery clash with another, may strike off a typical nova. A nova has far too little mass associated with far too superabundant a supply of energy to pose successfully as a star for more than a very, brief period. A star, though long-enduring, is far too massive to rival the intense activity of a nova. But why say more? Each of us has had his say. It would clearly be useless to prolong the discussion any further. The future must bo left to judge between us. For the.present, there is no doubt, your theory has the greater number of adherents. I feel, however, that mine makes up for this in thegrea,ter strength of the arguments it can adduce. . D.—We, can but seek the truth and leave its fuller disclosure to tho future. . .