December 23, 1922] 



NA TURE 



847 



Emission of Cathode and X-rays by Celestial Bodies. 1 



Bv Dr. Henri Deslandres. 



THE emission by the stars of X- and cathode rays 

 and similar radiations has already been con- 

 sidered and investigated by various writers. 2 The two 

 kinds of radiation, however — X- and cathodic — are 

 not separable, for each, when it meets an obstacle, 

 gives rise to the other, this interdependence having been 

 clearly pointed out by de Broglie. But their properties 

 are different : X-rays move in straight lines, and are 

 much the more penetrating, while cathode rays are 

 easilv deflected into helical paths by a magnetic field — 

 or, again, by an electric field. The paths of the elec- 

 trified particles forming cathode rays, under the in- 

 fluence of a magnetic field like that of the earth, have 

 been revealed by the detailed calculations of Stormer : 

 they are very interesting, and much more varied than 

 the trajectories due to gravitation. 



I. In several notes, from 1896 to 1922, 3 I have 

 suggested the emission of cathode and X-rays by the 

 sun, and also by the nuclei of nebulre. The rays of 

 the solar corona can thus be explained, and also the 

 aurora borealis and the magnetic disturbances of the 

 earth, their connexion with sunspots, and even the lag 

 of these disturbances behind the passage of a spot 

 across the central meridian of the sun's disc — a lag 

 due to the deviation imposed by the outer solar mag- 

 netic field. The same idea was put forward also in 

 1896 by Birkeland. who carried the investigation 

 further : he was able to reproduce, in the laboratory, 

 some of the phenomena of the aurora borealis by means 

 of a small sphere placed in a vacuum, magnetised like 

 the earth, and bombarded by cathode rays. Later, 

 the researches, both theoretical and experimental, of 

 Stormer made a great advance in the investigation, 

 and placed almost completely beyond doubt the 

 emission by the sun of ordinary cathode rays. In 

 terms of these rays. Stormer explains the smallest de- 

 tails of the aurora borealis, so rich in singular pheno- 

 mena. He has even been able to locate the origin of 

 the rays in the sun, and to determine the value of the 

 external solar magnetic field. This value, which is very 

 small and equal to 10 ~ 7 gauss, is exactly that which 

 I found in 191 1 by another method depending on the 

 radial velocities of the solar prominences recorded at 

 Meudon. 



The earth also emits these special radiations. The 

 radio-active bodies in its solid crust and in its atmo- 

 sphere emit a-, /?-, and y-rays, which ionise the atmo- 

 spheric gases and explain partly the permanence of the 

 terrestrial electric field. To explain the whole field, it 

 must be assumed that there enter, from the outside, 

 rays which are very penetrating — even more penetrat- 

 ing than any known X-rays. Further, if one ascends 

 in the atmosphere — as did Kohlhorster, who reached a 

 height 'if uooo metres — the number of ions formed per 



1 Translation of a paper read before the Paris Academy of Sciences on 

 October 2, 1922. 



- X-rays are constituted like the -y-rays of radium, except that the latter 

 have a greater frequency. The ultra X-rays, discussed in this note, have a 

 still .greater frequency. In the same way, 0-rays resemble cathode rays. 

 As for a- (positive or anode) rays, which play an important part in ionisa- 

 tion in general, they are absorbed very quickly, and move only a short 

 distance from their origin. 



3 Comptes renins, iz6, p. 1323, 189S ; 134, pp. ri34 and T486, 1902 ; 

 150, P- 65, 191°; '52, P- 1453, 1911; 155, p. 1573, 1912 ; 157, P- 517, 

 1913 ; 171, p. 45i. 1920; 172, pp. 405 and 709, 1921 ; 175, p. 121, 1922. 

 See also " Observations de l'Eclipse totale de 1893 " (Gauthier-Villars, r8g9). 



second in a closed chamber is found to increase rapidly ; 

 at 9000 metres it is eight times as great as at the surface 

 of the earth. The amount of this penetrating radia- 

 tion therefore increases rapidly with altitude. It pro- 

 ceeds probably from the sun, directly or indirectly, or 

 even from cosmic space, 4 but its exact origin has yet to 

 be determined. 



Such are the first results ; they are extremely in- 

 teresting, but still very incomplete. The investigation 

 thus begun should be pursued with every means at our 

 disposal. 



1 1 . Researches connected with the atmospheres of 

 yellow stars, carried on at Meudon during 1922 with 

 Burson, have led me to conclude that in these stars' 

 there is an extremely penetrating X-radiation, emitted 

 by the interior strata or the nuclei of the stars. These 

 results, which have been stated very briefly in former 

 communications, 5 are now given in detail. 



The sun, which is a yellow dwarf star, shows, as is 

 known, in its integrated spectrum, 6 three groups of 

 calcium lines — H,, K, ; IT 2 , K 2 ; H 3 , K 3 — weak, but 

 very distinct — which represent, respectively, the lower, 

 middle, and upper strata of its gaseous atmosphere or 

 chromosphere. Burson and I have discovered these 

 lines — in particular, the lines H 2 . K 2 . IT.,. K 3 in several 

 giant stars which are equally yellow. They have the 

 peculiarity that the lines, when compared with the 

 neighbouring continuous spectrum, are stronger and 

 wider than in the spectrum of the sun. The middle 

 and upper strata of the chromosphere are more lumin- 

 ous and important than the corresponding solar strata. 



Stars of the two types, giant and dwarf, have at the 

 surface, however, the same chemical composition, the 

 same temperature, and the same surface brightness. 

 How is the difference in the luminosities of their atmo- 

 spheres to be explained ? 



The atmospheric strata are represented l>v the radia- 

 tions IT and K, which, as is now known with certainty, 

 are emitted by the ionised atom of calcium. If, there- 

 fore, we consider, in each type of star, a tube normal 

 to the surface, having unit cross-section and extending 

 from the surface to the outer limits of the atmosphere, 

 the brightness of each stratum in the tube will be 

 proportional, or at least closely related, to the number 

 of ions formed in it per second. The number of ions 

 formed must therefore be greater in the giant stars. 

 Now one of the principal causes of ionisation already 

 pointed out is the intense emission of electrons by the 



* The earth also probably emits a very penetrating X-radiation — more 

 penetrating than the ^-rays of radium : this has been suggested by some 

 writers. But, if it exists, it is relatively weak, and ii has not yet been 

 clearly separated from the very penetrating X-radiation coming from 

 external sources. 



5 Sur la reconnaissance dans les etoiles des couches successives de leur 

 atmosphere et des variations periodiques de ces couches {Comptes renins, 

 171, p. 451, 1920, by Deslandres). Recherches sur 1'atmosphere des etoiles, 

 Reconnaissance d'etoiles qui ont les memes brillantes de 1'atmosphere que 

 le soleil {Comptes renins, 172, p. 405, 1921) ; Recherches sur 1'atmosphere 

 des etoiles, Reconnaissance de la couche superieure dans quelques etoiles 

 et comparaison avec le soieil [Comptes renins, 172, p. 479, 1921); Recherches 

 sur 1'atmosphere des etoiles, Proprieties des etoiles qui ont les memes radia- 

 tion, et les memes couches de la chromosphere que le soleil {Comptes rendus, 

 175, p. I2r, 1922, by Deslandres and Burson). 



Burson and I intend to publish shortly some new results. In particular, 

 we have discovered that, in certain giant stars, the lines H 3 , K 3 of the 

 upper stratum are displaced towards the red, and the lines H 2 , K 2 of the 

 middle stratum are displaced towards the violet, as in the case of the sun. 



6 The integrated spectrum is that which the sun would give if it were 

 as far from us as the stars. 



NO. 2773, VOL. IIO] 



