OoTOBBit 1, 1891.] 



KNOWLEDGE. 



193 



scattered by fine particles of dust, which ai-e most probably 

 driven away from the sun (not by electrical action), but by 

 evaporation from their heated sun-lit sides. Every mole- 

 cule evaporated from a particle towards the sun must give 

 the remnant of the particle a kick backwards (see 

 IvNowLEDdE for November 16th, 1888), which would tend 

 to drive the unevaporated remnant away from the sun. It 

 seems very improbable that gaseous matter is driven away 

 from the sun, or the gaseous part of the corona and 

 prominences would be driven away, and the sun would be 

 continually diminishing in bulk. 



We do not need to assume altogether different conditions 

 to account for the disappearance of a lunar atmosphere. 

 Our own ocean of atmosphere is continually being drained 

 by the absorption of gas which is stored up in solid form, 

 while there are other sources from which the atmosphere 

 is continually being recruited. If, owing to some change, 

 such as a change of temperature, the supply of gas to the 

 atmosphere were to fall below the quantity which is con- 

 tinually being taken up and stored in solid form, the 

 amount of our ocean of atmosphere would be diminished, 

 and it might ultimately be drawn so low as to fall below 

 the amount of the lunar atmosphere by the action of 

 chemical changes such as are at present going on. — A. ('. 

 Hanyakii.I 



ON SOME PECULIARITIES OF THE VARIABLE 

 STARS. 



By J. E. Gore, F.R.A.S. 



THE long period Variable Stars have periods ranging 

 from 100 to over 700 days, and with fluctuations 

 of light from about one magnitude to over eight 

 magnitudes. Dividing these into groups, I iind 

 that, the maximum number is found among those 

 with periods of 275 to 375 days. Chandler finds that the 

 longer the period the redder the tint. According to 

 Chandler's estimates of their colours, the reddest of all are 

 — in order — R Leporis (period 43(5 days), V Cygni (461 

 days), S Cephei (484 days), R Sculptoris (207 days), and 

 V Hydrfe (575 days). Duner, however, makes the reddest 

 variables V Hydrte, S Aurigse, and V Cygni. 



The Variable Stars of short period include 10 stars with 

 periods of less than 30 days. Of these I find that four 

 have periods of under five days, eight have periods of 

 under eight days, three have periods of less than 11 days, 

 and three under 18 days. The maximum number is, 

 therefore, under eight days. The variation of light is 

 usually small. In but few cases does it much exceed one 

 magnitude, and in several it is less. In some, as in 

 (3 Lyne, ^ Geminorum, and ij Aquilre, all the changes may 

 be observed with the naked eye alone, while in others 

 an opera glass is necessary to follow their fluctuations. 



The great majority of these short period variables are 

 found in a zone which nearly follows the course of the 

 Milky Way. Another curious pecuharity connected with 

 their distribution is that most of thein lie — like the Tem- 

 porary Stars— in the full' mi >i<i semicircle, that is, between 

 12 h. and 21 h. of Right Ascension. The most remark- 

 able exception to this rule seems to be ? Geminorum, 

 which has a period of about 10 days 3| hours. All the 

 Variable Stars of this class with shorter periods tlian 

 ? Geminorum conform to tills rule, except S (15) Mono- 

 cerotis, of which the n<iHlar variability seems very doubtful. 

 It is not easy to conjecture the cause of this peculiarity of 

 position, for the Variable Stars of other classes are found 

 scattered indifi'erently over all parts of the celestial vault. 

 Of the Algol type variables the brightest are Algol, 



X Tauri, and S Librie. The others are much fainter, only 

 two being visible to the naked eye when at their normal 

 brightness. Chandler finds that " the shorter the period 

 of the star the higher the ratio which the time of oscilla- 

 tion bears to the entire period." Thus, in U Ophiuchi, 

 with a period of about 20 hours, the light changes occupy 

 five hours, or about one-fourth of the period, while in 

 J Cancri, of which the period is about 227^ hours, the 

 fluctuations of light occupy 21^ hours, or about one-tenth 

 of the period. All the Algol variables are white, or only 

 slightly tinted, and it would therefore seem to be hopeless 

 to look for variables of this class among the highly- 

 coloured stars. In all cases in which the stars have been 

 examined with the spectroscope the spectrum is found to 

 be of the first or Sirian type, another peculiarity worthy of 

 notice. The same remark applies to the stars which have 

 been found by the spectroscope to be close binaries, such 

 as X, Ursse Majoris, /3 Auriga;, and Spica. These have 

 spectra of the first type, and may be considered as Algol 

 variables in which the plane of the orbit does not pass 

 through the earth. 



If we assume that the apparent variation of the Algol 

 variables is due to the transit of a dark or nearly dark 

 satellite, we seem logically compelled to conclude that 

 these stars are not really variable at all in the true 

 meaning of the word. Their light is merely obscured at 

 minimum in the same way that the Sun's light is reduced 

 during a partial Solar eclipse. It is simply a case of 

 occultation of one star by another, and probably these so- 

 called variables might more correctly be classed among the 

 binary stars. If, like the Temporary Stars, we reject the 

 Algol variables, we have then only three classes of true 

 Variable Stars, viz. : — (1) stars with regular periods of 

 considerable length, (2) irregular variables, and (3) variables 

 of short period. 



With reference to the general distribution of the 'N'ariable 

 Stars, I plotted some years since all the knowTi variables 

 on one chart for each hemisphere, and I do not find any 

 irnj marked tendency to aggregation in any particular 

 region of the sky. A marked pnuriti/ of Variable Stars is, 

 however, noticeable in the northern hemisphere in the 

 constellations of the Lynx, Coma Berenices and Canis 

 Venatici, and in the southern constellations, Canis ^lajor, 

 Coluinba, Pictor, Eridanus, Fornax, Horologium, Grus. 

 Microscopium, Indus, Toucan, Hydrus and Octaus. 1 

 notice, however, a tendency to congregate in small 

 subordinate groups. Tlie most remarkable examples of 

 this clustering tendency are as follows : — In and near Corona 

 Borealis, where, in a comparatively small region there are 

 five Variable Stars ; near Cassiopeia's Chair, five ; in Cancer 

 four, comparatively close together ; a small region near rj 

 Argus, containing six ; and a limited area near tlie head of 

 Scorpio, which contains no less than 15 small variables. 

 I find that if the whole sky were as rich in variables aS 

 this last-named region there would be about 3000 Variable 

 Stars. The number hitherto discovered has not yet 

 reached 300. 



A remarkable peculiarity about the Variable and 

 Temporary Stars is, that few of them show any appreciable 

 parallax. For a Cassiopeia' and a Herculis a parallax of 

 less than a tenth of a second of arc has been found, but for 

 X Orionis Dr. I^lkin finds a negative parallax. These are 

 irregular variables. Observations of Nova Cygni (1870) 

 by Sir Robert Hall, failed to show a measurable parallax. 

 For the new star of 1885 in the Andromeda -Nebula, Franz 

 also found a negative parallax. A negative parallax 

 implies either that the parallax is too small to be measured, 

 or else that the faint comparison stars are actually nearer 

 to the earth than their brilliant neighbour. As far as I 



