260 



KNOWLEDGE 



[November 1, 1890. 



E. Leonis. Hind says : — " It is one of the most fiery- 

 looking variables on our list — fiery in every stage from 

 maximum to minimum, and is really a tine telescopic 

 object in a dark sky about the time of greatest briUiancy, 

 when its colour forms a striking contrast with the steady 

 white hght of the 6 mag. a little to the north." This latter 

 star is 19 Leonis. 



In the Southern Hemisphere there are some fine examples 

 of red stars. Epsilon Crucis, one of the stars of the 

 Southern Cross, is very red. Mu Muscse is described by 

 Dr. Gould as of " an intense orange-red." Delta- Gruis 

 is a very reddish star of about the 4th magnitude. Pi' 

 Gruis was observed by Gould as " deep crimson," and 

 forming a striking contrast with its neighbour. Pi- Gruis, 

 which he notes as " conspicuously white." The variable 

 Lj Puppis is described as " red in all its stages, and re- 

 markably so when faint." Miss Clarke observing (at the 

 Cape of Good Hope) H. Doradus, another southern variable, 

 says ; — " This extraordinary object strikes the eye with 

 the glare of a stormy sunset,'' '■' and with reference to 

 the variable R Sculptoris, described by Gould as "an 

 intense scarlet," she says : — " The star glows Uke a live 

 coal in the field," a description I have found very appUcable 

 to other small red stars. 



An 8th magnitude star about 5 degrees north of Beta 

 Pictoris is noted by Sir .John Herschel in his " Cape Ob- 

 servations " as " vi^-id sanguine red, like a blood drop. A 

 superb specimen of its class." With reference to a star 

 about 8h magnitude io the field with Beta Crucis, Herschel 

 says : — " The fullest and deepest maroon red ; the most 

 intense blood red of any star I have seen. It is like a 

 drop of blood when contrasted with the whiteness of 

 Beta Crucis." 



Of stars of other colours the asserted green tint of 

 Beta Ijibr* has already been referred to. Among the 

 brighter stars of the Southern hemisphere Theta Eridani, 

 Epsilon Pavonis, Nu Pujipis, and Gamma Tucanae are said 

 to be decidedly blue. The wonderful cluster surrounding 

 the star Kappa Crucis contains several bluish, greenish, 

 and red stars, and is described by Sir .John Herschel as 

 resembling " a superb piece of fancy jewellery." 



Among the double stars we find many examples of 

 coloured sims. Of these may be mentioned Epsilon 

 Bootis, of which the colours are " most beautiful yellow, 

 superb blue,'' according to Secchi ; Beta Cephei, " yellow 

 and violet " ; Beta Cygni, " golden yellow and smalt 

 blue " ; Gamma Delphini, of which I noted the colours in 

 1874 as " reddish yellow and greyish lilac " ; Alpha Her- 

 cuhs, " orange and emerald or bluish gi'een," and de- 

 scribed by Admiral Smyth as a " lovely object, one of the 

 finest in the heavens " ; Zeta Lyr;e, " pale yellow and 

 lilac " (Franks), and Beta Piscis Australis, of which I 

 observed the colours in India as white and reddish 

 lilac. 



Some distant telescopic companions to red stars have 

 been described as blue. This may in some cases be due, 

 partly at least, to the effect of contrast. In others, the 

 blue colour seems to be real. This has been shown spec- 

 troscopically to be the case with the bluish companion of 

 Beta Cygni. 



The physical cause of the difl'erence in the colour of 

 stars is still more or less a matter of mysterj-. Although 

 we cannot consider it proved that the red stars are cool- 

 ing and " dying out " suns, as has been suggested, we may, 

 I think, conclude that their temperature, although doubt- 

 less very high, must be lower than that of the white stars. 

 We know that a bar of iron when heated to redness is not 



* OOservatorj/, Dec. 1888. 



so hot as when raised to a " white " heat ; and although 

 the analogy between hot iron and stellar photospheres may 

 not be a perfect one, it seems probable that the higher the 

 temperature of a star the whiter will be its colour. Most 

 of the white stars, as Sirius, Vega, and those only yellow or 

 slightly coloured, show spectra of Secchi's first and second 

 type, whUe the great majority of the red stars exhibit 

 banded spectra of the 3rd and 4th types. 



To this rule there are, however, like other rules, some 

 notable exceptions. For instance, Aldebaran, Alpha 

 Hydrte, Xi Cygni, and 31 Orionis, although distinctly 

 reddish stars, show well-marked spectra of the 2nd or 

 solar type. On the other hand, Ebo Ursse Majoris and 

 Omega Virginis, which, according to Dimer, are only 

 slightly yellow, have well-marked spectra of the 3rd 

 type. 



An apparent change of colour seems in some cases to 

 be well established. The supposed red colour of Sirius in 

 ancient times is well known, but that such a remarkable 

 change of colour has really taken place in this now bril- 

 Uantly white star is very far from being established. It 

 seems more probable that the idea of change is due to the 

 mistranslation of a word applied to the star by some of 

 the ancient writers, a word which probably referred to its 

 brightness, not to its colour. A certainly estabUshed 

 change is, however, found in the case of the famous 

 variable star Algol, which is distinctly described as red by 

 Al-Sufi in the tenth century. It is now pm-e white, and 

 this is probably the best attested instance on record of 

 change of colour in a bright star. 



Schmidt's Nova Cygni of 1H7G was noted as " golden 

 yellow " on the night of its discovery. When it had faded 

 to about the 8th magnitude. Dr. Copeland called it 

 " decided red "; but when examined at Lord Crawford's 

 Observatory in September 1877, its colour was recorded as 

 " faint blue " ! The new star in the Andromeda nebula 

 was considered to be yellowish or reddish by most observers 

 when near its maximum ; but about a month later its 

 colour was noted as " bluish." Possibly, however, these 

 Xor(C may be really nebulte and not stars at all. 



Among the red and variable stars there are many 

 suspected cases of colour variation. Espin and other 

 observers have noted that the wonderful variable Mira 

 Ceti is much less red at maximum than at minimum. My 

 own observations confirm this. When at its maximum 

 brightness Mira does not seem to me a very bighly- 

 coloured star, while at one of the minima I noted it as 

 " fiery red." Possibly, however, the great ditierence between 

 its maximum and minimum brilliancy may have an influ- 

 ence on estimations of its colour. The remarkable 

 variable Chi Cygni is said to be " strikingly variable in 

 colour." Espin s observations in different years show it 

 " sometimes quite red, at others only pale orange red." 

 The star Birmingham 118 was described by Schjellerup in 

 1863 as " decided red," but was found yellow by Secchi in 

 1868, " bluish " by Birmingham, 1873-1876, " no longer 

 red" by Schjellerup in March 1876, and '-white" by 

 Franks in 1885. Birmingham 169 was found red by 

 Struve, blue or bluish white by Birmingbam in 1874, and 

 white at Greenwich in the same year. Espin also saw it 

 white in March 1888. The star Birmingham 30, which 

 Ues close to Phi Persii (54 Andromeda-), was described by 

 Schweizer as " etoile rouge presentant un petit disque " in 

 •Jan. 1843; Birmingham noted it "light red" in Decem- 

 ber 1875 ; Copeland " deep red ' in .Jan. 1876, and Dreyer 

 "reddish " in September 1878; but Espin, in November and 

 December 1887, found it " certainly not red, and nothing 

 pecuhar in the star's appearance." It might be expected 

 that these cmious changes of colour, if real, would be 



