Feb., 1904.] 



KNOWLEDGE i^- SCIENTIFIC NEWS. 



the distance of Aldebaran. Hence the real brightness of 



Antares will be -^ , or 21-5 times greater than that of 



1-159 

 Aldebaran. The surface of Antares would therefore be 

 21-5 multiplied by 92, or 197S times the surface of the 

 companion of a Centauri, and its mass about <SS,ooo times 

 the mass of the Sun — a truly giant orb ! 



Betelgeuse (a Orionis) has a similar spectrum to 

 Antares, but as it is brighter and its distance greater it 

 is probably larger still. 



Rigel ( Orionis). Assuming a parallax of o"-oi fovmd 

 by Sir David Gill, and comparing it with the hvi<:;htey 

 component of a Centauri, which is of nearly the same 

 apparent (or stellar) magnitude, we have, since the 

 parallax of a Centauri is o"'75, 



Light of Rigel = 75- = 5625 times light of the Sun 

 (which is probably the same as that of a, Centauri). But 

 the spectrum of Rigel shows that it is hotter and brighter 

 than our Sun. The two bodies are therefore not exactly 

 comparable, and we must make an allowance for their 

 difference in intrinsic brightness. If we assume that the 

 Sun's light is reduced by absorption in its gaseous sur- 

 roundings to one-fourth of its real light — which is 

 probably a liberal allowance — we have, 



Surface of Rigel = -' — - = 1406 times surface of Sun. 

 4 

 From this it would follow that the volume of Rigel is 

 about 52,000 times that of the Sun. Rigel is, however, 

 probably of less density than our Sun, owing to its higher 

 temperature. Comparing it with Algol, which has a 

 similar spectrum, and of which the density and mass are 

 known, we have the surprising result that the mass of 

 Rigel is about 20,000 times the mass of the Sun ! The 

 parallax of Rigel is, of course, somewhat doubtful, but 

 Sir David Gill is confident that it does not exceed the 

 small quantity above stated. 



For ^ Centauri, Gill found a parallax of o"-046. Placed 

 at the distance indicated, the Sun would shine as a star of 

 about 6-75 magnitude, and as the photometric magnitude 

 of the star is o-86, we have a difference of 5-89 magnitude, 

 which would make jS Centauri 227 times brighter than 

 the Sun. This gives a volume 3420 times the Sun's 

 volume, and assuming the density at one fourth of the 

 Sun's, we obtam a mass for ^ Centauri equal to 855 times 

 the Sun's mass ! 



Crucis is of almost exactly the same brightness as 

 Aldebaran, but it is at double the distance from us, a 

 parallax of only o"-05 having been found by Gill. Its 

 spectrum (of the ■' Orion type") indicates, however, that 

 it is a much hotter and brighter body than Aldebaran. 

 Taking its greater distance into account, we may perhaps 

 conclude that it is comparable in size with Aldebaran, 

 and therefore a sun of great size. The star P Crucis, 

 whose stellar magnitude is i'50, but which has no measur- 

 able parallax, must also be a giant sun. Its spectrum is 

 the same as that of « Crucis. 



Arcturus and Pollux have similarspectra (K, Pickering). 

 The photometric magnitude of Arcturus is 0-24 and that 

 of Pollux I-2I. The parallax of Arcturus, as found at 

 Yale Observatory, is o"-o26, and that of Pollux o"-05r). 

 From these data it would follow that Arcturus is 

 11^ times brighter than Pollux. The Sun placed at the 

 distance of Arcturus would shine as a star of about the 

 eighth magnitude, or about 7-7 magnitudes fainter than 

 Arcturus appears to us. This would imply that Arcturus 

 is about 1200 times brighter than the Sun. It must there- 

 fore be a sun of gigantic size — probably one of the largest 

 bodies in the universe. The above calculation would 

 make Pollux about 100 times brighter than the Sun. 



The bright stars Canopus and Procyon have \ery 

 similar spectra, but the parallax of Canopus does not 

 exceed o"'Oi, while tliat of Procyon is about o"'32. Still 

 Canopus is a brighter star, its photometric magnitude being 

 — 0'86, while that of Procyon is + 0-48, a difiercnce of 

 I '34 magnitudes in favour of Canopus. From these 

 data 1 find that Canopus is 3500 times brighter than 

 Procyon, and it follows tliat its volume is 207,000 times 

 the volume of Procyon ! If the densities are the same, 

 the masses will be in this ratio, and as the mass of 

 Procyon, as computed from the orbit of its satellite, is 

 about five times the mass of the Sun, we have the mass 

 of Canopus more than that of a million of suns ! This 

 is probably the largest sun of which we know anything. 

 Sir David Gill's observations show that the parallax of 

 Canopus does not exceed the hundredth of a second as 

 above stated. A smaller parallax would, of course, 

 further increase its size. 



The observations of "spectroscopic binary stars" en- 

 able us to determine their mass although their distance 

 from us may remain unknown. As their actual orbital 

 velocity can be measured with the spectroscope in miles 

 per second, their distance from the earth is a matter of 

 no importance in the computation of their mass. One of 

 the most remarkable of these interesting objects is the 

 southern variable star known as V Puppis. It is a 

 variable of the Algol type, and also a spectroscopic 

 binary. The plane of the orbit must therefore neces- 

 sarily pass through the earth, or nearly so, and the mass 

 of the system can be easily computed. The spectro- 

 scopic observations show the enormous relative velocity 

 of 380 miles a second ! and indicate a mass equal to about 

 70 times the mass of the Sun. The variation of the star's 

 light shows, according to Dr. A. W. Roberts, that the 

 component stars revolve round each other in actual con- 

 tact, or nearly so, and that their mean density cannot 

 exceed i-5oth of the Sun's density, or about 0-028 that of 

 water. With such a small density and so large a mass 

 the components must evidently be greatly expanded 

 masses of gas, probably several millions of miles in 

 diameter. The period of revolution is about 34 hours 

 54 minutes, a wonderfully short period for a pair of 

 suns ! 



Let us now consider some suns of probably miniature 

 size. The star Lalande 21,185 (7'5 magnitude) in the 

 constellation Ursa Major has a parallax of about o"-47. 

 At the distance indicated by this comparatively large 

 parallax, the Sun would shine as a star of about 17 mag- 

 nitude, or over 200 times brighter than Lalande's star. 

 iVnother small star in the same constellation, Lalande 

 21,258 (8-5 magnitude), has a parallax of o"'24. This 

 distance would reduce the Sun to about 3-2 magnitude, 

 but it would still be 5-3 magnitudes, or over 130 times 

 brighter than the star. 



The small star Argelander-Oeltzen 17,415 of the 9th 

 magnitude has a parallax of o"-25. The Sun, if placed 

 in the same position, would be over 200 times brighter 

 than the star. 



Another small star with a comparatively large parallax 

 is Lacaille 9352. Its magnitude is 7-1, and the parallax 

 about o"-29. The Sun, if placed at the distance indicated 

 by this parallax, would shine as a star of about 27 mag- 

 nitude. This gives a difference of 4-4 magnitudes, and 

 implies that the Sun is over 50 times brighter than the 

 star. This star has the very large proper motion of 7" per 

 annum. It is a remarkable fact that the faint stars above 

 mentioned are actually nearer to the earth than Aldebaran, 

 which is one of the brightest stars in the sky. 



The famous double star 61 Cygni is also probably of 

 ! small mass. Taking its parallax at o"-39, the Sun, if 



