55 ] Physical Data 57 
Capella (a Aurigae), the second brightest star in the northern sky, is a 
binary system which has the rare distinction of being both a visual and 
a spectroscopic binary. Its parallax is 0'063" and its period 104 days. 
The brighter component is of visual magnitude 0 * 8 , its spectral type 
being GO, the same as that of the sun. From the shape of its energy curves 
Sampson and Abbott estimate its effective temperature as 5500 and 5800 
respectively. Its absolute magnitude, both visual and bolometric, is — 02, 
its luminosity and emission of radiation being about 105 times that of the sun. 
With an effective temperature of 5650, its surface must be 120 times that 
of the sun, and its diameter 11 times that of the sun, or say million miles. 
Its mass is 418 times the mass of the sun, whence its mean density must 
be 0-004. 
The fainter component is of spectral type F 0, representing an effective 
temperature of about 7400. Its visual magnitude is 1*1, so that its absolute 
magnitude, both visual and bolometric, is 0*1. Its radius is about 5| times 
that of the sun. Its mass being 332 times that of the sun, its mean density 
is 0 028. 
B.D. 6 C 1309 (Plaskett’s star)*. This is a spectroscopic binary with a 
period of 14-414 days, which is the most massive and the absolutely brightest 
star whose elements are,known with fair certainty*!*. The minimum values 
possible for the masses are 75‘6 and 63'2 times that of the sun. These are not 
computed masses, but values of M sinH’, where i is the inclination of the orbit, 
so that the actual masses are probably considerably greater. The spectral 
type is 0 8 , which on the theoretical scale of Fowler and Milne, would repre 
sent a temperature of about 28,000 degrees. 
H.D. 1337 (Pearce’s star)^;. This is a spectroscopic and eclipsing variable 
of period 3-5234 days, which has the distinction of being the most massive 
and the absolutely brightest star whose mass and luminosity are known with 
precision. The masses of the two components are found to be 36'3 and 
33'8 times the mass of the sun, with a probable error of about 5 per cent. 
The visual absolute magnitudes are — 5‘95 and — 4'84, the bolometric 
absolute magnitudes being estimated at — 8’82 and — 7‘71, so that the total 
emission of radiation by the two components are respectively 294,000 and 
73,000 times that of the sun. The spectroscopic type is 0 8 |, which may 
again be supposed to indicate an effective temperature of 28,000 degrees. 
An analysis of the light curve shews that the star consists of two ellipsoidal 
components which are almost in contact, their semi-major axes being 23'8 
* Dominion Astrophys. Observ. Publications, ii. (1922), No. 4. 
| Otto Struve (Astrophys. Journ. lxv. (1927), p. 278, and lxviii. (1928), p. 109) finds that the 
quadruple system 27 Canis Majoris has probably at least 950 times the mass of the sun. He 
discusses other possible interpretations of the observations, but can find no satisfactory explana 
tion which would give a total mass substantially less thun that just mentioned. 
7 Dominion Astrophys. Observ. Publications, in. (1926), No. 18.