56 
[ch. II 
The Light from the Stars 
Sun’s effective temperature 
(total radiation) = 5750 absolute 
(energy curves) = 6000 absolute. 
Betelgeux (a Orionis). This is the twelfth brightest star in the sky, 
its apparent visual magnitude being 0 92. Its spectral type is M0. From 
observations on the energy curve Sampson and Abbott estimate effective 
temperatures of 3400 and 2600 respectively; Coblentz, from direct bolometric 
measurements, estimates an effective temperature of from 2800 to 3300. The 
angular diameter of the star as determined by interferometer measurements 
at Mount Wilson, is 0047", which would represent an effective temperature 
of exactly 3000. Thus all methods agree in assigning to the star an effective 
temperature of about 3000°. 
The probable parallax of the star is 0‘017". Combining this with its 
angular diameter of 0‘047", the diameter of the star must be times the 
radius of the earth’s orbit, or about 250 million miles. With this parallax, 
the star’s luminosity must be 1320 times that of the sun, its bolometric 
absolute magnitude being — 4'6 and its total emission of radiation about 6000 
times that of the sun. The star is a variable, but shews only slight changes 
of luminosity. 
Antares (a Scorpii), the sixteenth brightest star in the sky, is in many 
respects similar to Betelgeux. Its spectral type is M 0, its apparent visual 
magnitude being T2. The angular diameter, as measured with the interfero 
meter, is 0 , 040 // , which corresponds to an effective temperature of about 
3020 degrees. Its trigonometrically measured parallax* of 0 026" gives the 
star a diameter of 140 million miles, absolute magnitude — T7 and luminosity 
about 440; the corresponding bolometric magnitude is — 3’4 and total emission 
of radiation about 2000 times that of the sun. 
Seares, Russell and others disregard its measured parallax, and, treating 
it as a member of the Scorpius cluster, assign it the parallax 0t)085" of the 
cluster. This gives it an absolute magnitude of—4*2, and an absolute bolometric 
magnitude of — 5'9, with an emission of radiation of about 20,000 times that 
of the sun. 
Arcturus (a Bootis) is the third brightest star in the northern sky, and 
the fifth brightest in the whole sky. Its spectral type is if 0 and its apparent 
visual magnitude 0‘2. Coblentz has determined its effective temperature by 
thermocouple measurements as from 3500 to 4500; Sampson from energy 
curves gives 4200. Its angular diameter, as measured with the interferometer, 
is 0 022", again indicating an effective temperature of 4200. 
Assuming an effective temperature of 4200, its parallax of 0 080" gives it 
a diameter of 26 million miles, an absolute bolometric magnitude of — 0 ' 8 , 
and a total emission of radiation equal to 180 times that of the sun. 
* General Catalogue of Parallaxes, Schlesinger (1924).