Prof. George C. Comstock finds that the average parallax of 67 selected stars ranging in brightness between the 9th and the 12th magnitude, is of the value of 0″·0051.[290] This gives a distance representing a journey for light of about 639 years!
Mr. Henry Norris Russell thinks that nearly all the bright stars in the constellation of Orion are practically at the same distance from the earth. His reasons for this opinion are: (1) the stars are similar in their spectra and proper motions, (2) their proper motions are small, which suggests a small parallax, and therefore a great distance from the earth. Mr. Russell thinks that the average parallax of these stars may perhaps be 0″·005, which gives a distance of about 650 “light years.”[291]
According to Sir Norman Lockyer’s classification of the stars, the order of increasing temperature is represented by the following, beginning with those in the earliest stage of stellar evolution:—Nebulæ, Antares, Aldebaran, Polaris, α Cygni, Rigel, ε Tauri, β Crucis. Then we have the hottest stars represented by ε Puppis, γ Argus, and Alnitam (ε Orionis). Decreasing temperature is represented by (in order), Achernar, Algol, Markab, Sirius, Procyon, Arcturus, 19 Piscium, and the “Dark Stars.”[292] But other astronomers do not agree with this classification. Antares and Aldebaran are by some authorities considered to be cooling suns.
According to Ritter’s views of the Constitution of the Celestial Bodies, if we “divide the stars into three classes according to age corresponding to these three stages of development, we shall assign to the first class, A, those stars still in the nebular phase of development; to the second class, B, those in the transient stage of greatest brilliancy; and to the class C, those stars which have already entered into the long period of slow extinction. It should be noted in this classification that we refer to relative and not absolute age, since a star of slight mass passes through the successive phases of its development more rapidly than the star of greater mass.”[293] Ritter comes to the conclusion that “the duration of the period in which the sun as a star had a greater brightness than at present was very short in comparison with the period in which it had and will continue to have a brightness differing only slightly from its present value.”[294]
In a valuable and interesting paper on “The Evolution of Solar Stars,”[295] Prof. Schuster says that “measurements by E. F. Nichols on the heat of Vega and Arcturus indicated a lower temperature for Arcturus, and confirms the conclusion arrived at on other grounds, that the hydrogen stars have a higher temperature than the solar stars.” “An inspection of the ultraviolet region of the spectrum gives the same result. These different lines of argument, all leading to the same result, justify us in saying that the surface temperature of the hydrogen stars is higher than that of the solar stars. An extension of the same reasoning leads to the belief that the helium stars have a temperature which is higher still.” Hence we have Schuster, Hale, and Sir William Huggins in agreement that the Sirian stars are hotter than the solar stars; and personally I agree with these high authorities. The late Dr. W. E. Wilson, however, held the opinion that the sun is hotter that Sirius!
Schuster thinks that Lane’s law does not apply to the temperature of the photosphere and the absorbing layers of the sun and stars, but only to the portions between the photosphere and the centre, which probably act like a perfect gas. On this view he says the interior might become “hotter and hotter until the condensation had reached a point at which the laws of gaseous condensation no longer hold.”
With reference to the stars having spectra of the 3rd and 4th type (usually orange and red in colour), Schuster says—
“The remaining types of spectra belong to lower temperature still, as in place of metallic lines, or in addition to them, certain bands appear which experiments show us invariably belong to lower temperature than the lines of the same element.
“If an evolutionary process has been going on, which is similar for all stars, there is little doubt that from the bright-line stars down to the solar stars the order has been (1) helium or Orion stars, (2) hydrogen or Sirian stars, (3) calcium or Procyon stars, (4) solar or Capellan stars.”
My investigations on “The Secular Variation of Starlight” (Studies in Astronomy, chap. 17, and Astronomical Essays, chap. 12) based on a comparison of Al-Sufi’s star magnitudes (tenth century) with modern estimates and measures, tend strongly to confirm the above views.