132 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1945 



the sun, at 8.1 miles; the earth at 18.5; and Mercury, nearest the sun, 

 has an average speed of nearly 30 miles a second. 



Our solar system of sun and planets has practically all its mass 

 at the center; 99% percent of it resides in the sun, only one-seventh 

 of 1 percent in the planets. Being so constituted, the rate of motion 

 of a planet at any known distance from the center is very simply 

 calculated by means of Kepler's great discovery of more than 300 

 years ago, his law of "harmonic motion." 



The rate of motion (and the period) of a star at any given dis- 

 tance from the center of the stellar system is not so easily found, 

 for the mass of the system is more widely distributed. There is 

 no great central star dominating all else. If the distribution of 

 material in the stellar system were quite uniform throughout, with 

 no concentration of stars toward the center, the whole flattened 

 Milky Way system would rotate as a wheel, and we could detect no 

 motion at all unless we could observe some outside landmarks, such 

 as extragalactic nebulae, that do not revolve with the stars. 



Actually, however, the stars of our galaxy are sufficiently con- 

 centrated near its center to give measurably different stellar veloci- 

 ties. Yet the galaxy is so large and the difference of velocity as 

 seen from the earth so small that our greatest telescopes are needed 

 to observe the relative motion; even with the largest telescopes we 

 cannot observe individual stars of our system as far away as the 

 center of the galaxy, and only with difficulty can star velocities be 

 measured as far as one-fourth the distance to the galactic center. 

 Since Oort's method of detecting and computing the rotation of the 

 stellar system depended upon measuring their relative motion, his 

 work had to be based upon observations of the most distant observ- 

 able stars. But even these are relatively close in terms of the dimen- 

 sions of the galactic whirlpool. 



Because the closest stars that can be used in obtaining data for 

 Oort's method had to be more than 1,000 light-years distant from us, 

 Plaskett and Pearce of the Dominion Observatory, Victoria, B. C.» 

 worked with those very hot and white giants of classes O and B. 

 Rigel, lying at the foot of Orion, is one of these and is one of our 

 brightest-appearing stars, but being only about 500 light-years away 

 is too close to be used in Oort's method. However, stars of this type 

 can easily be seen at much greater distances with such a telescope 

 as the one at Victoria. 



Others have used those reddest of all stars, the N type, which aver- 

 age several hundred times the brightness of the sun. The so-called 

 planetary nebulae are stars surrounded by gas, which makes them ex- 

 ceptionally luminous. The peculiar, brilliant gas, supposed to have 

 been thrown out by some violent explosion long ago, can be seen at 



