6 Introductory Chapter [CH. i 



space relatively to their centroid. Truman* and Young and Harperf find 

 respectively velocities of 670 kms a second and 598 kms a second. 



Not only are large velocities in space revealed by the spectroscope, but 

 also large velocities of rotation. The first discovery of rotation in a nebula 

 was Slipher'sj discovery in 1914 of the rotation of the nebula in Virgo 

 (N.G.C. 4594); Pease || has determined the velocity of rotation to be about 

 330 kms a second at a distance of 2' from the centre, the velocity increasing 

 proportionally to the distance from the centre. Velocities of the same order 

 have been found in other nebulae. By a comparison of photographs taken at 

 different dates Van Maanen^l has tfound a rotation in the nebula M. 101** 

 in Ursa Major which corresponds to a period of 85,000 years at 5' from the 

 centre; this nebula does not appear to rotate as a rigid body, the angular 

 velocity being greater near the centre. Van Maanen finds that in this nebula 

 the motion is along the arms and away from the centre, and similar results 

 have been obtained by Kostinsky (( for the spiral nebula in Canes Venatici 

 (M. 51 J+). Slipher suspects similar motion in the nebula N.G.C. 1068||||. 



Very large velocities such as we have been considering are a distinctive 

 property of the spiral nebulae. The large irregular nebulae, such as the 

 Orion and Trifid nebulae are found to be almost at rest relatively to the stars 

 of our system as a whole. The planetary nebulae have radial velocities 

 ranging up to 65 kms a second. The average radial velocity of thirteen 

 measured by KeelerHH is 27 '7 kms a 'second. If these velocities are corrected 

 for the solar motion***, their average numerical value is 26'8 kms a second, but 

 their average algebraic value is only 0*9 kms a second. Thus these thirteen 

 planetary nebulae, regarded as a whole, are almost at rest relative to our 

 system, while their individual velocities, although slightly larger than those 

 of ordinary stars, are small compared with the observed velocities of the 

 spiral nebulae. 



It must, however, be added that Campbell f f f has found quite exceptionally 

 large radial velocities for two planetary nebulae, namely a velocity of approach 

 of 141 kms a second for N.G.C. 4732 2 , and a velocity of recession of 202 kms 

 a second for N.G.C. 6644. These velocities are not greater than a few ex- 

 ceptionally high velocities observed for ordinary stars (e.g. 325 kms a sec. for 



* Pop. Astronomy, 24, p. 111. + Journal Royal Ast. Soc. Canada, 10, p. 134. 

 + Lowe.ll 06*. Bulletin, No. 62. See Plate III. 



II Ast. Soc. Pacific, 28, p. 191. t Astrophys. Journ. 44, p. 210. 



* See Plate II. ft M. N. Royal Ast. Soc. 77, p. 233. 

 *: See Plate II. Lowell Obs. Bull. 80 (1918). 



(Ill Two fine photographs of this nebula will be found in the paper by Pease already referred to. 

 Agtrophys. Journ. 46 (1917), p. 24, Plate IV. 

 1F1T Publications of Lick Observatory, 3, 201. 

 *** Perrine, Astrophys. Journ. 46 (1917), p. 176. 

 ftt Nat. Acad. Sci. Washington, 1 (1915), No. 9. 



