412 



Tables 636-637. 



ASTRONOMICAL DATA- 



TABLE 536. — Motions of the Stars. 



The individual stars are moving in all directions, but, for the average of considerable groups, there is evidence of a 

 drift away from the point in the heavens towards which the sun is moving (solar apex). The best determinations ol 

 the solar motion, relative to the stars as a whole, are given in Table 535. In round numbers this motion of the sun 

 may be taken as 20 km/sec. towards the point R. A. 18 h. om., Dec +30.0°. 



After allowance is made for the solar motion, the motions of the stars in space, relative to the general mean, present 

 marked peculiarities. If from an arbitrary origin a series of vectors are drawn, representing the velocities of the various 

 stars, the ends of these vectors do not form a spherical cluster (as would occur if the motions of the stars were at ran- 

 dom), but a decidedly elongated cluster, whose form can be approximately represented either by the superposition of 

 two intermmgling spherical clusters with different centers (Kapteyn's two-stream hypothesis) or by a single ellipsoidal 

 cluster (Schwarzschild), the actual form, however, being more complicated than is indicated by either of these hy- 

 potheses. The direction of the longest axis of the cluster is known as that of preferential motion. The two opposite 

 points in the heavens at the extremities of this sixis are called the vertices. The compxments of velocity of the stars 

 parallel to this axis average considerably larger than those parallel to any axis perpendicular to it. 



The preferential motion varies greatly with spectral type, being practically absent in Class B, very strong in Class 

 A, and somewhat less conspicuous in Classes F to M, on account 01 the greater mean velocities of these stars in all 

 directions. The positions of the vertices are nearly the same for all. 



Numerous investigators, from the more distant naked-eye stars, find substantially the same position for the 

 vertex, the mean being R. A. 6 h, 6 m., Dec. 4-9°. The nearer stars, of large proper motion, give a mean of 6 h. 12m., 

 -|-2S°. (See Stromberg's discussion, cited above.) 



In addition to these general phenomena, there are numerous clusters of stars whose members possess almost exactly 

 equal and parallel motions, — for example, the Pleiades, the Hyades, and certain large groups in Ursa Major, Scorpius, 

 and Orion. The vertices, and the directions toward which these clusters are moving, are all in the plane of the galaxy. 



Several faint stars are known which have radial velocities between 300 and 350 km/sec. (e.g. A. G. Berlin 1366 R.A. 

 1900 = 4* Sm 6, Dec. 1900 = -1-22.7°, mag. 8.9 velocity of recession ssg km/sec), and it is probable that the actual 

 velocity in space exceeds 500 km/sec. for some of these. 



The 9th magnitude star A. G. Berlin 1366 has a radial velocity of 404 km/sec. 



The greatest known proper motion is that of Barnard's star of the ninth magnitude in Ophiuchus, 10.3" per year, 

 position angle •'56°. The parallax of this star is 0.52", and its radial velocity about —100 km/sec. 



The averag^ radial velocity of the globular clusters is 100 km/sec. and that of the spiral nebulae 400 km. The 

 globular clust irs as a class are approaching the sun. The spiral nebulae, with a few exceptions, are receding. The 

 greatest individual values are —410 km for the cluster N. G. C. 6934 and -+- rSoo km for the nebula N. G. C. 584. 



Average vfelocities with regard to center of gravity of the stellar system, according to Campbell (Stellar Motion, 

 1913): 



TABLE 537. — Distances of the Stars. 



Distances. 



Alpha Centauri (nearest star) 



Barnard's Star 



Sinus 



Arcturus 



The Hyades 



Nebula of Orion (KapteyTi) 



Globular Clusters (Shapley): ome] 



Centauri (nearest) 



N. G. C. 7006 (farthest) 



*Parsec = 206,265 astronomical units = 3.08 X lo^' km = 3.26 light years, i astronomical unit = distance sun 

 to earth. 



Practically all the stars visible to the naked eye lie within 1000 parsecs of the sun, and most of them are more than 

 100 parsecs distant. In the vicinity of the sun, the majority of the stars lie within two or three hundred parsecs of the 

 galactic plane; but along this plane the star-filled region extends far beyond 1000 parsecs in all directions, and 

 may reach 30,000 parsecs in the great southern star clouds (Shapley). 



Average parallax 6 planetary nebulae, 0.018" (van Maanen, Pr. Nat. Acad. 4, p. 3941 1918). 



Smithsonian Tables* 



