634 VARIABLE STARS 



Table 824 



VARIABLE STARS— GENERAL CHARACTERISTICS 



(See Russell, Dugan, and Stewart, Astronomy, 1927; Ludendorff, Stratton, Das Stern- 

 system, Handb. Astrophys., 6, Berlin, 1928 ; Payne, Stars of high luminosity, Chap. 14, 

 1930.) 



Perhaps 5% of all stars are variable; number known, over 5000. Astronomische Gesell- 

 schaft acts as central bureau ; when a variable star is confirmed it there receives a definite 

 designation, e.g., R. T. Persei. Most recent list Astron. Nachr., 244, 82, 193 1, contains 

 873 additional thus named variables. The Harvard College Observatory (Doctor Shapley, 

 Cambridge, Mass.) keeps a record of variable-star data. A yearly list of stars with known 

 periods is published by the Berlin-Babelsberg Observatory. Note added in press : 5826 in 

 1933 volume. 



Classification 

 I. Periodic Variables. 



(1) Eclipsing variables: Generally B and A stars. Not true variables. See 



Table 815. 



(2) Short period: 100 to 10,000 sun's luminosity, large mass. Types B to M. 



Preferably F and G. 



(a) Period range about i day; about 10% of variables of regular period. 



Generally called cluster variables; quick rise in light, slow decline, 

 visual range generally less than 1.5 mag.; photographic range averages 

 50% greater ; \ day generally of class A ; peculiar velocities average 

 70 km/sec. ; variable radial velocity range small, proportional to range 

 in mag. Max. of approach invariably near max. mag., max. of reces- 

 sion near min. mag. Galactic concentration small. Shortest period 

 known (1932), 0.69746 days, 15 mag., range 1 mag. (8 h I9 m 38 s R. A., 

 18° 45' S. dec. van Gent). 



(b) Periods 1 to 32 + days; 15% of regular variables. Cepheids. Much 



like (2a) but periods 4 d, class F5; 8 d, Go; 20 d, G5. Peculiar veloci- 

 ties average about 12 km/sec. Galactic concentration strong. About 120 

 known. Long-period Cepheids are among the brightest stars known, 

 20,000 times brightness of sun. The following table 825 is due to 

 Shapley, 1931. 



(3) Long-period variables: Nearly all red stars 87% class M, 6% class N, 



5% class S, a few G and K. o Ceti typical. Abs. mag. — 2.0 (Oort, 1927) ; 

 periods 100 to 150 d, M = — 2.3 ; 250 to 340, M = -i.i; > 340, + 0.3 

 (Gerasimovic, 1928). Periods often irregular, proper motions small 

 (0.03" ±), radial velocities large (mean 35 km/sec). For S Librae, 385 

 km/sec. Heat radiation diminishes by 1 or 2 mag. while light by 5 or so. 



II. Irregular variables. 



(1) R V Tauri: Resembles Cepheids somewhat irregularly. 12 known (Gerasi- 



movic, 1929). 19 given by Ludendorff (1928). 



(2) R Coronae Borealis : About 11 known. Typical R. Cor. Bor. remains often 



for years of 6th mag. ; then may rapidly drop 6 mag. for indefinite period 

 then returns to original mag. Ludendorff gives II. 



(3) U Geminorum (type) : Normally faint but brighten up at irregular intervals 



to drop back to original magnitude. Some analogy to Novae. Ludendorff 

 gives 20. 



(4) T Pyxidis : Resemble Novae. Ludendorff gives 5. 



Smithsonian Tables 



