760 



TABLE 870.— VARIABLE STARS, GENERAL CHARACTERISTICS* 28 * 1 



The task of cataloging and naming variable stars was delegated in 1946 by the Inter- 

 national Astronomical Union to the Sternberg Astronomical Institute in Moscow. The 

 1948 General Catalogue lists 10,912 variable stars; a supplement lists 265 additional vari- 

 ables discovered in 1948. Several thousands of variable stars in globular clusters, in the 

 Magellanic Clouds, and in the nearest galaxies are not included in this catalog, nor are 

 thousands of stars whose variability has been announced, but which are not officially 

 recognized pending confirmation. The total number of variable-star discoveries announced 

 until 1950 probably amount to 20,000. 



Classification. — Variable stars, with the exception of eclipsing binaries (see Table 

 879), can be divided roughly into three major groups: (1) Pulsating stars. The variables 

 of this group are all giants, located above the main sequence in the Russell diagram. 

 (2) Explosive stars. The variables of this group are, as far as is known, dwarfish ; 

 located below the main sequence in the Russell diagram. (3) Erratic variables, whose 

 light, fluctuations, mostly of an erratic nature, are produced by external causes (nebulosity) 

 or by peculiar phenomena in their atmospheres. 



Pulsating stars. — Ccphcids. Usually divided into cluster-type variables, with periods 

 shorter than one day, and classical Cepheids, with periods longer than one day, although 

 at least five subgroups are indicated. 



Cluster-type variables belong to Population II, have spectra ranging from A to F, 

 absolute magnitudes close to zero; most variables found in globular clusters belong to 

 this group. Periods range from d .061 (CY Aquarii) to l d .35 (a star in the w Centauri 

 cluster), with the greatest concentration around 0''.53. Typical variable: RR Lyrae (7 m .l 

 — 8 m .0; period d .57 ; spectrum .42 — F0). About 1,700 galactic objects and 600 stars in 

 globular clusters are known to belong to this group. 



Classical Cepheids belong to Population I, have spectra ranging from F to K, with 

 marked dependence on period, and intrinsic luminosities increasing with the period (period- 

 luminosity law) from — M .5 to — 3 M (absolute visual magnitudes). Periods range from 

 l d .13 (BQ Coronae Austrinae) to 45 d .2 (SV Vulpeculae), with the greatest concentration 

 around 2 d .7. Typical variable: 5 Cephei (3 m .8 — 4 m .6, period 5 d .37, spectrum F 5 — G2). 

 About 500 galactic stars and 2,500 stars in the Magellanic Clouds and other extragalactic 

 systems are known to belong to this group. 



For both cluster-type and classical Cepheids the shape of the light curve is a function 

 of the period ; the rise to maximum is always faster than the decline. Average visual am- 

 plitude m .75 ; photographic amplitudes 50 percent larger. Radial-velocity curves are in 

 phase with light curves (maximum approach at maximum light) ; Average amplitude 

 30-40 km/ sec. 



Long-period variables. Typical variable: o (Mira) Ceti (2 m .0 — 10"'. 1 ; period 331 d ; 

 spectrum M 6c). Characterized by very large amplitudes (from 4 to 10 magnitudes, visual), 

 late spectra (M,S, R, N) with bright hydrogen emission lines near maximum light, un- 

 stable light curves and periods ranging from 120 d (W Puppis) to 1379 d (BX Monocerotis). 

 Greatest concentration of periods around 275' 1 . Long-period variables seem to fall into 

 two major groups, whose periods overlap to a great extent. Stars of the first group have 

 nearly symmetrical light curves with moderate amplitudes and periods ranging from 120 d 

 to 450 d ; they seem to belong to Population II. Stars of the second group have strongly 

 asymmetrical light curve (rise faster than decline), large amplitudes and periods upward 

 of 200 d ; they seem to belong to Population I. 



The enormous visual (and photographic) amplitudes are accounted for by a shift in the 

 effective wavelength of the radiation with phase and by the formation of strong absorption 

 bands at minimum light in the visual region of the spectrum. The total (bolometric) 

 radiation has an amplitude of only one magnitude. Absolute bolometric magnitudes near 

 — 4. About 2,600 stars are known to belong to this group. 



Scmircgular red variables. Typical variables: Af Cygni (6 m .3 — 8 m .O; period 89 d ; 

 spectrum M 6). Spectra similar to those of long-period variables, except for much weaker, 

 or entirely absent, hydrogen emission lines. Amplitude mostly comprised between 1 and 3 

 magnitudes (both visual and photographic). Light curves very irregular, often erratic; 

 periods ranging from 42 d (TX Tauri) to 810' 1 (S Persei), but mostly comprised between 

 100' 1 and 200 d ; several unrelated periods often occur in the same star and for many vari- 

 ables periods have only a statistical significance. Then mean brightness often changes 

 slowly, with cycles of 1,000-2,000 days. Absolute visual magnitudes high, between and 

 — 4. Their galactic distribution suggests Population II. Total number of recognized 

 variables 600. 



RV Tauri stars. Typical variable: RV Tauri (8'".7 — ll m .8 ; period 39 d .3 ; spectrum 

 AT IV). Spectra Cepheid-like, but light curves similar to those of the preceding group. 

 Deep and shallow minima often alternate. Periods (intervals between two successive 



* Prepared by L. Taechia, Massachusetts Institute of Technology. 



284 Kukarkin, B. V., and Parenago, P. P., Fiziceskie Peremennye Zvjozdy, 1937; Gaposchkin, C. P., 

 and Gaposchkin, S., Variable stars, 1938; Campbell, L., and Jacchia, L., The story of variable stars, 1941. 



(continued) 

 SMITHSONIAN PHYSICAL TABLES 



