Variable Stars - Pulsating
Hills Observatory: 1 January 2013 to 10 July 2018
Pulsating variables are stars showing periodic expansion and contraction of their surface layers. The pulsations may be radial or nonradial. A radially pulsating star remains spherical in shape, while in the case of nonradial pulsations the star's shape periodically deviates from a sphere, and even neighboring zones of its surface may have opposite pulsation phases. Depending on the period value, on the mass and evolutionary status of the star, and on the scale of pulsational phenomena, the following types of pulsating variables may be distinguished: ACYG   Variables of the Alpha Cygni type, which are nonradially pulsating supergiants        of Bep-AepIa spectral types. The light changes with amplitudes of the order of        0.1 mag often seem irregular, being caused by the superposition of many        oscillations with close periods. Cycles from several days to several weeks are        observed. BCEP   Variables of the Beta Cephei type (Beta Cep, Beta CMa), which are pulsating        O8-B6 I-V stars with periods of light and radial-velocity variations in the range        of 0.1 - 0.6 days and light amplitudes from 0.01 to 0.3 mag in V. The light curves        are similar in shape to average radial-velocity curves but lag in phase by a        quarter of the period, so that maximum brightness corresponds to maximum        contraction, i.e., to minimum stellar radius. The majority of these stars probably        show radial pulsations, but some (V649 Per) display nonradial pulsations;        multiperiodicity is characteristic of many of these stars.        BCEPS  A short-period group of Beta Cep variables. The spectral types are        B2-B3 IV-V; periods and light amplitudes are in the ranges 0.02 - 0.04 days and        0.015 - 0.025 days, respectively, i.e., an order of magnitude smaller than the        normally observed ones. CEP  Cepheids. Radially pulsating, high luminosity (classes Ib-II) variables with        in the range of 1-135 days and amplitudes from several hundredths to 2 mag in V        (in the B band, the amplitudes are greater). Spectral type at maximum light is F;        at minimum, the types are G-K. The longer the period of light variation, the later        is the spectral type. The maximum of the surface-layer expansion velocity almost        coinciding with maximum light. CEP(B) Cepheids (TU Cas, V 367 Sct) displaying the presence of two or more        simultaneously operating pulsation modes (usually the fundamental tone with the        period P0 and the first overtone P1).  The periods P0 are in the range from 2 to 7        days, with the ratio P1/P0 approx. 0.71.       CW  Variables of the W Virginis type. These are pulsating variables of the galactic        spherical component (old disk) population with periods of approximately 0.8 to 35        days and amplitudes from 0.3 to 1.2 mag in V. They obey a period-luminosity        relation different from that for Delta Cep variables (see DCEP). For an equal        period value, the W Vir variables are fainter than the Delta Cep stars by 0.7 - 2        mag. The light curves of W Vir variables for some period intervals differ from        those of Delta Cep variables for corresponding periods either by amplitudes or by        the presence of humps on their descending branches, sometimes turning into        broad flat maxima. W Vir variables are present in globular clusters and at high        galactic latitudes. They may be separated into the following subtypes: CWA  W Vir variables with periods longer than 8 days (W Vir); CWB  W Vir variables with periods shorter than 8 days (BL Her). DCEP   These are the classical cepheids, or Delta Cep-type variables. Comparatively        young objects that have left the main sequence and evolved into the instability        strip of the Hertzsprung-Russell (H-R) diagram, they obey the well-known        Cepheid period-luminosity relation and belong to the young disk population. DCEP        stars are present in open clusters. They display a certain relation between the        shapes of their light curves and their periods. DCEPS  These are Delta Cep variables having light amplitudes <0.5 mag in V (<0.7        mag in B) and almost symmetrical light curves (M-m approx. 0.4 - 0.5 periods);        as a rule, their periods do not exceed 7 days. They are probably first-overtone        pulsators and/or are in the first transition across the instability strip after leaving        the main sequence (SU Cas).        Traditionally, both Delta Cep and W Vir stars are quite often called Cepheids        because it is often impossible to discriminate between them on the basis of the        light curves for periods in the range 3 -  10 days. However, these are distinct        groups of entirely different objects in different evolutionary stages. One of the        significant spectral differences between W Vir stars and Cepheids is the presence,        during a certain phase interval, of hydrogen-line emission in the former and of        Ca II H and K emission in the latter.       DSCT   Variables of the Delta Scuti type. These are pulsating variables of spectral        types A0-F5 III-V displaying light amplitudes from 0.003 to 0.9 mag in V        (usually several hundredths of a magnitude) and periods from 0.01 to 0.2 days.        The shapes of the light curves, periods, and amplitudes usually vary greatly.        Radial as well as nonradial pulsations are observed. The variability of some        members of this type appears sporadically and sometimes completely ceases, this        being a consequence of strong amplitude modulation with the lower value of the        amplitude not exceeding 0.001 mag in some cases. The maximum of the surface        layer expansion does not lag behind the maximum light for more than 0.1        periods. DSCT stars are representatives of the galactic disk (flat component) and        are phenomenologically close to the SX Phe variables. DSCTC  Low amplitude group of Delta Sct variables (light amplitude <0.1 mag in V).        The majority of this type's representatives are stars of luminosity class V; objects        of this subtype generally are representative of the Delta Sct variables in open        clusters. L     Slow irregular variables. The light variations of these stars show no evidence of        periodicity, or any periodicity present is very poorly defined and appears only        occasionally. Like for the type I, stars are often attributed to this type because of        being insufficiently studied.  Many type L variables are really semiregulars or        belong to other types. LB   Slow irregular variables of late spectral types (K, M, C, S); as a rule, they are        giants (CO Cyg). This type is also ascribed, in the GCVS, to slow red irregular        variables in the case of unknown spectral types and luminosities.        LC    Irregular variable supergiants of late spectral types having amplitudes of about        1 mag in V (TZ Cas). M     Mira (Omicron) Ceti-type variables. These are long-period variable giants with        characteristic late-type emission spectra (Me, Ce, Se) and light amplitudes from        2.5 to 11 mag in V. Their periodicity is well pronounced, and the periods lie in the        range between 80 and 1000 days. Infrared amplitudes are usually less than in        the visible and may be <2.5 mag. For example, in the K band they usually do not        exceed 0.9 mag. If the amplitudes exceed 1 - 1.5 mag , but it is not certain that        the true light amplitude exceeds 2.5 mag, the symbol "M" is followed by a colon,        or the star is attributed to the semiregular class with a colon following the symbol        for that type (SR). PVTEL  Variables of the PV Telescopii type. These are helium supergiant Bp stars with        weak hydrogen lines and enhanced lines of He and C.  They pulsate with periods        of approximately 0.1 to 1 days, or vary in brightness with an amplitude of 0.1        mag in V during a time interval of about a year.        RR   Variables of the RR Lyrae type, which are radially-pulsating giant A-F stars having        amplitudes from 0.2 to 2 mag in V. Cases of variable light-curve shapes as well as        variable periods are known. If these changes are periodic, they are called the        "Blazhko effect."        Traditionally, RR Lyrae stars are sometimes called short-period Cepheids or        cluster-type variables. The majority of these stars belong to the spherical        component of the Galaxy; they are present, sometimes in large numbers, in        some globular clusters, where they are known as pulsating horizontal-branch        stars.  Like Cepheids, maximum expansion velocities of surface layers for these        stars practically coincide with maximum light. RR(B)  RR Lyrae variables showing two simultaneously operating pulsation modes,        the fundamental tone with the period P0 and the first overtone, P1 (AQ Leo).        The ratio P1/P0 is approximately 0.745; RRAB   RR Lyrae variables with asymmetric light curves (steep ascending branches),        periods from 0.3 to 1.2 days, and amplitudes from 0.5 to 2 mag in V;        RRC   RR Lyrae variables with nearly symmetric, sometimes sinusoidal, light curves,        periods from 0.2 to 0.5 days, and amplitudes not greater than 0.8 mag in V        (SX UMa). RV   Variables of the RV Tauri type. These are radially pulsating supergiants having        spectral types F-G at maximum light and K-M at minimum. The light curves are        are characterized by the presence of double waves with alternating primary and        secondary minima that can vary in depth so that primary minima may become        secondary and vice versa. The complete light amplitude may reach 3-4 mag        in V.  Periods between two adjacent primary minima (usually called formal        periods) lie in the range 30-150 days (these are the periods appearing in the        Catalogue). Two subtypes, RVA and RVB, are recognized:        RVA   RV Tauri variables that do not vary in mean magnitude (AC Her); RVB   RV Tauri variables that periodically (with periods from 600 to 1500 days and        amplitudes up to 2 mag in V) vary in mean magnitude (DF Cyg, RV Tau).        SR   Semiregular variables, which are giants or supergiants of intermediate and late        spectral types showing noticeable periodicity in their light changes, accompanied        or sometimes interrupted by various irregularities. Periods lie in the range from        20 to >2000 days, while the shapes of the light curves are rather different and        variable, and the amplitudes may be from several hundredths to several        magnitudes (usually 1-2 mag in V). SRA  Semiregular late-type (M, C, S or Me, Ce, Se) giants displaying persistent        periodicity and usually small (<2.5 mag in V) light amplitudes (Z Aqr).        Amplitudes and light-curve shapes generally vary and periods are in the range of        35-1200 days. Many of these stars differ from Miras only by showing smaller        light amplitudes; SRB  Semiregular late-type (M, C, S or Me, Ce, Se) giants with poorly defined        periodicity (mean cycles in the range of 20 to 2300 days) or with  alternating        intervals of periodic and slow irregular changes, and even with light constancy        intervals (RR CrB, AF Cyg). Every star of this type may usually be assigned a        certain mean period (cycle), which is the value given in the Catalogue. In a        number of cases, the simultaneous presence of two or more periods of light        variation is observed; SRC  Semiregular late-type (M, C, S or Me, Ce, Se) supergiants (Mu Cep) with        amplitudes of about 1 mag and periods of light variation from 30 days to several        thousand days; SRD  Semiregular variable giants and supergiants of F, G, or K spectral types,        sometimes with emission lines in their spectra. Amplitudes of light variation are in        the range from 0.1 to 4 mag, and the range of periods is from 30 to 1100 days        (SX Her, SV UMa). SXPHE  Phenomenologically, these resemble DSCT (Delta Sct) variables and are        pulsating subdwarfs of the spherical component, or old disk galactic population,        with spectral types in the range A2-F5. They may show several simultaneous        periods of oscillation, generally in the range 0.04-0.08 days, with variable-        amplitude light changes that may reach 0.7 mag in V. These stars are present in        globular clusters. ZZ   ZZ Ceti variables. These are nonradially pulsating white dwarfs that change their        brightnesses with periods from 30 s to 25 min and amplitudes from 0.001 to 0.2        mag in V. They usually show several close period values. Flares of 1 mag are        sometimes observed; however, these may be explained by the presence of close        UV Ceti companions.        These variables are divided into the following subtypes: ZZA  ZZ Cet-type variables of DA spectral type (ZZ Cet) having only hydrogen         absorption lines in their spectra; ZZB  ZZ Cet-type variables of DB spectral type having only helium absorption lines         in their spectra.
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