Hills Observatory: 1 January 2013 to 30 March 2020
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
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
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
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
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
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
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
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
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.