Semiregular variable star
Encyclopedia
Semiregular variable stars are giants
or supergiant
s of intermediate and late spectral type showing considerable periodicity in their light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to more than 2000 day
s, while the shapes of the light curve
s may be rather different and variable with each cycle. The amplitudes may be from several hundredths to several magnitudes
(usually 1-2 magnitudes in the V filter).
Semiregular variables
are classified in several subtypes:
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
or supergiant
Supergiant
Supergiants are among the most massive stars. They occupy the top region of the Hertzsprung-Russell diagram. In the Yerkes spectral classification, supergiants are class Ia or Ib . They typically have bolometric absolute magnitudes between -5 and -12...
s of intermediate and late spectral type showing considerable periodicity in their light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to more than 2000 day
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...
s, while the shapes of the light curve
Light curve
In astronomy, a light curve is a graph of light intensity of a celestial object or region, as a function of time. The light is usually in a particular frequency interval or band...
s may be rather different and variable with each cycle. The amplitudes may be from several hundredths to several magnitudes
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
(usually 1-2 magnitudes in the V filter).
Semiregular variables
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
are classified in several subtypes:
- SRA: Spectral-type (M, C, S or Me, Ce, Se) giants displaying persistent periodicity and usually small amplitude, less than 2.5 magnitudes in V. Z AquariiZ AquariiZ Aquarii is a M-type star in the constellation of Aquarius. It has an apparent visual magnitude which varies between 7.4 and 10.2....
is an example of this class. Amplitudes and light-curve shapes generally vary and periods are in the range of 35–1200 days. Many of these stars differ from Mira variables only by showing smaller light amplitudes.
- SRB: Spectral-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. Some may occasionally cease varying at all for a time. RR Coronae Borealis and AF Cygni are examples of this behavior. Every star of this type may usually be assigned a certain mean period. In a number of cases, the simultaneous presence of two or more periods of light variation is observed.
- SRC: Spectral-type (M, C, S or Me, Ce, Se) supergiants with amplitudes of about 1 mag and periods of light variation from 30 days to several thousand days. Mu CepheiMu CepheiMu Cephei , also known as Herschel's Garnet Star, is a red supergiant star in the constellation Cepheus. It is one of the largest and most luminous stars known in the Milky Way...
is a bright example of this class.
- SRD: 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 Herculis and SV Ursae Majoris are examples of this class. The globular clusterGlobular clusterA globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
M13Messier 13Messier 13 or M13 is a globular cluster of about 300,000 stars in the constellation of Hercules....
contains a dozen red variable stars from 11.95 to 12.25 visual magnitude, and with period of 43 days (V24) to 97 days (V43).
See also
- List of semiregular variable stars
- Low dimensional chaos in stellar pulsationsLow dimensional chaos in stellar pulsationsLow dimensional chaos in stellar pulsations is the current interpretation of an established phenomenon. The light curves of intrinsic variable stars with large amplitudes have been known for centuries to exhibit behavior that goes from extreme regularity,...
- Variable star designationVariable star designationVariable stars are named using a variation on the Bayer designation format of an identifying label combined with the Latin genitive of the name of the constellation in which the star lies...