Semiregular variable stars are giants or supergiants 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 days, while the shapes of the light curves 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).
The semiregular variable stars have been sub-divided into four categories for many decades, with a fifth related group defined more recently. The original definitions of the four main groups were formalised in 1958 at the tenth general assembly of the International Astronomical Union (IAU). The General Catalogue of Variable Stars (GCVS) has updated the definitions with some additional information and provided newer reference stars where old examples such as S Vul have been re-classified.
|Subtype||IAU definition||GCVS code||GCVS definition||Standard|
|SRa||semi-regular variable giants of late spectral classes (M, C and S), which retain periodicity with comparative stability and possess, as a rule, small (less than 2m.5) light-variation amplitudes. Amplitudes and forms of light curves are usually liable to strong variations from period to period. Many of these stars differ from Mira Ceti type stars only owing to the smaller amplitude of light variation.||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. 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||Z Aqr|
|SRb||semi-regular variable giants of late spectral classes (M, C and S) with a poorly expressed periodicity, i.e. with a different duration of individual cycles (which leads to the impossibility of predicting the epochs of maximum and minimum brightness), or with the replacement of periodical changes by slow irregular variations, or even by the constancy of brightness. Some of them are characterised by a certain mean value of the period, given in the catalogue.||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. 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||AF Cyg|
|SRc||semi-regular variable super-giants of late spectral classes||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.||μ Cep|
|SRd||semi-regular variable giants and super-giants belonging to spectral classes F, G, K||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||S Vul|
|SRS||Semiregular pulsating red giants with short period (several days to a month), probably high-overtone pulsators||AU Ari|
The semiregular variable stars, particularly the SRa and SRb sub-classes, are often grouped the Mira variables under the long-period variable heading. In other situations, the term is expanded to cover almost all cool pulsating stars. The semi-regular giant stars are closely related to the Mira variables: Mira stars generally pulsate in the fundamental mode; semiregular giants pulsate in one or more overtones.
Photometric studies in the Large Magellanic Cloud looking for gravitational microlensing events have shown that essentially all cool evolved stars are variable, with the coolest stars showing very large amplitudes and warmer stars showing only micro-variations. The semiregular variable stars fall on one of five main period-luminosity relationship sequences identified, differing from the Mira variables only in pulsating in an overtone mode. The closely related OSARG (OGLE small amplitude red giant) variables pulsate in an unknown mode.
Many semiregular variables show long secondary periods around ten times the main pulsation period, with amplitudes of a few tenths of a magnitude at visual wavelengths. The cause of the pulsations is not known.
η Gem is the brightest SRa variable, and also an eclipsing binary. GZ Peg is an SRa variable and S-type star with a maximum magnitude of 4.95. T Cen is listed as the next-brightest SRa example, but it is suggested that it may actually be an RV Tauri variable, which would make it by far the brightest member of that class.
There are numerous naked-eye SRb stars, with third-magnitude L2 Pup being the brightest listed in the GCVS. σ Lib and ρ Per are also third-magnitude SRb stars at maximum brightness. β Gru is a second magnitude star classified as a slow irregular variable by the GCVS, but reported to be of SRa type by later research. These four are all class M giants, although some SRb variables are carbon stars such as UU Aur or S-type stars such as Pi1 Gru.
Catalogued SRc stars are less numerous, but include some of the brightest stars in the sky such as Betelgeuse and α Her. Although SRc stars are defined as being supergiants, a number of them have giant spectral luminosity classes and some such as α Her are known to be asymptotic giant branch stars.
Many SRd stars are extremely luminous hypergiants, including the naked-eye ρ Cas, V509 Cas, and ο1 Cen. Others are classified as giant stars, but the brightest example is the seventh-magnitude LU Aqr.
2 Centauri is a single star in the southern constellation of Centaurus, located approximately 183 light years from Earth. It has the Bayer designation g Centauri; 2 Centauri is the Flamsteed designation. This object is visible to the naked eye as faint, red-hued star with an apparent visual magnitude of 4.19. It is moving away from the Earth with a heliocentric radial velocity of +41 km/s. The star is a member of the HD 1614 supercluster.This is an evolved red giant star with a stellar classification of M5 III. It is classified as a semiregular variable star and its brightness varies from magnitude +4.16 to +4.26 with a period of 12.57 days. The star has around 70 times the Sun's radius and is radiating 72 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 3,398 K.AK Pyxidis
AK Pyxidis is a semiregular variable star located in the constellation Pyxis. It varies between magnitudes 6.09 and 6.51, pulsating to multiple periods simultaneously of 55.5, 57.9, 86.7, 162.9 and 232.6 days. Located around 1228 light-years distant, it shines with a luminosity approximately 1500 times that of the Sun and has a surface temperature of 3410 K.CK Carinae
CK Carinae (CK Car / HD 90382 / SAO 238038) is a variable star in the constellation Carina, the keel of the Argo Navis. Apparent average magnitude +7.59, is a member of the star association Carina OB1-D, so the distance to CK Carinae can be estimated to be around 2,200 parsecs or 7,100 light-years.
CK Carinae is a red supergiant of spectral type M3.5Iab with an effective temperature of 3,550 K. It is one of the largest stars, with a radius over 1,000 times larger than the sun, which means that if it were in the place of the Sun, its surface would reach almost to the orbit of Jupiter, Earth being encompassed within the star. However, it is surpassed by size by stars like VY Canis Majoris, VV Cephei A, and Mu Cephei. Consequently, CK Carinae is also a bright star, being its luminosity 170,000 times that of the Sun.
Billed as a semiregular variable star SRC, CK Carinae's brightness varies magnitude between +7.2 - +8.5 with a period of approximately 525 days.HD 130144
HD 130144 (or EK Boötis) is a semiregular variable star in the northern constellation of Boötes. The variation in luminosity has an amplitude of 0.38 in magnitude with no apparent periodicity. This is an X-ray source, and was possibly the first M-type giant star to have a magnetic field directly detected. It is considered to be a single star, although there is nearby companion at an angular separation of 0.2023″ along a position angle of 82.2° (as of 2010.4812).IX Carinae
IX Carinae (IX Car) is a red supergiant and pulsating variable star of spectral type M2Iab in the constellation Carina. It is a member of the Carina OB1 association along the Carina Nebula. It is one of the largest stars with a radius of 920 R☉ (640,000,000 km; 4.3 au). If placed at the center of the Solar System, it would extend close to the orbit of Jupiter.
IX Car is a semiregular variable star with a maximum brightness range of magnitude 7.2 - 8.5 and a period of 408 or 4,400 days.Lambda Ursae Minoris
Lambda Ursae Minoris (λ UMi, λ Ursae Minoris) is a star in the constellation Ursa Minor. It is an M-type red giant with an apparent magnitude of +6.38 and is approximately 880 light years from Earth.
Lambda Ursae Minoris is an asymptotic giant branch (AGB) star, a star that has exhausted its core hydrogen and helium and is now fusing material in shells outside its core. AGB stars are often unstable and pulsate, and Lambda Ursae Minoris is classified as a semiregular variable star and its brightness varies by about 0.1 magnitudes. Its variability was discovered from Hipparcos astrometry and it was entered into the General Catalogue of Variable Stars in 1999.Omicron Orionis
Omicron Orionis refers to 2 distinct star systems in the constellation Orion:
ο1 Orionis (4 Orionis), an M3S semiregular variable star
ο2 Orionis (9 Orionis), a K2 giant starAll of them were member of asterism 參旗 (Sān Qí), Banner of Three Stars, Net mansion.RR Coronae Borealis
RR Coronae Borealis (RR CrB, HD 140297, HIP 76844) is a M3-type semiregular variable star located in the constellation Corona Borealis with a parallax of 2.93mas being a distance of 341 parsecs (1,110 ly). It varies between magnitudes 7.3 and 8.2 over 60.8 days. Located around 1228 light-years distant, it shines with a luminosity approximately 2180 times that of the Sun and has a surface temperature of 3309 K.RS Coronae Borealis
RS Coronae Borealis is a semiregular variable star located in the constellation Corona Borealis with a parallax of 2.93mas being a distance of 341 parsecs (1,110 ly). It varies between magnitudes 8.7 to 11.6 over 332 days. It is unusual in that it is a red star with a high proper motion (greater than 50 milliarcseconds a year). Located around 1072 light-years distant, it shines with a luminosity approximately 1839 times that of the Sun and has a surface temperature of 3340 K.RW Cygni
RW Cygni is a semiregular variable star in the constellation Cygnus, about a degree east of 2nd magnitude γ Cygni. Its apparent magnitude varies between 8.05 and 9.70 and its spectral type between M2 and M4.SRA
SRA may refer to:
The National Center for Biotechnology Information's Sequence Read Archive (previously Short Read Archive)
SRA, a type of semiregular variable star
Satanic ritual abuse
Senior Airman (SrA) in the US Air Force
SRA0 to SRA4 are ISO 217 standard paper sizes
Surveillance radar approach in aviation.
SRA-shooting, Sovellettu reserviläisammunta, Finnish practical shooting sports
Superhuman Registration Act fictitious law in Marvel Comics
Serotonin releasing agent
Stratford station, Australa station codeSS Virginis
SS Virginis is a Mira variable star that appears with a strong red hue. It varies in magnitude from a minimum of 9.5 to a maximum of 7.4 over a period of 361 days. It is also considered to be a semiregular variable star, as its minimum and maximum magnitude are themselves variable over a period of decades. Its spectral class is C63e. Because it is so rich in carbon, SS Virginis is classified as a carbon star, along with stars like T Geminorum. SS Virginis, like all carbon Mira variables, has a hydrogen-alpha emission line that varies widely, synchronized with the overall variations in light. The hydrogen-alpha emission line becomes far more prominent as the star becomes brighter. Observations made in the near-infrared spectrum indicate that it has a radius of 500 solar radii, and its temperature is between 2405 and 2485 kelvins.The location of SS Virginis is two degrees north-following of η Virginis (Eta Virginis, Zaniah).TT Corvi
TT Corvi (TT Crv) is a semiregular variable star in the constellation Corvus. It is a red giant of spectral type M3III and average apparent magnitude 6.48 around 923 light years distant. It shines with a luminosity approximately 993 times that of the Sun and has a surface temperature of 3630 K.T Microscopii
T Microscopii is a semiregular variable star in the constellation Microscopium. It ranges from magnitude 6.74 to 8.11 over a period of 352 days. Located around 700 light-years distant, it shines with a luminosity 7708 times that of the Sun and has a surface temperature of 2856 K.V763 Centauri
C1 Centauri (C1 Cen) is a star in the constellation Centaurus. It is approximately 720 light years from Earth.
C1 Centauri A is a M-type red giant with a mean apparent magnitude of +5.64. It is classified as a semiregular variable star and its brightness varies from magnitude +5.55 to +5.80 with a period of 60 days.VZ Camelopardalis
VZ Camelopardalis (VZ Cam) is a Semiregular variable star in the constellation Camelopardalis.
VZ Camelopardalis is a M-type red giant with a mean apparent magnitude of +4.92. It is approximately 473 light years from Earth. It is classified as a semiregular variable star and its brightness varies from magnitude +4.80 to +4.96 with a period of 23.7 days.X Cancri
X Cancri is a semiregular variable star located in the constellation Cancer. It varies between magnitudes 5.69 and 6.94 over 180 days. Located around 1116 light-years distant, it shines with a luminosity approximately 4695 times that of the Sun and has a surface temperature of 3317 K. It has a radius of 470 R☉.Y Centauri
Y Centauri or Y Cen (HD 127233, HIP 70969) is a semiregular variable star in the constellation of Centaurus.
The variability in the star was discovered by Williamina Fleming in 1895 and published in the Third Catalogue of Variable Stars. The photographic magnitude range was given as 7.7 - 8.8, but the variability was described as "somewhat doubtful". It was later given the designation HV 52 in the Harvard Catalogue of Variable Stars. The General Catalogue of Variable Stars lists it as a possible semiregular variable star with a period of 180 days and a photographic magnitude range of 8.9 - 10.0. A study of Hipparcos satellite photometry found a small amplitude range of 0.2 magnitudes at a visual magnitude of 8.53.The distance of the star is poorly known. The revised Hipparcos annual parallax of 3.50 mas gives a distance of 900 light years. A study taking into account the variability of the star found a parallax of 5.57 mas, corresponding to a distance of 585 light years. It is an asymptotic giant branch star 330 times as luminous as the sun. Its spectral type varies between M4 and M7 as it pulsates.The star has been observed to produce 22 GHz water maser emission, although later searches did not find any maser emission.Y Lyncis
Y Lyncis is a semiregular variable star in the constellation Lynx. It is an asymptotic giant branch star of spectral type M6S, with a luminosity class of Ib, indicating a supergiant luminosity. It is around 800 light years away.
Y Lyncis ranges in brightness from magnitude 6.8 to 8.9. Its changes in brightness are complex with at least two different periods showing. The General Catalogue of Variable Stars lists a period of 110 days. More recent studies show a primary pulsation period of 133 days, with and a long secondary period with an amplitude of 0.2 magnitudes and duration 1,300 days. The long secondary period variations are possibly caused by long-lived convection cells.Y Lyncis has a mass around 1.5-2.0 M☉ and a luminosity around 10,000 L☉. It is a thermally pulsing asymptotic giant branch star, an evolved star with a carbon-oxygen core that is fusing helium in a shell and hydrogen in a separate shell. It is also an S-type star, where third dredge-ups have brought some carbon to the surface, but not enough to create a carbon star.
Category:Stars · Stars portal