Classifying Variable Stars
The classification of variable stars has been evolving for more than a century. As our understanding grows (and new types of objects are discovered), the classification criteria change.
Many years ago, many classes of variable were described in terms of a prototype star; and astronomers would define new classes to encompass minor observational differences from known variables. Adding to the confusion, many variables would be in two or more classes depending on the criteria used to describe them.
Modern variable star taxonomy is more generic. Seven major categories are now recognised:
- Eruptive - variation caused by flares or shell ejection; eg: flare stars, T Tau variables, RCBs, S Doradus stars.
- Pulsating - radial or non-radial pulsation eg: Miras and semiregular variables, Cepheids, RV Tau stars.
- Rotating - variation caused by starspots, magnetism, or changing shape. eg: pulsars, elliptical stars and magnetic variables.
- Cataclysmic - variation caused by explosions of the star or an accretion disc. Includes dwarf novae, classic novae & supernovae.
- Eclipsing - binaries where one component passes in front of the other, as seen by the observer.
- X-ray - variable X-ray emission, usually from neutron star or black hole component of a binary, and often optically variable too.
- Unique - Unclassifiable & generally wierd variables!
There are numerous subcategories within each of these.
Some variables still fall into more than one class; this is indicated by a "+" joining the classification codes together. The following list describes all the classes mentioned in the 4th General Catalogue of Variable Stars (GCVS). The special suffix ":" in the GCVS and other lists is used to indicate a doubtful entry.
- FU = FU Ori. A slow rise (up to 7 mags) over many months to a maximum lasting for several years; followed by the development of an emission spectrum.
- GCAS = Gamma Cas. Rapidly rotating blue giants which occasionally eject a ring of matter from their equator, causing a fade up to 1.5 mags.
- I = Irregular. Poorly studied variable of unknown spectral type. Most of these get reclassified as knowledge improves.
- IA = Poorly studied blue to white irregular.
- IB = Poorly studied yellow to red irregular.
- IN = Orion variables, young stars & protostars in or near nebulae. Irregular variations up to several magnitudes. There are several subspecies indicated by one or more suffixes: "S" = rapid variations (>1.0 mags in 1 to 10 days), "A" = blue to white star, "B" = yellow to red star, "T" = T Tauri variables (intense iron emission lines), (YY) = spectral evidence for infalling matter. The letter "N" is omitted if there is no association with a nebula.
- RCB = R Coronae Borealis. Hydrogen-poor, helium and carbon-rich stars showing small cyclic pulsations; and irregular fades up to 9 magnitudes lasting for weeks/months caused by carbon ejection. These stars can disappear in just a few hours!
- RS = RS CVn. Close binaries with chromospheric activity, causing very small light variations. Eclipses and X-ray variablity often seen as well.
- SDOR = S Doradus. Massive, very luminous blue stars usually surrounded by expanding envelopes. Often associated with nebulae. Occasional outbursts up to 7 magnitudes, lasting for months, caused by ejection of a shell of matter.
- UV = UV Cet. Red dwarf stars showing outbursts up to 6 magnitudes lasting for only a few minutes, caused by flares.
- UVN = UV's in nebulae. They may actually be a subspecies of INB variables.
- WR = Wolf-Rayet stars. Emission lines of carbon & nitrogen and evidence for unstable mass outflow as a "stellar wind".
- ACYG = Supergiants with multiple non-radial pulsations and emission spectra. Very small variations with periods of weeks.
- BCEP = Beta Cep. Blue stars with variations 0.1-0.3 mags and periods 0.1-0.7 days, caused by radial pulsations. Multiple periods are common.
- BCEPS = Subspecies of BCEP, with much smaller variations and periods < 0.1 day.
- BLBOO = BL Boo. Anomalously bright RRAB variables (see below).
- CEP = Cepheids. Radially pulsating white to yellow giants with variations up to 2 magnitudes and periods from 1 to over 100 days. Most of these are reclassified into the next few classes.
- CW = W Vir stars. Old radially pulsating stars belonging to the galactic halo (Population II). Periods from 0.8 to 35 days and variations up to 1.3 magnitudes. A period-luminosity relationship applies: max absolute visual mag is about = -0.2-2.5*log(period). Lightcurves are superficially similar to DCEP for periods 3-10 days; but spectral features are different. The suffixes "A" or "B" indicate periods greater than or less than 8 days.
- DCEP = Delta Cep or "classic" Cepheids. Young radially pulsating stars belonging to the galactic disc (Population I). Their period-luminosity relationship is: maximum absolute visual mag = -1.67-2.54*log(period).
- DCEPS = Subspecies of DCEP with symmetrical lightcurves and periods < 7 days.
- DSCT = Delta Sct. Pulsating stars belonging to the galactic disc (Population I). Variations up to 0.9 mag with multiple radial & non-radial pulsations of periods < 0.2 day.
- DSCTC = Subspecies of DSCT with variations < 0.1 mag.
- L = Slow irregular variables. Many of these are reclassified after further study. LB and LC indicate giant and supergiant stars.
- M = Mira (Omicron Cet). Red giants with emission spectra and well-defined periods from 80 to over 1000 days. Variations from 2.5 to 11 magnitudes or more; and variations do not repeat exactly from one cycle to the next. Some of these stars have distinct multiple periods.
- PVTEL = PV Tel. Pulsating helium-rich supergiants with variations < 0.1 mag. May be related to WR class.
- RR = RR Lyr. Radially pulsating stars of the galactic halo (Population II); often found in globular clusters. Periods < 1.3 days and variations < 2 mags. Suffixes "AB" or "C" indicate asymmetric or symmetric lightcurves. Some of these stars exhibit the Blazhko Effect - periodic variations in period and lightcurve.
- RV = RV Tauri. Radially pulsating yellow to red supergiants with alternating primary and secondary minima. The "period" is actually the time between two adjacent primary minima. Variations up to 4 mags with periods of 30 to 150 days. Suffixes "A" and "B" indicate constant mean magnitude or varying mean magnitude (up to 2 mags with periods 500-2000 days).
- SR = Semiregular. Red giants with definite periodicity; but with irregularities. Variations up to 3 mags and periods from 20 days to several years. There is a continuum between these stars and class M.
- SRA = Semiregulars with persistent periodicity.
- SRB = Semiregulars with poorly defined periodicity.
- SRC = Red supergiant semiregulars.
- SRD = Orange to yellow supergiant semiregulars.
- SXPHE = SX Phe. Galactic halo (Population II) subdwarfs< resembling DSCT variables.
- ZZ = ZZ Cet. Non-radially pulsating white dwarfs with variations < 0.2 mag and periods < 30 minutes. Suffixes "A", "B" or "O" indicate spectral features: hydrogen lines, helium lines, or carbon lines.
The suffix "(B)" in classes BCEP, CEP, DCEP, RR indicates "beat frequencies" caused by two or more simultaneous pulsations.
- ACV = Alpha-2 CVn. White stars showing small variations due to large "starspots" generated by intense magnetic fields. Intense and variable spectral lines due to silicon, strontium, chromium and the lanthanide elements. The suffix "0" indicates rapid non-radial pulsations as well.
- BY = BY Dra. Red dwarfs with large "starspots". Many of these are also UV variables.
- ELL = elliptical. Tidally distorted close binary stars, but no eclipses are observed (see EB class). Variations < 0.1 mag.
- FKCOM = FK Com. Rapidly rotating yellow to orange giants with non-uniform surface brightness.
- PSR = pulsar with optical as well as radio variations. Optical variation up to 0.8 mag with period less than 5 seconds.
- R = reflection binaries. A large cool component is illuminated by a hot component, causing variations up to 1 mag as the system rotates.
- SXARI = SX Ari. High temperature helium-rich versions of ACV class.
- AM = AM Her. Sudden outbursts up to 5 mags, caused by accretion onto the magnetic poles of a compact object. Light is polarized. Probably identical to classes XPM and XPRM below.
- N = nova. Sudden outburst from 7 to 19 magnitudes; caused by a runaway thermonuclear reaction on the surface of a white dwarf component of a close binary system. Time to rise to maximum ranges from hours to weeks. Some suffixes indicate the rate of fade: "A" = 3 mags in < 100 days, "AB" = 3 mags in 100-150 days, "B" = 3 mags in > 150 days. Additional suffixes are: "C" = at maximum for more than 10 years (these may be related to ZAND class below), "L" = poorly studied nova-like stars, and "R" = recurrent. All novae are believed to be recurrent. However, only a few have been seen more than once; because the time between outbursts may be centuries(!) for most novae.
- SN = supernova. Sudden catastropic outburst of 20+ magnitudes in less than a week, destroying the star. Light output may exceed that of the entire host galaxy. The total detonation of a white dwarf forms Type I (suffix "I"), whereas the destruction of a red supergiant forms Type II (suffix "II"). In practice the types are defined spectroscopically. Recent research suggests several subclasses within these types. Supernova 1987A was unusual in several ways. The original star was a blue supergiant, the outburst took several weeks to reach maximum, and the explosion was unexpectedly faint. Even so, it was easily visible to the eye for several weeks!
- UG = U Gem or dwarf nova. Close binary systems with an accretion disc around the white dwarf component. Outbursts from 2 to 9 magnitudes, lasting for a day or two, occur at quasi-periodic intervals of days to years. Many of these stars are ultraviolet and X-ray variable. Most UG's are reclassified into the subspecies UGSS, UGSU and UGZ after further study.
- UGSS = SS Cyg. A UG subspecies with outbursts lasting for several days.
- UGSU = SU UMa. UG subspecies with both "normal" outbursts and occasional superoutbursts (up to 2 mags brighter and about 5-8 times longer).
- UGZ = Z Cam. UG subspecies which may remain at an intermediate near-constant magnitude for several cycles after an outburst.
- ZAND = Z And. Close binaries where a hot component actually orbits inside the extended envelope of its cool giant companion. Small irregular variations plus occasional outbursts up to 5 magnitudes. These may be related to the NC class above.
- E = eclipsing. Binaries where one component periodically passes in front of the other. Periods range from hours to years.
- EA = Algol. Spherical components, with eclipse times identifiable from the lightcurve.
- EB = Beta Lyrae. Tidally distorted components, with continuous changes in brightness. These are related to the ELL class above.
- EW = W UMa. Components almost touching, and primary & secondary eclipses near-equal. Periods < 1 day. Numerous suffixes may be added to the eclipsing classes:
- AR = AR Lac. Subgiant components.
- D = detached system.
- DM = detached main-sequence system.
- DS = detached subgiant system.
- DW = detached EW-type system.
- GS = giant or supergiant component.
- K = contact system - the components are joined, and either may exchange matter with the other.
- KE = contact system with blue or white stars.
- KW = primary is a yellow main-sequence star, secondary is a hotter subdwarf.
- PN = one of the stars has formed a planetary nebula.
- RS = RS CVn. Additional variation due to starspots, plus radio and X-ray emission.
- SD = semi-detached system, one component is losing matter to the other.
- WD = white dwarf component(s).
- WR = Wolf-Rayet component(s).
This category is characterised by X-ray emissions from binary systems which are not attributed to any other variable star mechanism. One of the components is a compact object (white dwarf, neutron star, black hole). The X-ray emission is caused by matter falling onto the compact object or its accretion disc. The X-rays then irradiate the companion star causing a variety of effects. The main classes recognised are:
- XB = X-ray burster. Flares lasting a few minutes, optical variation up to 0.2 mag.
- XF = X-ray flickerer. X-ray and optical variations on a timescale of tens of milliseconds.
- XI = X-ray irregular. Variations up to 1 mag on a timescale of minutes to hours. The companion star is a white dwarf.
- XJ = jets being emitted at several % of speed of light.
- XN = X-ray novae.
- XP = X-ray pulsars. Optical variation up to 4 mags with periods from 1 second to 2 hours.
- XPR = X-ray pulsars irradiating their companion, causing reflection variability (see rotating class R).
- XPRM = XPR's with emission of polarized light. Probably identical to class AM above. If no pulsar is seen, the "P" is omitted.
Additional classes of X-ray variables are still being discovered.
- BLLAC = BL Lac. Active galactic nuclei, mistakenly identified as variable stars. Actually related to class QSO.
- CST = constant. Stars mistakenly thought to be variable (very common in the early days of photographic surveys!).
- GAL = distant galaxies mistakenly thought to be variable (another photographic blunder!).
QSO = quasars. Now thought to be active galactic nuclei powered by a supermassive black hole.
- S = stars with rapid light changes, not yet studied adequately.
- * = generally wierd and unclassifiable variables!
Obsolete and Alternative Classifications
- RW Aur = reclassified to INT and IT.
- Flare stars = UV.
- Beta CMa stars = BCEP.
- Cluster Cepheids = RR.