Variable star

Variable stars are broadly classified into two major categories, stars in which the variability in brightness are attributable to physical processes within the star itself (eg. pulsating stars) and those in which the variations observed are caused by external factors more closely related to our perspective when viewing the stars (eg. eclipse variables).

General overview

History

Not counting the occasional supernova, the first variable star to be identified was Mira or ο Ceti which was found to be periodically invisible by German astronomer and theologian David Fabricius in the late 16th and early 17th centuries.

When Fabricius first observed Mira in 1596, he thought it to be a nova after its disappearance from naked eye visibility in October of that year. He saw the star again in 1609. It wasn't until Fokkens Holwerda of Friesland observed Mira in 1638 however that its periodicity was discovered and determined to be around 11 months. ο Ceti was given its popular name in 1642 by Hevelius who named it the wonderful or Mira and the star would serve as the prototype of the long-period variables still known as Mira-type variable stars. c Cygni, R Hydrae and R Leonis were the first three Mira-type variables discovered in the two centuries since Fabricius first saw his "nova".

By the turn of the 19th century about a dozen variable stars were known but with the advent of modern astro-photography the number of discoveries ballooned with tens of thousands known to exist in a variety of different classes.

Naming conventions

Some prominent variable stars like Mira and Algol have received popular names while others have Bayer designations like β Cephei. The majority of variable stars however, are named according to the system devised by the 19th century astronomer Friedrich Argelander. According to this system the first variable star discovered in any given constellation is designated R followed by the genitive of the constellation name (eg. R Andromedae). Subsequent discoveries in the same constellation receive the designations S through Z followed by RR through RZ, SS through SZ and so forth up to and including ZZ.

After these letter combinations have been exhausted the next variable star in the constellation receives the designation AA after which the system continues through to QZ while omitting the letter J from the sequence. this system leaves room for 334 variable stars to be so designated where after variables will simple receive the designation V335, V336 etcetera like in the case of the star V335 Sagitarii.

Organization

Kukarkin and a group of scientists at the Soviet Academy of Sciences published a General Catalogue of Variable Stars in 1948 containing well over 10.000 objects. The GCVS is also important in that it sets out a classification of the variable stars into different classes. The most recent edition of the GCVS contains in excess of 40.000 variables while supplementals list thousands more that are suspected of being variable stars.

In 1911, with the founding of the American Association of Variable Star Observers (AAVSO) records of variable star observations by professional and amateur astronomers alike could be collected centrally. Estimates of a variable star's brightness, compared to field stars with a known apparent magnitude are recorded and the resulting plot of various estimates for any given star can then be used to produce a light curve showing both the amplitude (difference between maximum and minimum brightness) and the period of variability. The AAVSO collects data from its contributors worldwide and makes the resulting light curves available for research purposes to professional astronomers.

The emphasis on variable star observing, at least in the professional field, is on intrinsic variables where the differences in apparent magnitude are due to physical processes within the star itself as opposed to the extrinsic types like eclipsing binaries where the variability is merely due to external factors.

Importance of variable stars

Variable stars can give astronomers important insights into stellar evolution and the physical processes at work within the interiors of stars. As well, certain variables, like the Cepheid variable stars, named after the prototype of the class, δ Cephei. Astronomer Henrietta Leavitt discovered in 1912 that Cepheid type variables exhibit a set correlation between their periodicity and their intrinsic brightness. By determining the period of a Cepheid the star's absolute magnitude can be determined and comparison with the apparent magnitude as seen from Earth allows astronomers to calculate the star's distance. Because this correlation is precisely known, Cepheid variables serve an important role in determining distances in the universe.

Intrinsic variable stars

Intrinsic variables fall into four main groups; eruptive, cataclysmic , pulsating and x-ray stars.

Eruptive variables

In eruptive variable stars the variations in brightness are caused by flares or some other process taking place in the outer layers of the star like its chromosphere or corona. Such events are often accompanied by the ejection of large amounts of matter or strong stellar winds. The cause of these eruptive events are often associated with rapid stellar rotation or strong magnetic fields. Eruptive variables are classified in the following subdivisions:

FU Orionis stars

Named after the prototype of this class, FU Orionis (GCVS code: FU), these stars are characterized by a slow outburst in which the brightness of the star increases up to 6 magnitudes over a number of months and stays at maximum brightness for up to several years after which a slow decline sets in that dims the star by a couple of magnitudes. During an outburst the spectral type of the stars can change significantly and an emission spectrum develops as well. At maximum light, FU Orionis stars are of spectral type A - G after which the spectral type becomes later. All FU Orionis stars are associated with reflecting nebulae.

FU Orionis stars are pre-main sequence stars somewhat similar to T Tauri stars. The prototype was first discovered in 1939 by A. Wachmann when the star increased some 100-fold in brightness. FU Orionis was studied in depth by George Herbig in the 1960s and 1970s. There are some 10 stars known of this type.

Gamma Cassiopeiae stars

Gamma Cassiopeiae type variables (GCVS code: GCAS) are blue giants of spectral type Be exhibiting rapid rotation which causes the star to eject matter from the equatorial region, forming a disk around the star and dimming it by one or two magnitudes with an irregular periodicity. The prototype, γ Cassiopeiae, was first studied by Father Angelo Secchi in 1867. It fluctuates between magnitude +1.5 and +3.0.

Irregular eruptive variables

Irregular eruptive variables (GCVS code: I) are not well understood and are broadly classified according to their spectral types. Those of spectral types O - A are classed IA while later spectral types (F - M) designated IB.

Orion variables

The class of Orion variables (GCVS code: IN) contain several different but related types of variable young stars that have not yet reached the main sequence stage in their evolution. Most of the Orion variables are associated with nebulosity. The stars fall into different subclasses depending on their period and amplitude as well as spectral properties.

Irregular variables of this class that exhibit sudden fadings are classed INA when their spectral types are B or A as in the star T Orionis or INB for F through M type stars. Some of the later types of these may also produce flares.

T Tauri stars (INT) are very young and low mass stars, perhaps no more than 10 million years old, that are still contracting under their own gravitational fields toward the main sequence stage. Many of the T Tauri stars known are associated with accretion disks that are a leftover from their earlier formative stages. Their young age is evidenced by the abundance of Lithium in their spectra, an element that is quickly destroyed as the star evolves further. The prototype of this class of stars, T Tauri varies erratically in brightness having been observed as bright as magnitude +9.3 and as dim as magnitude +14. For the most part T Tauri fluctuates around the magnitude +9.3 - +10.7 mark. spectral types for T Tauri stars range from Fe through Me.

YY Orionis stars (code INYY) are very young and low mass stars in a stage of stellar evolution just before that of the T Tauri stars. YY Orionis stars are embedded in the clouds of dust and matter from which they are in the process of forming.

Rapid irregular variables

Closely related to the Orion variables are the rapid irregular variable stars (GCVS code: IS) that show variations in the order of 0.5 to 1 magnitude with no apparent regularity. IS stars are never associated with nebulosity, however related Orion type stars are classified as INS stars and exhibit much the same behavior as regular IS stars. As is the case with the Orion variables and irregular eruptive variable stars, these stars are also subdivided according to their spectral types with "early" stars of spectral class B - A designated ISA and ISB stars defined as being of spectral types F through M

R Coronae Borealis stars

R Coronae Borealis stars (GCVS code: RCB) undergo periodic outbursts not by brightening but by fading by up to 9 magnitudes in brightness after which they slowly return to their pre-outburst brightness. These stars are supergiant stars of spectral class F or G that are rich in carbon and helium content but poor in hydrogen.

The cause of these fadings is thought to be due to a cloud of carbon dust that forms when expelled matter from the surface of the star condenses as it moves away from the star and cools. This dust cloud absorbs the radiation from the star resulting in its apparent magnitude to fall dramatically in a matter of weeks. As time goes on the dust particles are blown away by the pressure of the stellar winds and the star slowly recovers its former brilliance, usually on a time scale of up to one year. During the period of minimum light emission lines appear in the spectrum of R CrB stars. During the period the stars are at their maximum brightness they undergo small scale fluctuations in the order of some tenths of a magnitude with periods of 30-100 days.

The prototype of this class of variable stars, R Coronae Borealis, was discovered to be variable by the English amateur astronomer Edward Pigott in 1796 when he observed the star to be invisible even though it was found to be around 6th magnitude previously. R CrB returned to visibility a year later. Various studies were done on the star but it was not until the 1930s that the generally accepted views on the nature of this class of stars was first proposed. Only about 40 or so R Coronae Borealis stars are known suggesting that this stage of stellar evolution is of a rather short duration and might be associated with the star's final helium flash which happens when a relatively low mass star exhausts the hydrogen in its core and starts the helium-to-carbon conversion in its interior instead.

RS Canum Venaticorum stars

Variables of the RS Canum Venaticorum type (GCVS code: RS) are close binary stars where a type G or K type star whose rotation is sped up by the presence of a companion star. The main component star usually exhibits large star spots (analogous to sunspots) due to the strong magnetic fields present in these stars, causing their brightness to fluctuate (in the range of 0.2 magnitude or so) with a period that is somewhat similar to that of the orbital period of the binary system as a whole. Some stars in this class may also show variability due to eclipses between the two stars.

S Doradus stars

Luminous blue variables, also known as S Doradus stars (GCVS code: SDOR) are among the most luminous known stars. These massive, supergiants show brightness variations that range from a few hundredths of a magnitude to 5 magnitudes or more with greatly varying periods. Some S Doradus stars undergo outbursts in which large quantities of matter (sometimes as much as one solar mass) are driven off the star. Because these massive stars have relatively short life spans only a handful of them are known to exist. Among the more familiar examples of the class are η Carinae and S Doradus which actually lies within the Large Magellanic Cloud. The mass of a typical LBV star may exceed 50 solar masses while their absolute magnitudes can be -9.6 of brighter. Many S Doradus stars are associated with diffuse nebulae, the results of the matter that these stars lose during their outbursts.

UV Ceti stars

Flare stars, also known as UV Ceti stars (GCVS code: UV) are low mass red dwarfs of spectral types K or M that periodically increase in brightness, often within a minute or so, and then slowly return to their pre-flare magnitudes, usually within minutes up to roughly an hour. The flare-ups are not equally strong in all frequencies of the electro-magnetic spectrum. A flare that is observed in the visual (red) end of the spectrum that may be in the order of one magnitude may be as much as 5 magnitudes in amplitude in the blue or ultra-violet part of the spectrum. Emission lines in the spectrum of flare stars tend to become more apparent during outbursts.

Flare star outbursts are believed to be similar in nature to the flares that occur on the sun when under the influence of the sun's magnetic field charged particles are accelerated and radiate brightly, particularly in ultra-violet light. While flares on the sun are dim when compared to the overall radiation output of our home star, similar events on the cool (surface temperature 2.500 - 4.000 K) and intrinsically dim red dwarfs will produce a significant elevation in total energy output for the duration of a flare. Flares may occur at irregular intervals and on occasion more than one flare at a time can be observed. Flares of a smaller amplitude also occur.

Flare stars were first observed in 1924 by astronomer Willem Jacob Luyten who observed AT Microscopii and V1396 Cygni while in 1948 the star Luyten 726-8 was observed going through an outburst from the Mt. Wilson Observatory. Today, the class is named after this prototype which was designated with the variable star "name" of UV Ceti.

A class of Orion variables, (code: UVN) associated with nebulae, exhibit a greater luminosity on average than regular UV Ceti stars and their flare-ups occur on longer timescales.

Wolf-Rayet stars

Wolf-Rayet stars (GCVS code:WR) are massive and hot O type stars of up to 25 solar masses, nearing the end of their evolution. these stars display small amplitude fluctuations of perhaps a tenth of a magnitude. These stars exhibit strong stellar winds which may cause them to loose a significant percentage of their mass during the Wolf-Rayet stage. Wolf-Rayet stars typically show prominent emission lines in their spectra with lines of helium, carbon, oxygen and nitrogen being particularly strong. French astronomers Charles Wolf and Georges Rayet were the first to observe these emission lines in several stars and the class was subsequently named after these two scientists.