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 but whose variability has not yet been established.

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 stars 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.