A variable star is any star which undergoes changes to its apparent brightness, caused by the physical behavior of the star itself (not including terrestrial causes like clouds and scintillation). There are thousands of known variable stars, and their variability has many different causes.

Types of variable stars

Eclipsing binary stars are variable because one star may obscure the surface of another star during their orbit, if the plane of their orbit lies on or near our line of sight. Algol is a common example of this kind of binary star. Binary stars which do not eclipse one another may have variable stellar spectra caused by the Doppler effect as they orbit around one another, and are called spectroscopic binaries. Some binary stars are so close together that they touch, and form a contact binary.

Another class of variable binary star is the accreting binary star. In this case, one star loses mass to the other component in the binary system when the donor star overflows its Roche lobe. When the matter spirals into the companion star, it often forms an accretion disk, which becomes very hot and luminous due to friction. There are many kinds of accreting stars, including cataclysmic variables, novae, and X-ray binary stars. The stars that actually do the accreting can be anything from normal stars to white dwarfs, neutron stars, and black holes. Very young stars known as T Tauri stars also have accretion disks, and can be variable.

A third class of variable star is the pulsating variable. These stars vary because their surfaces pulsate in and out. Their variability then comes from a change in radius or a change in temperature, or both. The pulsations can be driven by a number of things, including stochastic excitation by convection. The Sun is variable for this reason, and it pulsates in thousands of independent modes (with very low amplitude). Stars which lie on the instability strip of the Hertzsprung-Russell Diagram pulsate because their atmospheres act like pistons, periodically blocking and releasing radiation. Stars on the instability strip include Cepheid variables, RR Lyrae stars, delta Scuti stars, and pulsating white dwarf stars. Other stars may pulsate because of periodic changes in the strength of nuclear reactions in the center of the star, rather than changes to the outer regions of the star.

A final class of variable star are stars which are magnetically active. These stars can have magnetically driven explosions on their surface which cause flares, and are known as flare stars. Young red dwarf stars are commonly flare stars, and their ultraviolet brightness can change by several magnitudes during a flare. Other stars may have large star spots on their surfaces, just like sunspots, and the star's brightness may change because the spot rotates out of our view, or because it can fade away over time.

Naming variable stars

Variable star names are descended from an old naming system for bright stars within constellations. The system was invented by Johannes Bayer around the year 1600; it used the familiar Greek alphabet for the 24 brightest stars in a given constellation, and the Roman alphabet for fainter stars. Bayer never got past the letter Q in any of his catalogs, and in the mid-1800's the German astronomer Friedrich Wilhelm Argelander came up with the naming system we use today -- designating variable stars in a given constellation with the letters R through Z, followed by the Latin genitive case of the constellation name. Argelander and others soon realized that there were far more than nine (R-Z) variable stars within each constellation, so once they got to Z, they started using double-letter combinations: RR-RZ, SS-SZ, TT-TZ, and so on. When those ran out, they went back to A again: AA-AI:AK-AZ, and so on up to QQ-QZ, but excluding AJ-QJ and JJ-JZ because J might be confused with I. After those ran out, astronomers gave up and sensibly started using numbers preceded by the letter V beginning with V 335 (as there are 334 possible combinations of the letters and letter pairs above).

So, to summarize, variables are named:

  1. R - Z (9 stars)
  2. RR:RZ, SS:SZ, and so on (45 stars)
  3. AA:AZ-QQ:QZ,excluding J* and *J (280 stars)
  4. V335 - (to however many are needed)

What is surprising about the naming system is that so many variable star names are needed at all -- with 88 constellations and on the order of 334 variable stars per constellation, there are tens of thousands of variable stars known, just in the solar neighborhood. Bayer's system of using Roman letters for bright stars has died out, but the variable star naming convention is still the standard. However, the numerical system has been extended backwards so that "R Orionis" is also known as "V1 Orionis" and so on. The number of observed variable stars per constellation can be over 1500 -- Sagittarius alone has over five thousand known variable stars.

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