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Introduction

Variable Stars
Variable stars are stars showing brightness and/or color (temperature) changes on time scales from seconds to a few centuries.
All stars will show changes in brightness and temperature in the course of their evolution, but this of course happens on time scales much too long to be observed. A star is only considered as variable if it displays changes in brightness or temperature on time scales roughly comparible with human lifetimes.


Discovery
Ancient Chinese chronicles already reported "guest stars", stars which suddenly appeared and disappeared. These stars are now known as novae. There are however no indications that the ancient Chinese performed systematic observations.

During the middle ages western astronomers considered stars as fixed and unchangeable, until in 1572 Tycho Brahe noted a star as bright as Jupiter in the constellation Cassiopeia. This supernova brightened quickly until it rivalled the brightness of Venus and was visible during daytime for about 2 weeks.
In 1596 David Fabricius discovered the variable star Mira (omicron Ceti). Since Mira became soon too faint to be seen it was thought to be a nova. Only in 1638 Holwerda discovered that Mira changed its brightness with a period of 11 months. Initially the number of variable star discoveries remained small. In 1786 there were 12 known variable stars, by 1896 this number increased to 393. Variable star research became more serious when Argelander called for systematic observations for these interesting stars.

The table below lists the first known variable stars, the column indicated with con contains the constellation to which the variable star belongs.

Name                        Con  Type       Year    Discoverer(s)
SN 1572                     Cas  SN         1572    W. Schuler, Tycho Brahe
Mira, Omicron Ceti          Cet  Mira       1596    David Fabricius
P Cygni, Nova Cygni 1600    Cyg  S Dor      1600    Willem Janszoom Blaeu
SN 1604                     Oph  SN         1604    Brunowsky, Joh. Kepler
Algol, Beta Persei          Per  Algol      1669    Geminiano Montanari
Nova Vulpeculae 1670, CK    Vul  Nova       1670    Dom Anthelme
SN 1680 ? 1667 ? (Cas A)    Cas  SN         1680    John Flamsteed ? 1667 ?
Chi Cygni                   Cyg  Mira       1687    Gottfried Kirch
R Hydrae                    Hya  Mira       1704    Giacomo Filippo Maraldi
R Leonis                    Leo  Mira       1782    J.A. Koch
Nova Sagittae 1783, WY Sge  Sge  Nova       1783    D'Agelet
Eta Aquilae                 Aql  Delta Cep  1784    Edward Pigott
Beta Lyrae                  Lyr  Beta Lyr   1784    John Goodricke
Delta Cephei                Cep  Delta Cep  1784    John Goodricke

Alpha Herculis              Her  SRc        1795    William Herschel
R Coronae Borealis          CrB  R CrB      1795    Edward Pigott
R Scuti                     Sct  RV Tau     1795    Edward Pigott
R Virginis                  Vir  Mira       1809    Harding
R Aquarii                   Aqr  Mira       1810    Harding
Epsilon Aurigae             Aur  Algol      1821    Fritsch
R Serpentis                 Ser  Mira       1826    Harding
Eta Carinae                 Car  S Dor      1827    Burchell,
                                                    discovered 1677 E. Halley
S Serpentis                 Ser  Nira       1828    Harding
R Cancri                    Cnc  Mira       1829    Schwerd
Alpha Orionis (Betelgeuse)  Ori  SRc        1836    John Herschel
       

The number of known variable stars increased significantly with the advent of photography. By 1912 about 4000 variable stars were discovered, and by 1970 there were 22650 stars catalogued as variable.

The last decenium thousands of variable stars have been discovered in the course of all sky surveys like ASAS and as by-product of projects searching for massive compact objects causing gravitational lensing events. These projects include MACHO and OGLE among others. Also the astrometrical Hipparcos satellite discovered a lot of new variable stars. In the future new missions are planned in which the research of variable stars is an important topic. Among these missions are COROT and GAIA.

Why researching variable stars?
Variable star research is important since it provides fundamental information on stars, galaxies and the universe in general. By studying variable stars one can obtain important information on the physical properties and evolution of stars. Distance, mass, radius, internal and external structure, composition, temperature and luminosity can be derived from the observations. Variable stars are important as distance indicators, and are the base of the cosmological distance scale. The luminosity of Cepheid variable stars is proportional with their period, implying that once the period is known the distance can be calculated from the luminosity. Type I supernovae all have (almost) equal maximum luminosity. Supernovae are very luminous and can be detected in very distant galaxies.

Classification
In science a good classification according certain properties is often a very important step to get some knowledge. This is also the case with variable stars. Classification is often very difficult. During the research new properties will be discovered which might not fit in the scheme and sometimes even seems to belong to more than 1 type.
In variable star research initially one could only classify the stars according their lightcurve. This resulted in inhomogeneous classes of stars with very different physical properties. Also now this problem still arises in certain types of stars.

The General Catalogue of Variable Stars (GCVS) groups variable stars in 6 main classes which are each subdivided in numerous types and subtypes. In the literature one can find further subtypes which are not (yet) included in the GCVS.

The 6 main GCVS classes are:

  • Rotating variables
  • Intense variable X-ray sources

The VVS Working group Variable Stars mainly observes stars of the last 4 classes listed above. The amplitudes of the light variations of the stars from the first 2 classes are very small and therefore very difficult to observe.

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