Theodor Koutsotheodoris
Variable Stars
Variable stars are stars that change in
brightness. These brightness changes can range from a few
hundredths to as much as twenty magnitudes over periods of a
fraction of a second to years, depending on the type of variable
star. Stars change in brightness when they are very young, or
when they are very old or dying.
Left: chi Cyg, near
minimum. Right: chi Cyg, near maximum
Variable Stars in
History: The
observations of differences in the brightness of variable stars,
start from the antiquity. In 134 B.C, Iparchus observed the flash
of a nova. Many nova and supernova who appeared in the antiquity,
had posed questions to ancient astronomers for the brightness of
stars, but all of them hesitated even to mention such issues.
However, Arabians must have observed the variations in brightness
of ß Perseus, hypothesis that can explain the Arabian name of
Algol (demon). In all cases, regular observations of variations
in brightness of stars have not been reported until 1572 A.D.
This year, Fabricius observed a star in Cetus that was not shown
in any atlas. Some months later, the star disappeared and it was
rediscovered in 1603. This is star the Mira, o (omicron) Ceti,
which was the first variable star known. In 1669, Montanari
discovered the variation of Algol (ß Per). Discoveries became
more and more frequent subsequently: in 1844, 18 variables were
known and 2054 in 1920. There are now over 28,000 stars
known to be variable, and 14,000 more that are suspected
to be changing in brightness in our galaxy, the Milky Way.
Classification of variable
stars: Variables are divided into four main
classes: pulsating and eruptive, where the variability is
due to physical changes in the star or star system, and eclipsing binary and rotating stars, where the variability is
due to an eclipse of one star by another, or the effect of
stellar rotation.
Some of the more famous variable stars include supernovae
(such as the supernova in the Large Magellanic Cloud in 1987) and
Cepheid variables (helpful in determining distances to far away
galaxies and the age of the universe), the long period stars like
Mira, and eclipsing binaries, such as Algol (the demon star) in
Perseus.
Research on variable stars is important because it can
provide much information about stellar properties, such as mass,
radius, luminosity, temperature, internal and external structure,
composition, and age. This information can then be used to
understand other stars. Professional astronomers have neither the
available time nor the unlimited telescope access needed to
gather data on the brightness changes of thousands of variable
stars. But amateur astronomers utilizing visual, photographic,
photoelectric, and now CCD techniques, are making a real and
highly useful contribution to science by observing variable stars
and submitting their observations to organizations like the AAVSO,
that collect and supply them to the Astronomical community.
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