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:
- R - Z (9 stars)
- RR:RZ, SS:SZ, and so on (45 stars)
- AA:AZ-QQ:QZ,excluding J* and *J
(280 stars)
- 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.