STARS THAT VARY IN LIGHT
All stars brightness change with time but some stars undergo noticeable changes in a human lifetime, sometimes even on the order of seconds. There are three broad classes of variables, the important eclipsing binary stars such as Algol, the pulsating variables such as d Cepheus, and eruptive variables (supernovae, novae, flare stars).Nomenclature
Variable stars are named and typed at the time of discovery in a pretty strange fashion. If the star already has a name or greek letter designation it retains that name eg Algol, Betelgeuse (a Ori), d Cep, W Vir. Otherwise, the first unnamed variable star in a constellation is designated R followed by the possessive Latin name of the constellation eg R Leporus (R Lep). what happens when the next variable after Z is discovered? Call it RR then RS, RT, ..., SS, ..., SZ and so on up to ZZ. Whew! Next go to AA, AB, ..., AZ, BB, ..., up to QZ (The letter J is omitted to avoid confusion with I). We have wrapped around the alphabet and come back to RR so what next, RRR? Any reasonable person might conclude that a new nomenclature system is needed. We have named 334 variable stars and it is too bad that R Cyg was not just renamed V1 Cyg but this has not been done. The next variable star after the 334th variable in Cygnus, QZ Cyg, is named V335 Cyg and now we can go to infinity.
In addition, the type of variable is often indicated through a prototype variable star such as "the Cepheids" or d Cephei stars, UV Ceti or flare stars, RR Lyr or cluster variables, etc.
The light curve
Variable stars are frequently discovered by comparing starfields taken at different epochs. This includes visual inspection of a known field, and many novae and supernovae are discovered by amateur astronomers in this fashion. Once a variable is discovered the first bit of scientific work involves the production of a light curve, a graph that shows the magnitude of a star as a function of time. Amateur astronomers have and continue to provide a great volume of important observational data and many belong to the American Association of Variable Star Observers (A.A.V.S.O.) a world wide organization that collates data and suggests observing schedules and programs. Principal data gleaned from a light curve include period, maximum, minimum and median magnitudes. The amplitude is the difference in magnitudes between maximum and minimum and median brightness is halfway between maximum and minimum for the pulsating variables.
Pulsating Variables
During the lifetime of a star there comes a time when the surface layers become unstable to large amplitude radial oscillations. This is simply "natures way", the easiest way for radiation to be transported out of the star for a particular combination of gas parameters (T, P, state of ionization).
These pulsations are not small, the visible radius of a star can easily double, or range in size from the orbit of Mercury to that of Venus over a one month interval! This can be seen by onserving the doppler shift in spectral lines as the photosphere approaches and recedes as the star expands and contracts.
The largest group of pulsating stars belong to the Mira (omicron Ceti) or red long period variables. The Mira variables have long (about a year) somewhat irregular periods and a brightness range of several magnitudes. The Mira's are practically owned by the A.A.V.S.O.
The Cepheid variables are bright hot giant stars which can be seen in distant galaxies. Though rather rare, the cepheids are of great value as yardsticks. The protype d Cep was discovered by John Goodricke in 1784 and their great importance was discovered by Henrietta Leavitt in 1912 while analyzing the light curves of 25 cepheids in the Small Magellanic Cloud (SMC). She found that the periods of the cepheids were related to their brightness, the brighter the star the longer the period. Subsequent investigations showed the relation held for all cepheids in the Large and Small Magellanic Clouds.
The Cepheids range in period from over one day up to 100 days and the absolute magnitudes range from -1.5 to -6, making the longer period cepheids among the most luminous of stars. The only problem is the determination of the zero point of the cepheid scale. If we knew the distance to the LMC or any other cepheids we would be able to obtain the distances to many nearby galaxies with confidence. The calibration remains an important problem today. I estimate the absolute scale is good to 0.2 magnitudes (July 1998).
W Virginis and RR Lyrae stars are quite similar to the cepheids and will be discussed in detail later.