Variable Star
From Slackerpedia Galactica
If you look closely at stars in the sky with your naked eye, you probably see them twinkle and shimmer a bit. That's not a variable star -- those variations are caused by the Earth's very turbulent atmosphere. A true variable star is one whose brightness changes due to physical changes associated with that particular star. Those "physical changes" can be almost anything, including eclipses of the star by a stellar or planetary companion, pulsation, accretion, magnetic activity, and dust formation. Even supernovae and gamma-ray bursts are a type of variable star.
Novae and supernovae were probably the first variable stars observed by human beings, (unless you count eclipses of the Sun by the Moon, but that's just being pedantic). Such phenomena were observed long before the invention of the telescope, so their physical understanding of what these objects were was non-existent. The eclipsing binary star Algol might have been the first "permanent" star suspected of being variable, but the first variable star with a recorded history of observations is the prototype of the Mira variables, Omicron Ceti. It was discovered by David Fabricius in 1596, and though it was assumed to be a nova, it was discovered again in 1609 and has been followed regularly since. The invention of the telescope in the first decade of the seventeenth Century led to several new discoveries of variable stars, but it was the work of stellar cartographers like Friedrich Argelander and the invention of photography in the mid-nineteenth Century that really led to the modern study of variable star astronomy.
Depending upon your level of tech and the type of variability, variable stars can be easy to observe. The primary way variable stars are studied is by measurement of their light curve -- a graph of their apparent brightness measured as a function of time. Some variable stars exhibit changes that can be detected by the human eye, while others require sensitive electronic cameras or photoelectric detectors to measure their variations. Regardless of the instrumentation used, the basic technique for measuring a variable star's brightness is the same: you compare the brightness of the variable to that of a nearby star known to be "constant" at a given moment in time, and simply continue to make that comparison as time goes by. Some stars change on timescales of seconds (or even milliseconds in the case of some pulsars), while others may take years or decades to change. A very small number of variable stars -- mainly the long-period Mira variables -- have recorded light curves spanning more than a century and a half.
Variable stars were one of the earliest classes of object to help usher in the field of astrophysics -- the study of astronomical objects with an eye towards understanding the physics of their behavior. Variable stars change on observable timescales, and these changes provide clues on why the variations occur and ultimately on what these objects are like. Astronomers have used variable stars to determine the masses of stars, determine their sizes and distances, and even probe their interior structure. Our own Sun is perhaps the most remarkable variable star in that regard: even though its pulsations are not detectable with the naked eye, the careful measurement of the thousands of oscillation frequencies of the Sun allowed astronomers to probe the Sun's interior, in the same way that geologists can use earthquakes to probe the interior of the Earth.
There are several dozen classes and subclasses of variable star, and although there are often common reasons for the observed variability (like accretion from one star to another in a binary, or pulsations) the behavior of a specific class is unique, and provides insight into the behavior of the object. As an example, we can take the group of binary stars known as cataclysmic variables composed of a normal star and a White Dwarf, and divide them into a number of classes. Are the stars orbiting very close to one another, or farther apart? Is the mass-losing star a giant star or a dwarf star? And is it losing lots of mass to the white dwarf, or just a trickle? Is the white dwarf magnetic? And on, and on. Depending upon the answers to these questions, the variable star can exhibit enormous differences in behavior,
The scientific study of variable stars is one of several sub-fields of astronomy where amateur astronomers have made valuable contributions over time. The earliest organized efforts to involve amateurs in variable star observing began in the latter half of the nineteenth Century, with parallel but cooperative efforts on opposite sides of the Atlantic Ocean. Organized efforts to recruit amateur observers of variable stars began at Harvard University in the mid-1880's, which eventually led to the formal founding of the American Association of Variable Star Observers (AAVSO) in 1911. The Variable Star Section of the British Astronomical Association was founded in 1890 with similar goals. Amateur variable star observing is now a global effort, with observers on every continent, and independent variable star research organizations and data archives operating world-wide.
External Links
General Catalogue of Variable Stars (GCVS)
L'Association Française des Observateurs d' Etoiles Variables (AFOEV)
The Variable Star Observers League of Japan (VSOLJ)
The Royal Astronomical Society of New Zealand (RASNZ) Variable Star Section
