INTRINSIC VARIABLE STARS are stars that vary in brightness or other respects for reasons that are internal to the star rather than Aside from their pathological interest they provide important information about the structure, evolution and proper-of the stars. For instance, the timescale of the variation may be related with the luminosity of the star. If so, observations of the variation can tell us the luminosity, and hence the distance of the star. Compared to constant stars, variable stars provide an extra dimension of interest and information.

The brightness variations in some stars can easily be seen by the unaided eye, yet the constancy of the stars was part of Aristotelian dogma, which persisted for centuries. Not until 1572 was a variable star recorded in Western literature. In that year, Tycho Brahe observed a ‘new star’ in the constellation of Cassiopeia. It rose rapidly in brightness to rival Venus, then slowly faded. Johannes Kepler observed another ‘new star’ in 1604. We now know that these ‘new stars’ (called supernovae) were actually old stars in the terminal stages of their evolution. Oriental astronomers had been recording such GUEST STABS since BC, and their catalogues provide a valuable record for astronomers today. The Crab Nebula, for instance, has been identified as the remnant of a super¬nova observed by Oriental astronomers in AD 1054 .

Some variable stars are more predictable than supernovae. In 1596, David Fabricius noted that o Ceti (subsequently called Mira – the wonderful) was sometimes visible, sometimes not; further¬more, the cycle of visibility repeated regularly every 11 months. By 1850, Friedrich Argelander could list an assortment of 22 variable stars (variables for short). Most of these had been discovered by accident : astronomers measuring the brightness of a star had found that their result differed from previous measurements .These discoveries probably mark the beginning of true astrophysics –the study of the nature of the stars as opposed to their positions and motions.

These early measurements were all made by eve, Despite the importance of cameras, spectrographs and photometers, their is still a place for the eye in astronomy. Amateur astronomers make hundreds of thousands of useful visual observations each year. These observations are coordinated, collected and analysed by such organizations as the British Astronomical Association, and the American Association of Variable Star Observers and they are circulated to professional astronomers around the world.

In the last few decades, new techniques have revolutionized variable star research and every ,other branch of astronomy . The camera ,in a long time-exposure, can record thousands of faint star images on a single permanent photographic plate Astronomers then use a BLINK MICROSCOPE To compare this plate with a plate of the same field taken at a different time, to discover which stars have changed in brightness. They then use an IRIS PHOTOMETER to measure the size of the image, and hence the brightness of each star.

The PHOTOELECTRIC PHOTOMETER is a device that measures the brightness of a star, through a telescope, using a photomultiplier. If used with care, it can measure brightness with an accuracy of 0.001 mag. This has permitted a detailed study of brightness and colour variations, and has resulted in the discovery of many micro-variables – including the Sun – whose variations could not be detected by visual or photographic techniques. At least a third of all bright stars turn out to be variables when examined carefully.

Spectroscopy provides detailed information about the spectrum of a variable. From the absorption or emission lines that are present, the astronomer can classify the star according to tempera¬ture, luminosity, chemical composition and other properties. From the Doppler shift of these lines, he can study the expansion and contraction which occur in the outer layers of variable stars.

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