Once a variable has been discovered and announced in an astronomical journal, it is catalogued arid named according to the fol¬lowing system. Stars lacking a proper name (like Polaris) or a Greek letter name (like 8 Cephei) are designated by one or two capital letters, followed by the genitive case of the constellation’s Latin name. The letters are assigned in the order in which the variables are discovered, starting with R,S… .Z, RR… .RZ, SS… .SZ through to ZZ, then AA… .AS through to QZ, omitting J. This system provides 334 designations per constellation; subsequent variables are called V335, V336, etc., followed by the constellation name.

Millions of variables must still remain undiscovered. Of the 25000 known variables listed in the General Catalogue of Variable Stars, only a small fraction has subsequently been studied in detail. The most basic property of a variable is its LIGHT CURVE – a graph of its brightness against time. The light curve can be described by its shape or form, its RANGE from maximum to mini¬mum brightness, and (if the variation is regular) its PERIOD, the time for one complete cycle. Careful, repeated observations may reveal details or changes in the shape of the light curve, or changes in the range or period. Observations through colour filters reveal changes in the temperature of the outer layers of the variable. It is also particularly important to try to determine the ‘normal’ brightness and temperature of a variable – that is to say, the brightness and temperature which it would have if it were not variable. These can be compared with the brightness and temperature of non-variable stars.

The simplest classification scheme for variables is based on whether the light curve is PERIODIC or NON-PERIODIC. Periodic variables are subclassified according to cause into eclipsing, pulsating and rotating variables, and are further classified according to period, range and shape of light curve. The most numerous non-periodic variables are the eruptive variables, which suddenly brighten, then fade. These are further classified according to the magnitude and duration of the eruption.

The normal absolute magnitude and temperature of an intrinsic variable are usually correlated with the properties of the light curve. They therefore strengthen the classification scheme, and also provide clues to the cause of the variation. This leads finally to the most fundamental classification scheme: one based on the physical nature and cause of the variations.

Unfortunately, the understanding of variable stars has not yet reached this final exalted state. Through ignorance and historical accident, a cumbersome and sometimes misleading system of classification and terminology has arisen. In this chapter, for instance, we shall discuss pre-main sequence variables separately, even though their light curves could be classified as eruptive. We shall also discuss flare stars as a subclass of eruptive variables, even though they :M- probably related to pre-main-sequence variables. There are some variables which still defy classification.

Variability may been environmental or genetic in origin, if a star has a Companion in orbit around it, then its brightness may vary due to eclipse or tidal distortion by the companion. An eclipse occurs only if the observer is situated near the plane of the mutual orbit of the two stars. Hence an eclipse is not an intrinsic property of the stars but is a properly of the position of the observer. Occasionally, if the star and its companion are close, tidal forces may pull material from one star onto the other. If the star gaining the material is a compact star with a strong gravitational field then the results of the transfer of material can be quite spectacular the infalling material may generate X-rays, radio waves, rapid and irregular light variations and occasionally a full-scale eruption.

ROTATING VARIABLES are the least conspicuous of the intrinsic variables, but may possibly be the most numerous. Any star that rotates and has permanent or semi-permanent ‘surface’ features will appear to vary in brightness, unless the rotation axis points to the observer or unless the surface features are symmetrical about the axis. The peculiar A stars, are the best-known rotating variables. They have extremely strong magnetic fields, which may cause their peculiar chemical composition, and segregate different elements in patches on the star. As rotation carries different parts of the star under the observer’s line of sight, the magnetic field, apparent composition, brightness and colour all vary in synchronism with the rotation (figure 4.2). Seen from a distance, these effects are quite small, but it is interesting to speculate on the appearance of these stars as seen by observers (if any) on planets (if any) in orbit around them. Even our own Sun — which is certainly spotty and rotating — varies slightly in brightness due to rotation. High-precision photoelectric photometry may eventually prove that every star is a rotating variable.

Gravitation can convert an innocuous rotating variable into an exotic one, by amplifying both the rotation and the magnetic field. Gravitation dominates the last phases of a star’s evolution, converting it into a compact white dwarf, an even more compact neutron star or an infinitely compact black hole. Rotational variability has been observed in several white dwarfs and in the only visible neutron star – the one in the centre of the Crab Nebula. This latter object was first discovered because it was a pulsing radio source, or PULSAR. The term pulsar is another example of an historical misnomer :pulsars rotate ,and do not pulsate.It is also an example of our bias towards optical wavelengths .Many strong radio sources are variable ; some of them are associated with eruptive (light) variables .In this chapter ,however ,we concentrate on stars that vary in light

Pulsating and eruptive variability are examples of genetic variability: A star is born with certain properties, particularly the mass, that determine the future evolution of its luminosity and surface temperature. This evolution results from the finite nature of the star’s energy supply. As the star radiates energy, it depletes, and eventually exhausts its thermonuclear energy supply. This causes changes in the internal chemical composition and structure, and in the luminosity and radius, which can make the star unstable against pulsations or eruptions. During its lifetime a star may become unstable several times, and thus exhibit different types of variability. However, evolution is so slow that we rarely see evolutionary changes take place. We see stars as in a snapshot, for a brief moment in their long lives.

Evolution, and its relation to variability, can best be illustrated in the Hertzsprung- Russell (HR) diagram, a graph of luminosity, or absolute magnitude, against effective surface temperature,’ or some measure thereof. Most stars lie on or near a narrow diagonal band called the MAIN SEQUENCE ; variable stars lie on or near a narrow diagonal band called the MAIN SEQUENCE ;Variable stars lie in other specific areas of the HR diagram .Most pulsating variables lie in a vertical band which merges into a broad region in the upper right .Pre-main-sequence variable lie in the lower right ,supernovae in the upper right ,novae in the lower left ,and flare stars on the main sequence in the lower right

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