Observations of variable stars
What are variable stars?
Simply put variable stars are stars that varie their brightness (magnitude) through either intrinsic, events occurring on the star itself, or extrinsic processes, events occurring in the region of the stars.
Why observe them?
There are thousands of variable stars in the night sky. They can be observed with the naked eye, binoculars and telescopes. Variable star observing is one of the last bastions of amateur astronomy where the amateur can make a valuable contribution to our understanding of stellar evolution with only the most modest of equipment. You don’t need expensive CCD’s, telescopes or complex computer programmes, in some cases all you need is a paper, pen, star charts and your naked eye!
Amateur observation of variable stars is critical. There are not enough professional astronomers to observe all the stars needing observation so the task falls upon the shoulders of the amateur community who observe and report their findings to organisations like the AAVSO (aavso.org). Professional astronomers then use the data submitted by amateurs to further their understanding of stellar life cycles.
If you want to make a contribution to serious amateur astronomy then becoming a variable star observer could be for you!
“...the amateurs are doing the work the professionals cant. They are providing a vital role and making discoveries all the time so hopefully that will continue as the continued observations of variable stars is very important” Sir Patrick Moore (Patrick Moore on Variable Stars. An interview with David Scanlan. Popular Astronomy Magazine. April-June 2008.
Some of the types of variable stars that we observe can be:
Pulsating variables are stars showing periodic expansion and contraction of their surface layers. The pulsations may be radial or non-radial. A radially pulsating star remains spherical in shape, while in the case of non-radial pulsations the star's shape periodically deviates from a sphere, and even neighbouring zones of its surface may have opposite pulsation phases.
Eruptive variables are stars varying in brightness because of violent processes and flares occurring in their chromospheres and coronae. The light changes are usually accompanied by shell events or mass outflow in the form of stellar winds of variable intensity and/or by interaction with the surrounding interstellar medium.
Variable stars with non-uniform surface brightness and/or ellipsoidal shapes, whose variability is caused by axial rotation with respect to the observer. The non-uniformity of surface brightness distributions may be caused by the presence of spots or by some thermal or chemical inhomogeneity of the atmosphere caused by a magnetic field whose axis is not coincident with the rotation axis.
These are variable stars showing outbursts caused by thermonuclear burst processes in their surface layers (novae) or deep in their interiors (supernovae). We use the term "nova like" for variables that show nova like outbursts caused by rapid energy release in the surrounding space (UG-type stars) and also for objects not displaying outbursts but resembling explosive variables at minimum light by their spectral (or other) characteristics. The majority of explosive and nova like variables are close binary systems, their components having strong mutual influence on the evolution of each star. It is often observed that the hot dwarf component of the system is surrounded by an accretion disk formed by matter lost by the other, cooler, and more extended component.
Above: Chandra X-Ray Observatory image and illustration of a Cataclysmic Variable. The white dwarf is drawing matter away from its supergiant companion, forming an accretion disc in the process. When Hydrogen builds up in excess the white dwarf ignites the hydrogen in massive thermonuclear explosions and variable star observers are treated to the views of a nova. ©NASA Chandra X-Ray Observatory
Close Binary Eclipsing:
We adopt a triple system of classifying eclipsing binary systems: according to the shape of the combined light curve, as well as to physical and evolutionary characteristics of their components. The classification based on light curves is simple, traditional, and suits the observers; the second and third classification methods take into account positions of the binary-system components in the (MV ,B-V) diagram and the degree of inner Roche lobe filling. Estimates are made by applying the simple criteria proposed by Svechnikov and Istomin (1979). The symbols for the types of eclipsing binary systems that we use are given below.
Classification based on the shape of the light curve
E Eclipsing binary systems. These are binary systems with orbital planes so close to the observer's line of sight (the inclination of the orbital plane to the plane orthogonal to the line of sight is close to 90 deg) that the components periodically eclipse each other. Consequently, the observer finds changes of the apparent combined brightness of the system with the period coincident with that of the components' orbital motion.
EA Algol (Beta Persei)-type eclipsing systems. Binaries with spherical or slightly ellipsoidal components. It is possible to specify, for their light curves, the moments of the beginning and end of the eclipses. Between eclipses the light remains almost constant or varies insignificantly because of reflection effects, slight ellipsoidality of components, or physical variations. Secondary minima may be absent. An extremely wide range of periods is observed, from 0.2 to >= 10000 days. Light amplitudes are also quite different and may reach several magnitudes.
EB Beta Lyrae-type eclipsing systems. These are eclipsing systems having ellipsoidal components and light curves for which it is impossible to specify the exact times of onset and end of eclipses because of a continuous change of a system's apparent combined brightness between eclipses; secondary minimum is observed in all cases, its depth usually being considerably smaller than that of the primary minimum; periods are mainly longer than 1 day. The components generally belong to early spectral types (B-A). Light amplitudes are usually <2 mag in V.
EP Stars showing eclipses by their planets. Prototype: V0376 Peg.
EW W Ursae Majoris-type eclipsing variables. These are eclipsers with periods shorter than 1 days, consisting of ellipsoidal components almost in contact and having light curves for which it is impossible to specify the exact times of onset and end of eclipses. The depths of the primary and secondary minima are almost equal or differ insignificantly. Light amplitudes are usually <0.8 mag in V. The components generally belong to spectral types F-G and later.
All of these types of variable stars listed above have numerous other facets and sub categories. Only the types of variable stars that are the easiest and most enjoyable stars to observe have been listed here. More information of all the differing types of variable stars can be found by visiting the pages of the General Catalogue of Variable Stars at http://www.sai.msu.su/gcvs/gcvs/vartype.htm