Observation Variable Stars

Variable stars are stars which vary in luminosity for a reason or another. These variations are slight however. Variable stars observation is mostly an amateur domain and may be a rewarding activity although CCD photometry and automated sky surveys are bringing a progressive change into variable stars observation. Amateurs are entering the field of CCD anyway, their famous association, the American Association of Variable Star Observers (AAVSO), might turn to manage the overwhelming archives produced by these new techniques. Such a time is still ahead however and amateur role is thought to remain important. Amateurs work about variable is mainly using magnitude charts to assess a variable magnitude: two stars in the field, magnitude of which are estimated each side of the variable, allows to give the latter a magnitude value

Variable stars are sorted into intrinsic variables (star vary due to physical changes) and extrinsic (star vary due to orbital effects). These two categories part in turn into two categories each: intrinsic variables are either pulsating or eruptive. Asteroseismology, or the study of the interior of stars, is also used in such studies because the variability of stars depends in complex ways on the properties of their interiors. Extrinsic variables are either eclipsing binaries or rotating stars. Variable stars generally might be more prone to stellar flares of hot plasma as they are rotating fast, or in rapidly orbiting binary systems

Intrinsic Variables

• Pulsating Variables: these stars vary due to periodic expansion and contraction of their external layers. Most common types of pulsating are: Cepheids (period:1-70 days, variation: 0.1-2.0 mag), RR Lyrae (0.2-1.0 days, 0.3-2.0 mag), RV Tauri (30-100 days, up to 3.0 mag), Long period variables (80-1000 days, 2.5 to 5.0 mag), and semiregular (30-1000 days, 1.0 to 2.0 mag). Generally, long-period variables are giant pulsating stars whose brightness can take up to, and even over, a thousand days to change. Most red giants are placid and calm, but some pulsate with periods between 80 and 1,000 days like the star Mira and undergo large changes in brightness. This subset of red giants is called 'Mira variables.' Kepler mission allowed to discover a vast sea of low-amplitude pulsation stars that were undetectable from Earth
• Eruptive Variables (or Cataclysmic Variables). these stars have violent occasional outbursts due to surface or interior processes). Most common types are: supernovae (no period, up to 20 mag), novae (1-300 days or more, 7-16 mag), recurrent novae (1 or 200 days or more, 7-16 mag), dwarf novae (2-6 mag) -parted themselves into three types (U Geminorum (30-500 days, 2-6 mag), Z Camelopardalis (alternating variation and constant brightness periods), SU Ursae Majoris (alternating frequent, faint, 1-2 days and rarer, bright, 10-20 days variations), symbiotic stars (semi-periodic, up to 3 mag), R Coronae Borealis (irregular, up to 9 mag)

Extrinsic Variables

• Eclipsing binaries: these variables are due to binary systems periodically eclipsing each other. Period is the orbital period of the system. Such variable are usually short-period eclipsing binaries
• Rotating Stars: variability of these stars might be due to spots or patches existing on their surface. When these are in the observer's line of sight, there is a dimming

Some additional types of variables may be found too: flare stars (or UV Ceti stars; outbursts occur at localized areas of their surface), irregular variables (most red giants are of this type; they are of the pulsating type). Some stars raise questions about the origin of the variations, with a unexpected pattern of tiny regular variations of the order of 0.1 percent, with a period between about two and 20 hours. Such a new class of variable stars is yet to be given a name as such a precise study was made possible by a high quality of instrument and observation as a spin at speeds that are more than half of the critical velocity, which is the threshold where stars become unstable and throw off material into space, might be a explanation

Variable stars most known amateur association is the American Association of Variable Star Observers (AAVSO). You will find there futher information. Most of above data have been taken from one of their page

Website Manager: G. Guichard, site 'Amateur Astronomy,' http://stars5.6te.net. Page Editor: G. Guichard. last edited: 6/3/2016. contact us at ggwebsites@outlook.com