Understanding stars that blink
“ Maybe that ’ s what life is … a wink of the eye and winking stars .”
– Jack Kerouac
By GABRIEL CRISTIAN NEAGU
Guest Contributor
In the 16th century a lot of discoveries and new theories were arising on Earth , though one thing was known for sure : the stars are fixed . The sun was moving , the planets , the moon . During the winter of 1572 that myth was shattered by Tycho Brahe who observed a dazzling star , brighter than Venus where nothing was observable before , in Cassiopeia . This started a revolution in cosmology still ongoing . When the Chandra space telescope observed the coordinates of the object it saw the fascinating turbulent debris of the supernova observed by Tycho in 1572 . A lot of other similar objects were discovered in the past . Those are called cataclysmic variable stars . Those are stars in which brightness increases by a large factor irregularly by a large factor and then drops back to quiescent state . The first ones discovered were called “ novae ” meaning “ new ” in Latin because we observed them as new stars on the sky . Cataclysmic variables are binary systems that consist of a white dwarf primary and a mass transferring secondary ( most times a red giant ). Because of gravity , the matter from the companion creates an accreting disk around the primary and drops on it . When density and temperature on the white dwarf rise enough to start runaway hydrogen fusion reactions , which convert the hydrogen layer into helium in a very fast way , the system transforms into a nova . If the process continues long enough so
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A Chandra view of the Tycho ’ s Nova remnant
the dwarf reaches the Chandrasekhar limit , runaway carbon fusion starts and we see a type Ia supernova that completely destroys the white dwarf . If you think this is all , there is always something new in the world of variable stars . There are four more types : eclipsing variables , rotating variables , pulsating variables and eruptive variables . The eclipsing variable stars eclipse each other from the Earth ’ s vantage point creating a dip in brightness with the period equal to the period of the stars rotating around each other . This is the case of Algol , also known as the “ Demon star ”, one of the first variable stars discovered that are not novae or supernovae . Algol ’ s magnitude is constant at 2.1 but periodically drops to 3.4 every 2.86 days . The oldest
COURTESY OF NASA / CXC / SAO
documentation of this star comes from an Ancient Egyptian “ Calendar of Lucky and Unlucky Days ” composed about 3200 years ago . Figure I on the following page is a “ phase plot ”. This means on the y axis the brightness is presented , and on the x axis we see the phase of the variability . Rotating variable stars are binary stars that do not eclipse each other but their brightness still changes because of changes in the amount of light emitting area visible to us . Those fluctuation do not exceed 0.1 magnitudes so we can ’ t observe them with the naked eye , needing special equipment for this : CCD / CMOS cameras mounted on a telescope . My favorite type of variable stars are the pulsating ones . Those stars swell and shrink in a periodic manner .