LIFE OF THE STARS
VARIABLE STARSThe stars do not shine as constantly as they appear to at first sight. Stars that vary in brightness are known as variable stars. In some variables, such as pulsating, eclipsing, and rotating ones, there is a regular pattern or period to their variation. Others, such as eruptive and cataclysmic variables, are more unpredictable in their behavior. A star may vary because it gives out changing amounts of light is obscured by shifting dust clouds or a companion star. By plotting graphs, or light curves, of the stars brightness, astronomers can work out why the brightness varies.
The brightness of eat Carinae has fluctuated dramatically since it was recorded by Edmond Halley in 1677. By the middle of the 19th century, it had become the second brightness star in the sky at magnitude 0.8, but then suddenly plunged to below magnitude 6. Eta Carinae had thrown out a thick cloud of obscuring dust now known as the Homunuculus Nebula. The shifting dust and the stars unstable outer layer account for the variations in its brightness. Eta Carinae is classed as an eruptive variable.
Stars that burst into brilliance when they undergo sudden, violent changes are cataciymic variables. They include novas and supernovas. A nova occurs when a while dwarf in a double, or binary, star system pulls hydrogen gas off its companion. The gas builds up until there is a nuclear explosion. In 1975, a nova appeared in Cygnus, briefly making the binary star 40 million times brighter
Stars that brighten or fade with no regular pattern are
called eruptive variables. Their brightness varies, as violent changes occur in their
outer atmospheres. Some puff out clouds of smoke that make them suddenly fade. Other, such
as T Tauri, are young stars still shirking to a stable size as stellar winds blow away the
dust and gas from which they formed.