ellow Giants are former high or low mass stars. The highly luminous yellow giants are former High Mass Stars while those less luminous are former Low Mass Stars.
o matter their size, all yellow giants have the unusual property of pulsating. Their size is in a state of constant flux which varies their luminosity which is why astronomers refer to them as "variables" or "pulsating stars"
he time it takes the star to complete a pulsation cycle (bright to dim and then back to bright) is called a "period."
iant stars pulsate because of the radiated gas which is trapped by their atmospheres. This trapping heats the outer layers which causes increased pressure which in turn makes the layers expand. The expanded gas cools and gravity pulls the layers back down to be recompressed. The recompressed gas starts to absorb energy and the cycle is repeated.
he property necessary in the star's atmosphere to trap radiation is called "opacity." These special absorbing properties can only occur when the surface temperature and radius fall within a certain range. That range is commonly referred to as the "instability strip." A star will start pulsating when it reaches that instability strip and won't stop until its temperature or radius changes enough to move it out of that range. The time spent in this instability strip depends on its mass.
