Match the word to its description. 1: Stars that oscillate in size and shape, causing them to change in brightness. These stars obey a period-luminosity relation that predicts that a star of a given period of oscillation has a certain intrinsic brightness, but that stars of different period have differing brightnesses. 2: A special type of star that gets bigger and smaller, causing an apparent change in brightness. The pulsation comes from the fact that the star itself has entered an internally unstable state in its evolutionary history. This phase causes the star to “bounce” or oscillate back and forth across a stable size and brightness becoming alternately too large and fainter, and then too small and brighter.  3: Two stars that orbit one another. Often, one is large with a low surface brightness, and the other is small but very bright. As the stars orbit one another, the bright star is alternately visible and invisible. When both stars are visible the brightest light level is seen. 4: A graph of the brightness of a star over a period of time. It can be used to determine the shape of a pulsation to identify different types of variable stars. 匹配题