Astronomy — Variable stars
- The evolution of a star after it leaves the main sequence swells it into a giant and moves it back and forth in the giant region of the H-R diagram. If a star passes through a region called the instability strip, it can become unstable and start to pulsate. Stars in the instability strip have the right temperature and luminosity to produce an energy absorbing layer of gas in their outer envelopes. This layer stores and releases energy much as a spring does and makes the star pulsate like a beating heart, expanding and contracting. The changes in surface area and temperature cause the brightness of these stars to vary. We will refer to the time a star takes to go from bright to faint and back to bright as the period of the star. The periods of these variable stars range from a few hours to hundreds of days. Through there are many types of variable stars, two kinds are especially important to our story of the discovery of the Milky Way galaxy. The RR Lyrae variable stars, named after variable star RR in the constellation Lyrae, have periods from 12 to 24 hours and are common in some star clusters. All RR Lyrae stars have about the same intrinsic brightness.
- Another type of variable star is the Cepheid, named after delta Cephei, the first star of this type to be found. Cepheids have periods of from 1 to 60 days, but they do not all have the same absolute magnitude. More massive stars cross the instability strip high in the diagram and are thus more luminous. Because of their greater mass and larger radius, they pulsate more slowly.
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