Star (astronomy)

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Pulsating Variables

Pulsating variables are stars that rhythmically brighten and fade due to changes in the stars’ outer layers. Pulsating variables change in luminosity, temperature, and other characteristics as they expand and contract. Astronomers have identified two types of pulsating variables: periodic stars, such as Cepheid variables and RR Lyrae stars, and semiregular stars. Light emitted by periodic stars increases and decreases in regular cycles, while light emitted by semiregular stars fluctuates in irregular cycles.

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Cepheid Variables

Cepheid variables are a type of yellow supergiant star that vary in size and brightness in regular intervals, or periods, that last from 1 to 50 days. Typical Cepheid variable stars contain from 5 to 20 times more mass than the Sun does and shine about 10,000 times more brightly than does the Sun. As a Cepheid variable star pulsates, it expands beyond its equilibrium size, the size at which the inward force of gravity is offset by the outward pressure of the burning gases. The star’s rate of expansion slows as the force of gravity exceeds the outward pressure of the burning gases. At a certain point, the star begins contracting and continues until the outward pressure inside the star forces it to begin expanding again. This process causes the star to expand and contract, changing up to 30 percent in size over regular intervals. Delta Cephei, a member of the northern constellation Cepheus, is a Cepheid variable about 950 light years from Earth with a period (the time it takes for a cycle of expansion and contraction) of 5.37 days. Delta Cephei expands and contracts rhythmically, heating and cooling as a result of internal instability. The longer the period of a Cepheid variable is, the greater its intrinsic or absolute brightness is. Astronomers use this correlation to calculate the distance between Earth and remote Cepheid variable stars, which provide valuable distance indicators for measuring the extent of the Milky Way and the distance to other galaxies.

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RR Lyrae Stars

Named after the prototype star in the constellation Lyra, these pulsating variable stars have periods ranging from 88 minutes to just less than a day. RR Lyrae stars are old, giant stars mostly found in globular clusters. These stars all have about the same luminosity, regardless of the length of their period. As a result, these stars are important distance indicators for establishing the size of our Milky Way galaxy.

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Mira or Long Period Variables

Mira variables, named after Mira Ceti, the type star, are cool but luminous stars, with absolute brightness typically 3000 times that of the Sun. Mira variables change in brightness by at least 10 times, and sometimes by as much as 1000 times. These pulsational changes go in cycles of 100 to 500 days, so they are often called long period variables.

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Semiregular and Irregular Variables

Nearly all cool giants and supergiants show some variability, and are classified as semiregular or irregular variable stars. The brightness variations of these stars are neither quite punctual nor regular. Some irregular stars have variations that appear to be random or chaotic rather than appearing as pulsations. Betelgeuse, a red supergiant star marking the right shoulder of the constellation Orion the Hunter, is an irregular variable that has random fluctuations of brightness superimposed on a six-year cycle of brightness variation that is so small it is seldom noticed by visual observation.

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