Astronomy: Variable Stars

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Variable Stars A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes systemati cally with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either: Intrinsic variables, whose inherent luminosity changes; for example, because the star swells and shrinks. Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star has an orbiting companion that sometimes eclipses it.

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Why Variable Stars matters: Stars are the engines of the cosmos -- they forge the chemical elements, light up galaxies, and create the conditions that make planets and life possible. Intrinsic variables, whose inherent luminosity changes; for example, because the star swells and shrinks. Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Ea rth; for example, because the star has an orbiting companion that sometimes eclipses it. Depending on the type of star system, this variation can include cyclical, irregular, fluctuating, or transient behavior. Changes can occur on time scales that range from under an hour to multiple years. Many, possibly most, stars exhibit at least some oscillation in luminosity: the energy output of the Sun, for example, varies by about 0.1% over an 11-year solar cycle. At the opposite extreme, a supernova event can briefly outshine an entire galaxy. Of the 58,200 variable stars that have been catalogued as of 2023, the most common type are pulsating variables with just under 30,000, followed by eclipsing variables with over 10,000. Variable stars have been observed since the dawn of human history. The first documented periodic variable was the eclipsing binary Algol. The periodic variable Omicron Ceti, later named Mira, was discovered in the 17th century, followed by Chi Cygni then R Hydrae. By 1786, ten had been documented. Variable star discovery increased rapidly with the advent of photographic plates. When Cepheid variables were shown to have a period-luminosity relationship in 1912, this allowed them to be used for distance measurement. As a result, it was demonstrated that spiral nebulae are galaxies outside the Milky Way. Variable stars now form several methods for the cosmic distance ladder that is used to determine the scale of the visible universe. The periods of eclipsing binaries allowed for a more precise determination of the mass and radii of their component stars, which proved especially useful for modelling stellar evolution.

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Deep dive: Variable Stars Intrinsic variables, whose inherent luminosity changes; for example, because the star swells and shrinks. Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star has an orbiting companion that sometimes eclipses it. Depending on the type of star system, this variation can include cyclical, irregular, fluctuating, or transient behavior. Changes can occur on time scales that range from under an hour to multiple years. Many, possibly most, stars exhibit at least some oscillation in luminosity: the energy output of the Sun, for example, varies by about 0.1% over an 11-year solar cycle. At the opposite extreme, a supernova event can briefly outshine an entire galaxy. Of the 58,200 variable stars that have been catalogued as of 2023, the most common type are pulsating variables with just under 30,000, followed by eclipsing variables with over 10,000. Variable stars have been observed since the dawn of human history. The first documented periodic variable was the eclipsing binary Algol. The periodic variable Omicron Ceti, later named Mira, was discovered in the 17th century, followed by Chi Cygni then R Hydrae. By 1786, ten had been documented. Variable star discovery increased rapidly with the advent of photographic plates. When Cepheid variables were shown to have a period-luminosity relationship in 1912, this allowed them to be used for distance measurement. As a result, it was demonstrated that spiral nebulae are galaxies outside the Milky Way. Variable stars now form several methods for the cosmic distance ladder that is used to determine the scale of the visible universe. The periods of eclipsing binaries allowed for a more precise determination of the mass and radii of their component stars, which proved especially useful for modelling stellar evolution. Source: https://en.wikipedia.org/wiki/Variable_star (Wikipedia, CC BY-SA)