A star whose brightness changes over time due to intrinsic physical processes like pulsation or eruption, or extrinsic causes like eclipses by companion stars. These brightness variations can range from barely detectable to dramatic changes of several magnitudes over periods from minutes to years.
From Latin 'variabilis' (changeable) and 'stella' (star), first used when astronomers in the 16th century noticed stars that appeared and disappeared. The term became formalized in the 18th century as systematic observations revealed the periodic nature of many stellar brightness changes.
The first recorded variable star was actually a nova observed by Chinese astronomers in 1181, but they thought it was a 'guest star' visiting from heaven! Today we know of over 150,000 variable stars, and they've become cosmic lighthouses for measuring distances across the universe.
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