Not all stars shine with a constant magnitude. Many are variable, their brightness changing in cycles that may last hours or years, or may be completely unpredictable. This variability can have many causes.
pulsating variables many stars pass through a period of instability at some point in their lives. T-Tauri stars, which vary in size and brightness as they settle onto the main sequence, are one example. The most common type, however, is a star that leaves the main sequence and swells into a giant. Changes in its internal composition can make its outer layers transparent or opaque, altering how they are affected by the outward pressure of radiation, and causing the star to
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