Variable stars are celestial beacons whose brightness fluctuates over time, creating a cosmic lighthouse effect that has captivated astronomers for centuries. These stellar performers change their luminosity through two main mechanisms: intrinsic variability, where the star itself physically pulsates, expands, or undergoes internal changes, and extrinsic variability, where external factors like eclipsing companions or rotating starspots cause apparent brightness variations.</p><p>The most famous example is Polaris, our North Star, which subtly pulsates with a period of about 4 days, changing by roughly 0.15 magnitudes. Delta Cephei, discovered in 1784, gave its name to the crucial Cepheid variable class—massive stars that pulsate predictably, with periods ranging from 1 to 100 days. These "standard candles" revolutionized astronomy when Henrietta Swan Leavitt discovered their period-luminosity relationship in 1908, enabling astronomers to measure cosmic distances and ultimately leading to the discovery of the expanding universe.</p><p>Another dramatic example is Algol, the "Demon Star," which dims by over one magnitude every 2.87 days when its fainter companion eclipses it. Variable stars serve as cosmic laboratories, revealing stellar evolution, internal structure, and even hosting planets. Today, automated surveys monitor millions of variables, with some changing brightness in minutes while others cycle over decades.