A red giant is a massive, luminous star in a late stage of stellar evolution that has dramatically expanded and cooled after exhausting the hydrogen fuel in its core. When a star like our Sun runs out of core hydrogen (typically after 8-12 billion years), it begins fusing hydrogen in a shell around an inert helium core. This process causes the star's outer layers to expand enormously—sometimes growing 100 to 1,000 times larger than their original size—while the surface temperature drops to around 3,000-4,500K, giving them their characteristic reddish appearance.</p><p>Famous examples include Arcturus, the fourth-brightest star in our night sky located 37 light-years away, and the massive Aldebaran in the constellation Taurus. Perhaps most fascinating is Betelgeuse in Orion, a red supergiant so enormous that if placed at our Sun's position, it would engulf the orbits of Mercury, Venus, Earth, and Mars.</p><p>Red giants were first systematically studied in the early 1900s when astronomers like Ejnar Hertzsprung and Henry Russell plotted stellar brightness against temperature, revealing these cool yet luminous stellar behemoths. Understanding red giants is crucial because they represent our Sun's fate in approximately 5 billion years, when it will swell beyond Earth's orbit before eventually shedding its outer layers to become a white dwarf.