the.com/red giant

means A red giant is an aging star that has exhausted the hydrogen fuel in its core, causing it to swell to many times its original size while its surface cools and reddens.

from The name is straightforwardly descriptive, stitched together in early 20th-century astronomy once astronomers began classifying stars by color and size using spectroscopy. 'Giant' marks the star's enormous expanded radius — up to hundreds of times the Sun's — and 'red' describes the cooler surface temperature (around 3,000–4,000 K) that shifts its emitted light toward the red end of the spectrum, following the same physics that makes a cooling ember glow red instead of white. The term crystallized alongside the Hertzsprung-Russell diagram, the plot that sorts stars by brightness and color and revealed giants as a distinct population separate from smaller, hotter 'dwarf' stars.