Stars, Galaxies and the Universe

A star's color is a thermometer you can read from light years away. Hot surfaces radiate more energy at short wavelengths, so the hottest stars look blue or white, while cooler stars peak at longer wavelengths and look orange or red. No spectroscope needed, just color: blue means hot, red means cool. This is the first thing to pull out of any star description.

The Hertzsprung-Russell diagram plots temperature on the horizontal axis, running backwards (hot on the left, cool on the right), against luminosity on the vertical axis. Most stars sit on a diagonal band called the main sequence, running from hot-and-bright in the upper left to cool-and-dim in the lower right, because for ordinary stars hotter also means more luminous. A star spends most of its life on this band, fusing hydrogen in its core.

Once a star uses up the hydrogen in its core, it leaves the main sequence and moves up and to the right: it swells up, so luminosity rises, while its outer surface cools, so it reddens. That track lands it among the red giants, which are huge and bright but cool, unlike the small, hot, dim white dwarfs that form later when the core collapses. This is why apparent brightness in the sky can mislead: brightness also fades with the square of distance, so a dim but close star can outshine a brilliant but distant one.