Stellar classification and the Hertzsprung-Russell diagram

A star's colour is a direct, visible consequence of its surface temperature, via the same blackbody radiation principle that governs any hot, glowing object: a hotter surface radiates more strongly toward shorter wavelengths, shifting its overall colour from red (cooler) through yellow and white to blue (hotter). Astronomers use this relationship to classify stars into spectral types, ordered from hottest to coolest, and quantify a star's brightness independent of its distance from Earth using luminosity, the total power the star radiates. Plotting a large sample of stars with temperature (or equivalently spectral type) along one axis and luminosity along the other produces the Hertzsprung-Russell diagram, one of the most information-dense and widely used diagrams in astrophysics.

The striking feature of this diagram, once enough stars are plotted, is that stars do not scatter randomly across it. The large majority fall along a single, fairly narrow diagonal band running from hot, highly luminous stars in the upper left to cool, dim stars in the lower right, called the main sequence. This pattern is not a coincidence: a star spends the great majority of its life fusing hydrogen into helium in its core, and for a star in this stable, hydrogen-burning phase, both its surface temperature and its luminosity are set almost entirely by a single underlying variable, its mass — a more massive star has a hotter, more luminous core, burns hotter at its surface, and shines with far greater total luminosity than a lower-mass star, producing the tight, orderly band seen on the diagram. The Sun, a fairly ordinary, middling-mass star, sits roughly in the middle of the main sequence.

Not every star lies on the main sequence, and the diagram's other regions correspond to distinct evolutionary stages away from stable hydrogen burning. Once a star exhausts the hydrogen fuel in its core, it evolves off the main sequence: its outer layers expand and cool while its luminosity can actually increase, moving it into the upper-right region of the diagram populated by giant and supergiant stars — larger, cooler, but very luminous due to their enormous surface area. Later still, many stars end their lives as white dwarfs, small, extremely hot, faint stellar remnants that occupy the lower-left region of the diagram — hot in surface temperature but dim because of their tiny radiating area. Because a star's position on the Hertzsprung-Russell diagram reflects both its mass and its evolutionary stage, the diagram serves simultaneously as a tool for classifying stars observed today and for reconstructing the life story of stars in general.