By Virginia Grant • Updated March 24, 2022
The Hertzsprung–Russell diagram is the gold standard for classifying stars by their luminosity and temperature, and it also reflects their physical sizes. From the gigantic red supergiants that dwarf our solar system to the faint, low‑mass brown dwarfs, a star’s radius can span several orders of magnitude. Apparent size on the sky is also influenced by distance and brightness, so a nearby white dwarf can appear brighter than a distant red supergiant.
Supergiants are the most luminous and massive stars, with masses exceeding ten times that of the Sun. As their cores exhaust hydrogen, they contract and heat, igniting helium fusion and subsequently heavier elements such as carbon, oxygen, neon, magnesium, and silicon. During this advanced stage, the outer envelope expands dramatically—often to the scale of the outer planets’ orbits—producing the iconic red supergiant. Some supergiants can contract again, heating their surfaces and shifting blueward on the H‑R diagram.
Giant stars have masses roughly 0.8 to 10 M☉. When core hydrogen runs out, the helium core contracts and ignites, while the envelope swells. The star brightens and cools, moving into the red‑giant branch. This phase can last tens of millions to a few hundred million years, depending on the star’s mass.
Stars on the main sequence—including our Sun—are in hydrostatic equilibrium, fusing hydrogen into helium in their cores. Their masses range from about 0.75 M☉ to 1.2 M☉ in the examples above. Once core hydrogen is depleted, they evolve into giants or supergiants. Higher‑mass stars exhaust their fuel more rapidly; a 10 M☉ star may live only a few million years, whereas a 1 M☉ star can burn for billions of years.
Brown dwarfs occupy the mass gap between the heaviest planets and the lightest stars. With masses between roughly 13 M_Jup and 75–80 M_Jup, they fuse deuterium (heavy hydrogen) but cannot sustain the proton–proton chain required for full stellar fusion. Objects below ~13 M_Jup never ignite fusion and cool steadily over time.
