The main sequence and stellar lifespans
Stars spend most of their active lives on the main sequence, fusing hydrogen in their cores. The location on the main sequence depends entirely on mass: a 10-solar-mass star appears upper left (hot, bright, short-lived), while a 0.5-solar-mass star appears lower right (cool, dim, long-lived). Sirius A and Rigel exemplify opposite ends of the main sequence. A star's position shifts only slightly during its main-sequence lifetime because the gradual depletion of core hydrogen is a slow process. For a sun-like star, the main sequence phase lasts roughly 10 billion years. For a massive star, it can be as short as a few million years. This is why massive stars are rare and transient, while low-mass stars are numerous and nearly eternal.
Red giants, white dwarfs, and the stellar graveyard
Once a star exhausts its core hydrogen, it expands and cools, becoming a red giant. Despite the name, red giants are cooler and dimmer than main-sequence stars of the same mass, but they are so much larger that their total luminosity can exceed their younger selves. Betelgeuse and Aldebaran are classic red giants. After shedding its outer layers as a planetary nebula, the hot, dense core that remains is a white dwarf, a stellar remnant roughly the size of Earth but with the mass of the sun. Sirius B, the famous white dwarf companion to Sirius, demonstrates this extreme density. White dwarfs cool slowly over billions of years, eventually becoming dark cinder-like objects called black dwarfs. The HR diagram's upper left to lower right progression thus tells the story of stellar death: the hottest, brightest objects in the upper left are either massive main-sequence stars destined for supernovae or white dwarfs still radiating leftover heat from stellar collapse.
Supergiants and the most extreme stars
The most massive stars never become red giants; instead, they swell into red supergiants before exploding as supernovae. Betelgeuse and Deneb represent different supergiant types. Their positions on the HR diagram are distinct from red giants by their extreme luminosity despite cool temperatures, reflecting enormous surface areas. Supergiants are unstable objects near the ends of their lives, fused up to iron and ready to collapse. The HR diagram ultimately maps stellar physics across 10 orders of magnitude in luminosity and nearly a factor of 10 in temperature, cataloging the universe's most extreme furnaces and its remnants.