Plot stars on the HR diagram—explore the main sequence, giants, and white dwarfs
The H-R diagram plots stellar luminosity versus temperature. It reveals that stars are not randomly distributed—they cluster into distinct populations: the main sequence, giants, supergiants, and white dwarfs. This structure reflects stellar evolution: stars spend most of their lives on the main sequence fusing hydrogen, then evolve off to become giants or supergiants before ending as white dwarfs, neutron stars, or black holes.
The diagonal band from hot, luminous O-stars to cool, dim M-dwarfs is the main sequence. Stars here fuse hydrogen in their cores. Mass determines position: massive stars are hot and bright (upper left), low-mass stars are cool and faint (lower right). The Sun is a G-type main sequence star. Stars spend 90% of their lives here—about 10 billion years for solar-mass stars, 100 million years for O-stars, trillions of years for M-dwarfs.
After exhausting core hydrogen, stars evolve off the main sequence. They expand enormously while surface temperature drops, becoming red giants or supergiants (upper right). These stars fuse helium and heavier elements in shells around an inert core. Betelgeuse and Antares are red supergiants—hundreds of times the Sun's radius, destined for supernova.
Low- and intermediate-mass stars end as white dwarfs (lower left): hot but faint because they're Earth-sized. Supported by electron degeneracy pressure, not fusion, they slowly cool over billions of years. Sirius B is a famous white dwarf, orbiting Sirius A. Eventually, all white dwarfs fade to black dwarfs (theoretically—none exist yet, the universe is too young).