PX158 - I2 - stellar classes

main-sequence

star is fusing hydrogen in its core
covers the majority of a star's lifetime: $\sim M y r \to 100 - 1000 G y r$
the sun will be in the main-sequence for $\sim 12 G y r$
using luminosity-temperature-radius of stars relation, $R_{s} \approx 0.1 R_{⊙} \to 10 R_{⊙}$

red giant

after hydrogen in the core has depleted, helium fusion begins
star expands and outer layers cool
for the sun, $R_{⊙, r e d g i a n t} \approx 1 A U$
$R_{R G} \approx 10 R_{⊙} \to 1500 R_{⊙}$
a star will not spend much time in this phase
for the sun, it will be about $1 G y r$

white dwarf

the end states of most main-sequence stars
$T_{W D} \sim 25, 000 K \to 3000 K$
$R_{W D} \sim 0.01 R_{⊙} \approx R_{\oplus}$
in an isolated state, they remain in this state