The Red Giant Phase

The red giant stage is one of the most dramatic transformations in stellar evolution:

• The star swells to about 100 times its original diameter when core hydrogen runs out
• The helium flash ignites helium fusion at over 100 million Kelvin in lower-mass stars
• After helium runs out, the outer layers drift off as a planetary nebula
• The leftover core becomes a white dwarf — Earth-sized, holding much of the Sun's mass
• Over far longer than the universe's current age, white dwarfs should eventually fade into black dwarfs

Every Sun-like star travels this path. The glowing planetary nebulae we see through telescopes are the death shrouds of stars that once shone like our own.