First, when stars of at least 0.4 solar masses exhaust their supply of hydrogen, their outer layers expand to form a red giant.
Evolution towards the red giant branch for the first time is very rapid while stars can spend much longer on the horizontal branch.
Most stars retain more of their hydrogen after the first red giant phase, and eventually become asymptotic giant branch stars.
This is the first ionized nebula to be have been discovered around a red giant star.
Serpentis, a cool red giant that pulsates between magnitudes 5.89 and 7.07 in 87 days.
Although many of those sources are expanding dust envelopes around red giant stars, a few are found to be associated with solar-type stars.
Sun-like stars will enter the red giant branch as subgiants.
Once a star has evolved sufficiently to become a red giant, its circumstellar habitable zone will change dramatically from its main-sequence size.
