Presented at the AAVSO
Session on Mira Stars (part of the 85th Annual Meeting), November 1, 1996
Some aspects of the nature
and evolution of Mira stars and their small-amplitude relatives are briefly described.
As stars ascend the asymbiotic giant branch, it appears that they begin to
pulsate when their effective temperature drops below 3800 K. The amplitude
tends to increase with decreasing temperature. The stars show appreciable
periodicity, perhaps beginning to pulsate in a high radial overtone, then in
lower overtones as they approach the Mira stage. Mode-switching is occasionally
observed in the small-amplitude variables. There is appreciable irregularitiy
in the latter stars, including long-term variability of unknown cause. Our
understanding if these stars will certainly increase as a result of new
Hipparcos parallaxes and other advances. The observation and analysis of these
and other variables provides fruitful projects for students.