Spheroids are generally dominated by stars older than 1 Gyr. In such
populations, changes with age are less dramatic than they are for
younger systems. The best age diagnostics come from photometry
reaching low-mass (~ 0.8
M
) main
sequence (MS) stars; such
photometry enables the reconstruction of the complete star formation
history with an age resolution of ~ 1 Gyr, but it is difficult to
obtain such photometry outside the Milky Way system, due to crowding
and depth limitations. Age constraints are also available through
photometry of later evolutionary phases, such as the horizontal branch
(HB), asymptotic giant branch (AGB), and red giant branch (RGB);
relative to MS stars, these brighter stars can be detected
in more distant and crowded regions, but the age resolution is poorer,
allowing one to distinguish between young (< 3 Gyr),
intermediate-age (3-8 Gyr), and old (> 8-13 Gyr) stars.