Stellar Halo

Essentially spherical population of stars and globular clusters thought to surround most disk galaxies and the cD class of elliptical galaxies. Only ~1% of a galaxy’s stellar mass is in its halo, and consequently observation of halos of other galaxies is extremely difficult. In contrast to the thin and thick disks of disk galaxies (q.v.), the halo generally has no net rotation and is supported almost entirely by velocity dispersion.
Halo stars in the Milky Way are generally old, most with ages >12 Ga. These ages are similar to those of bulge and globular cluster stars, indicating that halo stars were among the first Galactic objects to form. The halo stars are generally metal-poor, with a metallicity distribution peaking at ~1/30 of the solar value. The lowest metallicity star found in the Milky Way is a halo star with a metallicity of ~1/200,000 of the solar value.