The stellar halo extends far outside a galaxy's brightest regions and typically contains its oldest and most metal-poor stars.
Early studies, investigating the shape of the stellar halo of the Milky Way, found some evidence that it may vary with increasing distance from the galaxy.
[3][4] More recent studies have found the halo to be flattened with a broken power law radius dependence; evidence for triaxiality is unclear.
[5] As a result of their faint brightness, observations of stellar halos in distant galaxies have required very long exposure times, the stacking of data from numerous galaxies to obtain averaged properties, or observing only the resolved stellar populations.
[8][9] As a result of the buildup from an assortment of satellite galaxies, variations in properties such as metallicity are present across stellar populations in halos.