The galactic halo
Published online by Cambridge University Press: 14 August 2015
The objects from which the structure of the galactic halo may be derived by optical means are the extreme population II objects such as RR Lyrae variables with periods longer than 0-4 days, the sub-dwarfs, globular clusters, stars similar to those found in globular clusters such as the bright red giants, and the blue stars found in the galactic polar regions. The longperiod variables with periods shorter than 200 days and the ordinary highvelocity stars are now regarded as belonging to an intermediate population II, somewhat younger than the halo population II proper.