The
Halo
The galactic halo has a spherical shape and is composed by stars that
are even older and metal poor than those in the central bulge.
Globular Clusters
In the halo, the stars are packed in globular clusters, which hold
higher or lower metallicities according to their
position in regard to the galactic centre (those located closer to the centre
hold lower metallicities and those located closer to it hold higher
metallicities).
The globular clusters can display very high densities (like 1 million
stars tightly packed in a sphere only 30 light years wide) or can be very
rarefied. They are composed by a range between 50 000 to several million stars
located around a nucleus and have a vaguely spherical distribution.
It's estimated that the globular clusters that are most distant from the
galactic centre have been formed during the epoch when occurred the collapse of
the gaseous mass that generated the primordial body of the
Milky Way. On the other hand, those who are closer to the centre would have
initially been dwarf galaxies that were later captured by our galaxy.
The movement of the globular clusters, cyclically crossing the plane of
the disk, would have stripped the clusters
from the early gas content, so none of it was left in order to forge new stars.
It's possibly for that reason that the stars of the globular clusters are the
oldest known.
A globular cluster
(Davidson E. Soper)
Dark Matter and Interstellar Material
Given the high rotation speed of the outer layers of the spiral
galaxies, it's thought that, along with the globular clusters, the halo
concentrates huge amounts of dark matter, which still has an
uncertain nature, and some other rarefied interstellar material mainly composed
by protons and electrons.
_