The Galactic Stellar Halo 

The third luminous component of the Galaxy is the stellar halo, comprised of globular clusters and field stars which have high space velocities that can take them far out of the galactic disk.

Originally it was thought that all globular clusters were part of the halo. Now, however, it is realized that two distinct populations of globulars exist. Old, metal-poor clusters ([Fe/H] < -0.8) are part of an extended, spherical halo, while younger clusters with [Fe/H] > -0.8 are in a more concentrated and flattened distribution.
 
 

The less-metal-poor clusters have a scale height similar to that of the thick disk, and they may be associated. Other ideas have them related to the Galaxy's bulge instead..

Globular Cluster ages:

Young: 9-12 billion years old
Old: 12-14 billion years old
but there's still a lot of controversy about absolute ages here...
Field Stars
How do we find field halo stars? Look at the space velocities of stars with respect to the Sun. If they are low, they are probably disk stars (like the Sun). If they are high, they are usually associated with the halo.

Like the GCs, the halo field stars are also metal-poor. This tells us something about the formation of the Galaxy!

If we add up all the mass in the field stars and the metal-poor GCs, we can come up with a rough density distribution for the Galaxy's stellar halo:
Where n0 is about 0.2% of the thin disk's central density.

The total mass of the halo is about 109 M sun, about 1% of which is the globular clusters, and the rest in field stars. So there's not a lot of stuff in the stellar halo, but what is there holds a lot of information about the early history of the Galaxy!
 
Below: Star streams in the galactic halo from the SDSS survey (Belokurov et al 2006). Upper main sequence/turnoff stars have been selected via a color cut; these stars should have similar luminosities, so their apparent magnitude is a measure of distance. In this plot, color is not the color of the star, but distance (blue nearer, red further).


 
On larger scales, these streams can be overlaid on 2MASS measurements of structure in the galactic halo, showing how the big SDSS stream connects with the larger Sagittarius stream detected by 2MASS: