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.

Globular Clusters:

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 (slightly) 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 clusters are generally old, with ages ranging from 9-12 billion years. However, there's still a lot of uncertainty 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 108 - 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!

The metallicity distribution of field stars in the Milky Way's halo shows lots of very metal poor stars, with an long tail to extremely metal poor values of [Fe/H] < -3.5. (Figure taken from Prantzos 2008)

Spectra of extreme metal-poor stars (solar, -4, -5.3, zero metals), courtesy ESO:

Stars this chemically unenriched must trace the early history of star formation in the Galaxy!

Halo substructure

The distribution of stars in the Milky Way's halo is not smooth, but shows evidence for stellar streams.

Below: A plot showing the distribution of halo stars on the sky. In this plot, color is not the color of the star, but distance (blue nearer, red further).

(courtesy Vasily Belokurov)


On larger scales, these streams can be overlaid on other measurements of structure in the galactic halo, showing how the big SDSS stream continues around the whole galaxy:

(RA and dec are coordinates on the sky, like latitude and longitude on Earth)

We see similar streams around the nearby Andromeda Galaxy:

(Geraint Lewis, U Sydney)
And around other galaxies:

(David Martinez-Delgado)

Making satellite streams:

So the halo has a significant component which comes from tidally shredded satellite galaxies:

(Vasily Belokurov, Cambridge U)