Digital Universe Guide - Hayden Planetarium

3.3. MILKY WAY DATA GROUPS 131
3.3.33 The Galactic Halo
Group Name halo
Reference Merrifield, M. & Binney, J. 1998, Galactic Astronomy
(Princeton: Princeton University Press)
Prepared by Brian Abbott (AMNH/Hayden)
Labels No
Files halo.speck
Dependencies none
Surrounding the disk of the Milky Way is the spherical Galactic halo. Unlike the population of the
disk, where stars continue to form, the halo population is devoid of gas and dust and, therefore, star
formation. It contains cooler, dimmer stars left over from an era of star formation in the Galaxy’s early
history. There are no young, luminous stars in this population.
The main player in the Galactic halo is the system of globular clusters (turn them on with the gc
button). Numbering about 150, the Milky Way’s globulars are spherically distributed about the Galactic
center, and most are within the halo. The stars in the halo are so intrinsically dim that it is difficult to see
them unless they are very close to the Sun. These stars travel in elliptical orbits about the center of the
Galaxy that are not confined to the disk but are distributed throughout the spherical halo.
Many properties of the halo are still under consideration by astronomers. With the introduction of
dark matter and the discovery of more small satellite galaxies around the Milky Way, the halo seems to
come under continual scrutiny. We place the radius of the halo at about 134,000 light-years (41 kpc);
however, some astronomers believe it to be far larger, perhaps as much as
300,000 − 800,000 light-years in radius.