Spiral galaxies like our own Milky Way display a characteristic central bulge and "arms," along with a distinct glow arcing above and below the galaxy's flat disc. This glow is called the galactic halo.
The halo's inner regions closer to the galaxy contain globular clusters of very old stars called halo stars, while its outer regions are likely occupied by large amounts of dark matter.
A globular cluster is a collection of stars orbiting a galactic core. These clusters are tightly bound by gravity, giving them their spherical shapes and the high concentration of stars in their centers. They are essentially clumps of stars that are all gravitationally attracted to one another, and then orbit a larger body together, maintaining their grouping. Globular clusters contain many more stars and are much older than the thinner open clusters found in a spiral galaxy's disk.
Dark matter, lurking in the outer regions of a galactic halo, is still not wholly understood. Astronomers say it may consist of dust, planets, intergalactic gas or of MACHOs (Massive Astrophysical Compact Halo Objects), which are non-luminous bodies like extinguished stars or black holes. It comprises more than 90 percent of the mass of the universe but can be detected only indirectly, such as through the bending of light rays from distant stars by its gravity.
Halo stars are the oldest stars in their galaxies, and are usually grouped in a cluster formation. But for every star in a globular cluster, there are roughly 100 halo stars on their own, at least in the Milky Way. Around 1 to 2 percent of these primitive stars contain abnormally high quantities of the heaviest elements - like gold, platinum and uranium.
Early supernovae may have been able to expel these heavy elements in jets in different directions, allowing the elements' incorporation into some of the gas clouds that formed the stars we see in the halo today.
A galactic halo contains no gas, dust or star formation because halo stars formed when the galaxy did, and the nearby gas and dust that contribute to star formation was used up long ago. The halo of the Milky Way Galaxy is hard to measure precisely, thanks to its elusive dark matter, but according to current estimates, it extends up to 200,000 light years from the center of the galaxy.