Galactic spheroid
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Spiral galaxies have a typical structure related to their history. These stars have random orbits: the difference between their orbital velocities (or velocity dispersion) is much greater than their mean velocity. They are as likely to orbit in the opposite direction as the sun as in the same direction or perpendicular. These stars are typically old and have a low abundance of heavy elements such as carbon, oxygen, and iron, with abundances typically 0.01 of solar abundance. Spheroid stars are relics from the early turbulent history of the galaxy before most of the gas settled into a galactic disk. The galactic halo also contains many globular clusters, tight knots of old stars.
At present, it is controversal to what extent halo stars formed monolithically with the developing Milky Way, or are remnants of other smaller galaxies which merged with the Milky Way. This process is continuing to this day: the Sagittarius Dwarf Elliptical Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it.
As the gas of the galaxy cooled, it settled into a disk and later formed stars. Thus, the bulk of the stars in a spiral galaxy are located either close to a single plane (the Galactic plane) in more or less conventional circular orbits around the center of the galaxy (the galactic centre), or in a spheroidal galactic bulge around the galactic core. Stars in the galactic halo are known as Population II, much older and with much lower metallicity than their Population I cousins in the galactic disk.
Since the sun and other nearby disk stars are all orbiting the galaxy together in the same direction, their relative velocity is low. In contrast, Halo stars will typically have high velocities relative to the sun and low metallicities. They can thus found in samples of high proper motion stars. Prominent nearby halo stars are Kapteyn's Star and Groombridge 1830.
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