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. A globular cluster is a spherical bundle of stars (star cluster) that orbits a galaxy as a satellite. ...

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. The Sagittarius Dwarf Elliptical Galaxy (Sag DEG) is a satellite galaxy of the Milky Way Galaxy. ...

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. The galactic plane is the plane in which the majority of a flattened galaxys mass lies. ... In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... The Galactic Center is the rotational center of the Milky Way galaxy. ... A spheroid is a quadric surface in three dimensions obtained by rotating an ellipse about one of its principal axes. ... It has been proposed below that Bulge (astronomy) be renamed and moved to galactic bulge. ... Stars can be grouped into two general types called Population I and Population II. The criteria for classification include space velocity, location in the galaxy, age, chemical composition, and differences in distribution on the Hertzsprung-Russell diagram. ... In astronomy, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. ...

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 Barnard's Star, Kapteyn's Star and Groombridge 1830. The proper motion of a star is the motion of the position of the star in the sky (the change in direction in which we see it, as opposed to the radial velocity) after eliminating the improper motions of the stars, which affect their measured coordinates but are not real... Barnards star is a star in the constellation Ophiuchus which is notable for having the largest proper motion (10. ... Kapteyns Star (also known as GJ 191, HD 33793 or CD -45 1841) is a class M0 subdwarf discovered by Jacobus Kapteyn in 1897. ... Groombridge 1830 is a star. ...