One of the categories into which stars may be classified, based on their place in the evolution of the galaxy that they occupy. The stellar component of the Milky Way Galaxy consists of three populations: the thin disk, the thick disk, and the halo. The thin disk, originally referred to as population I, is the youngest, located amid most of the molecular and cold atomic gas, and confined to a height of order 1 kiloparsec (3.3 × 10³ light-years or 3.1 × 10¹⁶ km or 1.9 × 10¹⁶ mi) from the plane. The bulge and halo, roughly corresponding to the original population II, is a much older, far more extended structure, having an approximately spherical distribution with a scale length of order 3.5 kpc.