The conventional picture of the formation of the Galaxy was developed to explain the spatial distribution, motions, and chemical properties of the stars that are found in the Galaxy. Initially, two distinct groups of stars, or stellar populations, were recognized by their very different properties.
The most distinct component of what was defined as Population I are the open clusters and associations whose brightest stars are the luminous, blue, and young O and B stars. Such clusters are often associated with the interstellar material out of which these stars recently formed. On the other hand, the globular clusters representing Population II are very different stars, containing no O and B stars or gas and dust, but filled with old red giant stars.
Population clusters' differences include more factors than simply their time of formation, however, because they differ significantly in their space distribution and motions. Open clusters, for example, are located in the disk and have small velocities relative to the Sun. On the other hand, globular clusters are located in a spheroidal halo concentrated in the galactic center and are generally observed to have large velocities relative to the Sun. Chemically, the open clusters are similar to the Sun, possessing a fraction of heavy elements that range from about one‐third to twice the solar abundance. In contrast, the globular clusters are relatively metal poor, with heavy element abundances between 0.001 and 0.5 times solar abundance.
The characteristics of these two classes of star clusters are indicative of the overall characteristics of other stars in the halo and disk. Astronomers now understand that their properties characterize not two truly distinct populations, but rather the extremes of a continuous distribution of stellar types, whose properties range from the spheroidally distributed, metal‐poor stars to those metal‐rich stars confined to a very thin plane in the disk. Stars with an even smaller content of heavy elements are the nearly pure hydrogen‐helium stars, which have been discovered and represent the once hypothetical Population III, the first generation of stars in the Galaxy.
In the standard model for the formation of the Galaxy, the motions of the stars and their spatial distribution as observed at the present time reflect the conditions during the phase in which they formed. This is postulated to have begun very early in the history of the universe when some 10 12 solar masses of primordial hydrogen and helium gas began to collapse under its own self‐gravitation. The first stars to form would have been pure hydrogen and helium; but rapid stellar evolution of massive stars and their subsequent supernovae would have “polluted” the remaining interstellar material with heavy elements. The next generation of stars (Population II) would have had a small fraction of heavy elements, but their stellar evolution would have lead to ever greater additions to the heavy element content of the interstellar medium. The earliest generations of stars (including the globular clusters) forming during the collapse phase retain a memory of this in their nearly radial orbits. The gas, still the largest fraction of the mass of the Galaxy at this era, progressively flattened into a rotating disk because of angular momentum conservation, with each successive generation of stars being marked by a spatial distri‐bution indicative of the gas from which they formed. During the flattening, collisions between the gas particles regularized motions until only circular motions survived. This process has continued to the present day, with the remaining interstellar gas, now significantly enriched with metals, in a very thin plane, in which the most recent Population I stars continue to form.
Many aspects of the present Galaxy, however, suggest that the true process of formation has been more complicated. A major alternative theory suggests that the collapse of pre‐existing gaseous material again formed very flat disks, smaller galaxies similar to, but not quite the same, as the spiral galaxies identified in the present universe. Assemblages of these proto‐spiral galaxies merged over time to form the large Milky Way Galaxy of today. Regardless of which process best describes the past of the Galaxy, it is apparent that the capture or cannibalism of other smaller galaxies has played a significant role in the history of the Galaxy.