According to a classification introduced by Walter Baade (1893-1960), stars can be subdivided into two categories according to their age: Population I comprising young stars; and Population II comprising old stars.
Baade came to this distinction by observing that distant galaxies, such as Andromeda, have a circular halo characterized by old red stars (Population II) and spiral arms characterized by younger stars (Population 1).
The old stars of Population II have formed around ten billion years ago, when galaxies began to condense and the first stars were born inside. At that time, the spiral arms had not yet formed and the stars, now transformed into red ones, are approximately in a circular-shaped halo.
The young stars of Population I are, instead, in a thin layer that coincides with the galactic plane, in which gases and dusts are accumulated: from the condensation processes still in progress, precisely the new stars that are made are born visible in the form of blue stars.
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