What are OB associations and T associations?
Most of the brightest stars are older yellow stars, but a few young blue stars are also visible. This picture is a composite of three images taken by the Hubble Space Telescope.
Stellar associations are groups of stars that share similar physical characteristics and are relatively close to one another in space, though they are often more dispersed than open clusters.1
OB Associations
An OB association is characterized by a high concentration of hot, massive blue-giant stars of spectral classes O and B. These associations are relatively young, range from 100 to 700 light-years in diameter, and often contain supergiant stars, binaries, and gaseous nebulas. They are typically short-lived, as tidal forces tend to break them apart in less than 10 million years.1
T Associations
T associations (or T Tauri associations) consist of cooler, low-luminosity dwarf stars known as T Tauri stars. These stars are newly formed and still contracting from interstellar matter. Unlike OB associations, T associations are found exclusively in or near regions of galactic nebulosity containing dust. They are considered a primary source of low-luminosity stars in the solar neighborhood.12