Star Associations

The spiral arms of our galaxy contain, in addition to open clusters, associations, which are groups of stars that appear to be moving as a group but are too widely separated to be bound to each other by gravity. The associations identified by astronomers include

O-Associations of hot young spectral-class O stars
B-Associations of hot young spectral-class B stars
T-Associations of young T-Tauri stars

(We shall discuss T-Tauri stars in Chapter 20.) Since these all involve young stars, the stars cannot be far from where they were born, suggesting that they formed together from the same molecular cloud.