Association (astronomy)

In astronomy, an association is a very loose grouping of stars, gas, and dust that are spatially related but are not as gravitationally bound as an open cluster or globular cluster. They are typically young, hot, massive stars (O and B type stars being common) that share a common origin in a giant molecular cloud. Due to their relatively weak gravitational binding and the stellar winds and supernova explosions from their massive stellar members, associations tend to disperse over time, usually on timescales of tens to hundreds of millions of years.

Associations are categorized based on their dominant stellar population:

• O Associations (OB associations): These are associations dominated by massive O and B type stars. They are the most common type of association and are often found in spiral arms of galaxies. They are typically associated with regions of active star formation.

• T Associations: These associations primarily contain young, pre-main sequence T Tauri stars. These stars are lower-mass than O and B stars and are still in the process of contracting and heating up to become main sequence stars. T associations are indicative of relatively recent star formation.

• R Associations: These associations are characterized by reflection nebulae surrounding the member stars. The dust in the reflection nebulae scatters the light from nearby stars, making the association visible. These are also associated with young stellar populations.

The study of associations helps astronomers understand the processes of star formation, stellar evolution, and the dynamics of the interstellar medium. The fact that the stars in an association share a common age and origin provides a valuable laboratory for testing stellar evolution theories. Their dispersal also allows astronomers to study the effects of stellar feedback on the surrounding gas and dust.