Stars are not formed separately, but in groups within molecular clouds. All the observed members of such primitive, embedded clusters suffer heavy extinction and reddening because of interstellar dust. Additionally, they exhibit varying degrees of excess infrared emission created by circumstellar matter, i. e., dust immediately adjacent to the stars themselves. The ensemble of observed spectral energy distributions fall into four classes that appear to define an evolutionary sequence.
Somewhat older, visible objects are generally located in either T or OB associations. The former contain classical and weak-lined T Tauri stars along with residual cloud gas, and remain intact for periods up to 107 yr. OB associations, so called because they also include a few massive objects, are already dispersing into the field population. Here, the molecular gas was driven off violently by the winds and radiation pressure of the highest-mass members. Finally, a relatively small fraction of stars survives dissipation of their parent cloud as gravitationally bound clusters.
The observed luminosities of field stars, together with their main-sequence lifetimes, allow one to deduce the statistical distribution of masses at birth. This initial mass function peaks between 0.1 and 1.0 MQ, a fundamental and unexplained fact. The distribution within specific groups, obtained by placing members in the HR diagram, agrees broadly with the field-star result.
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