Stellar birthline

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Stellar birthline is a predicted line on HR diagram of initial stellar radii as a function of mass at the end of the initial phase of accretion on protostar. Before that phase a protostar is deeply embedded in the cloud of dust and gas, therefore radiating only in the far infrared region. When the evolutionary effects rarefy envelope, the star becomes visible as pre-main sequence object. A set of places on HR diagram, where can reside those "firstly visible" stars is called the "birthline". Physically it resides to the right of main sequence.

The introduction of the term was firstly made by Steven W. Stahler in 1983.

The location of a birthline depends on the stellar structure, accretion rate and geometry. This means, that the location of birthline is not uniquely defined. In that context, we can call it even a "birthband".