Stellar birthline

The stellar birthline is a line in the Hertzsprung–Russell diagram that relates the effective temperature and luminosity of pre-main-sequence stars on the start of their contraction. To this point, the objects are protostars, and are so deeply in the cloud of dust and gas from which they are forming that they radiate only in far infrared and millimeter wavelengths. 
When stellar winds disperse this cloud, the star becomes visible as pre-main-sequence object. The set of locations on the Hertzsprung–Russell diagram where these newly visible stars form is called the birthline and is found above the main sequence.
Location.
Location of the stellar birthline depends on detail in the accretion rate onto the star and geometry of this accretion, i.e. whether or not it is happening through an accretion disk. This means that the birthline is not an infinitely thin curve, but has a finite thickness on the Hertzsprung-Russell diagram.