The stellar birthline is a predicted line on the Hertzsprung–Russell diagram that relates the colour and luminosity of protostars at the end of the initial phase of accretion onto them. Prior to this point, the protostar is deeply embedded in the cloud of dust and gas from which it formed, and so radiates only in the far infrared. Once evolutionary effects rarefy and disperse this cloud, the star becomes visible as a pre-main-sequence object. The set of locations on the Hertzsprung–Russell diagram where these newly visible stars reside is called the birthline, and is found to the right of the main sequence.
The location of the stellar birthline depends on the stellar structure, the accretion rate onto the star and geometry of this accretion, i.e. whether or not it is occurring through an accretion disk. This means that the location of the birthline is not uniquely defined.
- http://jila.colorado.edu/~pja/stars02/lecture29.ps – several low-quality plots with the stellar birthline