These objects include a variable subclass whose members increase up to 6 magnitudes in brightness; that subclass is named for the prototype FU Orionis, while another subclass consists of the Herbig Ae/Be objects.  
These objects include a variable subclass whose members increase up to 6 magnitudes in brightness; that subclass is named for the prototype FU Orionis, while another subclass consists of the Herbig Ae/Be objects.  They evolve from deeply embedded Class 0 ["zero"] sources which have material infalling from the original molecular cloud core onto the surrounding accretion disk.  At the low mass end they include the T Tauri stars, which evolve into spectral types G, K and M. For 10 points, name these objects that, depending on their mass, evolve along the Henyey and/or Hayashi tracks towards a major feature on the HR diagram.
These objects include a variable subclass whose members increase up to 6 magnitudes in brightness; that subclass is named for the prototype FU Orionis, while another subclass consists of the Herbig Ae/Be objects.  
{young stellar} objects [or {YSOs}; or {pre}-{main sequence} stars or objects; or {PMS} stars or objects; accept {T Tauri stars} or {objects} before “{Herbig Ae/Be objects}”; accept {protostars}, {baby stars}, etc.]