A young stellar object (YSO) is any star that has evolved past the protostar stage (i.e. is shining by way of internal nuclear reactions) but has yet to arrive on the main sequence. YSO's come in a variety of forms depending on their age, mass, and environment, and include Herbig Ae/Be stars, T Tauri stars, and, in general, immature stars prone to irregular brightening, embedded in nebulosity, and associated with bipolar outflows.