In this work, we propose that the Herbig-Haro objects can be divided into two groups. Objects in the first group have luminosities which can be quantitatively explained by reflection of the light from a neighboring source. Objects in the second group are not illuminated from outside. For the objects in the second group we propose the following hypothesis. There is an analogy between the nature of the Herbig-Haro objects and the post-FUOR's, in the sense that these objects are FUOR's in which the ejected envelope has expanded and is now quite tenuous. In support of this hypothesis, we mention the following points: the negative velocity of the material being ejected from the central star; the polarization of the light from the Herbig-Haro objects; and finally, the fact that it is possible to interpret the maximum in the spectrum of H-H 1 around 6500 A (a maximum which is apparently real) in terms of emission from neutral hydrogen atoms which form in a flux of protons streaming through the material.