Young Stellar Objects:
Any star that has evolved past the protostar stage (i.e. is producing energy due to internal nuclear reactions) but has yet to arrive on the main sequence is called a Young Stellar Object (YSO). YSO's come in a variety of forms depending on their age, mass, and environment, and include Herbig-Haro (HH) objects, T Tauri stars, and, in general, immature stars prone to irregular brightening, embedded in nebulosity, and associated with bipolar outflows. Observations of Herbig-Haro objects provide a dramatically clear look at collapsing circumstellar disks of dust and gas that build stars and provide the ingredients for planetary systems. Jets of hot gas are funneled from deep within these embryonic systems, and bursts of material are ejected from the young stellar objects at speeds of nearly a half-million kilometers per hour. The Herbig-Haro object HH111 shows the fast-moving jet of material from a newborn star colliding with the interstellar medium. As the bipolar flow from a young star plows into the surrounding gas, it generates strong shock waves that heat and ionize the gas. In the cooling gas behind the shock front, electrons and ions recombine to give an emission line spectrum characteristic of Herbig-Haro objects. All known Herbig-Haro objects have been found within the boundaries of dark clouds, and are strong sources of infrared radiation.
The Trifid Nebula is one of the most prominent nebulas in the night sky. Radiation from the powerful central star is eating away at the surrounding dense interstellar material. The field of view of this Hubble image includes a region of star formation that will be destroyed by the advancing ionization front in the next ~20,000 years. A prominent jet from a young stellar object and a long finger with a possible young stellar object at its tip are apparent in the image. The stellar jet is emerging from the wall of a cloud in the Trifid Nebula. The jet is remarkable because, unlike most stellar jets, it can be seen along its entire length. This is because the jet is being lit up by radiation from the massive, luminous star that powers the Trifid. The tip of a finger-like Evaporating Gaseous Globule, or "EGG", points back at the Trifid's central star. A tiny jet emerging from the EGG and a patch of reflected light suggest that a young stellar object is buried in the tip of the jet. This young stellar object was uncovered a few tens of thousands of years ago as radiation from the Trifid’s central star disrupted the dense cloud from which the star formed.
A T Tauri star is a very young, lightweight star, less than 10 million years old and under 3 solar masses, that it still undergoing gravitational contraction; it represents an intermediate stage between a protostar and a mid-mass main sequence star like the Sun. T Tauri stars are found only in nebulas or very young clusters, have low-temperature (G to M type) spectra with strong emission lines and broad absorption lines. They are more luminous than main sequence stars of similar spectral types, and they have a high lithium abundance, which is a pointer to their extreme youth, as lithium is rapidly destroyed in stellar interiors. T Tauri stars often have large accretion disks left over from stellar formation. Their erratic brightness changes may be due to instabilities in the disk, violent activity in the stellar atmosphere, or nearby clouds of gas and dust that sometimes obscure the starlight. Two broad T Tauri types are recognized based on spectroscopic characteristics that arise from their disk properties: classical T Tauri and weak-lined T Tauri stars. Classical T Tauri stars have extensive disks that result in strong emission lines. Weak-lined T Tauri stars are surrounded by a disk that is very weak or no longer in existence. The weak T Tauri stars are of particular interest since they provide astronomers with a look at early stages of stellar evolution unencumbered by nebulous material. Some of the absent disk matter may have gone into making planetesimals, from which planets might eventually form. According to one estimate, about 60% of T Tauri stars younger than 3 million years may possess dust disks, compared with only 10% of stars that are 10 million years old. T Tauri stars represent an evolutionary stage between protostar and the main sequence for mid-mass sized stars and are located just above the main sequence on the H-R diagram.