YSOs

Young stellar objects (YSOs) are stars in the earliest phases of formation, found in molecular clouds. They arise from the gravitational collapse of dense cores, during which material from an infalling envelope and a circumstellar disk accretes onto a central protostar. The surrounding dust and gas emit strongly in the infrared, making YSOs prominent targets of infrared astronomy.

YSOs are commonly classified by their spectral energy distributions (SEDs) into four broad classes: Class 0, I, II, and III. Class 0 are the youngest, deeply embedded in their envelopes with most of their mass still in the envelope and little emission at wavelengths shorter than about 20 micrometers. Class I objects have prominent envelopes and ongoing accretion. Class II objects, often called classical T Tauri stars, have optically visible central stars with circumstellar disks and weaker envelopes. Class III objects, or weak-line T Tauri stars, have little or no disk emission and resemble pre-main-sequence stars more closely in appearance.

YSOs show observational signatures such as infrared excess due to dust, molecular outflows and jets, and occasionally Herbig-Haro objects. Their typical ages range from roughly 0.1 to a few million years. They are found in star-forming regions within giant molecular clouds and serve as laboratories for studying early stellar evolution and planet formation in protoplanetary disks.

Observational programs with space- and ground-based facilities, including Spitzer, Herschel, ALMA, and the James Webb Space Telescope, have expanded the census and detail of YSOs. Understanding YSOs informs theories of accretion, disk evolution, and the initial conditions for planet formation.