circumstellar

Circumstellar refers to material, structures, or phenomena that surround a star. It encompasses disks, envelopes, winds, and shells located in the immediate stellar environment, from distances comparable to the solar system to much larger scales around evolved stars. The term is used across stellar and planetary astrophysics to distinguish material associated with a star from that of the broader interstellar medium.

Circumstellar disks are a prominent example and come in several forms. Protoplanetary or primordial disks surround young stars and contain gas and dust from which planets may form. Debris disks surround more mature stars and consist mainly of dust produced by the collisions of planetesimals. Circumstellar disks are often detected through infrared excess emission, as dust reprocesses stellar light, and through direct imaging and spectroscopy that reveal gas, dust composition, and kinematics.

Circumstellar envelopes and shells surround stars at different evolutionary stages. In evolved stars such as asymptotic giant branch stars and red supergiants, substantial mass loss creates dusty circumstellar envelopes that can extend far beyond the star. In some cases, these envelopes evolve into planetary nebulae, while others dissipate into the interstellar medium. Stellar winds, outflows, and jets are also considered circumstellar phenomena, shaping the immediate surroundings of the star.

Observationally, circumstellar matter is studied via infrared and submillimeter astronomy, spectroscopy of molecular and atomic lines, and high-resolution imaging and interferometry. Circumstellar material plays a central role in processes such as planet formation and stellar mass loss, influencing both the evolution of the star and its environment.