Gasdisks

Gas disks are rotating, flattened distributions of gas that orbit a central mass. They occur in a range of astrophysical settings and can span from sub-AU scales around young stars to tens of kiloparsecs in galaxies. The gas, often with embedded dust, provides the raw material for accretion and formation processes.

Common contexts include protoplanetary disks around young stars, where planets form; accretion disks around compact objects such as black holes and neutron stars; circumnuclear disks surrounding galactic centers; and large-scale galactic gas disks containing much of a galaxy's neutral gas.

Composition and structure vary with environment but are typically dominated by hydrogen and helium. Molecular gas (for example CO) traces cold regions, while dust absorbs and re-emits starlight. Temperature and density decline with radius, and disks often possess a warmer, ionized surface and a cooler interior.

Dynamics are governed by gravity and rotation. Angular-momentum transport—through viscosity or magnetic stresses—drives accretion. In planet-forming disks, young planets can open gaps and rings; outflows and jets may accompany accretion in inner regions.

Observations rely on spectral lines such as CO in the millimeter/submillimeter, the HI 21 cm line in radio, and infrared dust emission. Interferometers like ALMA or the VLA provide high-resolution images that reveal rotation, mass distribution, and disk structure.

Gas disks are central to theories of star and planet formation, black-hole growth, and galaxy evolution. Their lifetimes, compositions, and dynamics constrain how planetary systems form and how material is transported to central engines.