GAIA will have a key role to play in future studies of the ISM for gas components within 10 kpc of the Sun. Once a large fraction of the intrinsically luminous stars have been accurately pinpointed within the ISM, it will be possible to examine in detail how they interact with their environment. With the aid of a self-consistent dynamical model for the Galaxy derived from the GAIA data, the spectral information can be inverted to produce accurate density distributions for the different phases in Galactic coordinates, . However, unlike stars, the extended sources cannot be inverted directly to provide a 3-d emissivity distribution. Stars are the principle source of energy for the interstellar medium.
Starlight photons produce photoelectric emission from dust grains which heat the neutral gas.
Ultraviolet photons from the youngest stars ionize atoms and dissociate molecules. The component properties of the interstellar medium.