<Dl> <Dt> Photoelectric heating by grains </Dt> <Dd> The ultraviolet radiation emitted by hot stars can remove electrons from dust grains . The photon is absorbed by the dust grain, and some of its energy is used to overcome the potential energy barrier and remove the electron from the grain . This potential barrier is due to the binding energy of the electron (the work function) and the charge of the grain . The remainder of the photon's energy gives the ejected electron kinetic energy which heats the gas through collisions with other particles . A typical size distribution of dust grains is n (r) ∝ r, where r is the radius of the dust particle . Assuming this, the projected grain surface area distribution is πr n (r) ∝ r . This indicates that the smallest dust grains dominate this method of heating . </Dd> </Dl> <Dt> Photoelectric heating by grains </Dt> <Dd> The ultraviolet radiation emitted by hot stars can remove electrons from dust grains . The photon is absorbed by the dust grain, and some of its energy is used to overcome the potential energy barrier and remove the electron from the grain . This potential barrier is due to the binding energy of the electron (the work function) and the charge of the grain . The remainder of the photon's energy gives the ejected electron kinetic energy which heats the gas through collisions with other particles . A typical size distribution of dust grains is n (r) ∝ r, where r is the radius of the dust particle . Assuming this, the projected grain surface area distribution is πr n (r) ∝ r . This indicates that the smallest dust grains dominate this method of heating . </Dd> <Dl> <Dt> Photoionization </Dt> <Dd> When an electron is freed from an atom (typically from absorption of a UV photon) it carries kinetic energy away of the order E − E . This heating mechanism dominates in H II regions, but is negligible in the diffuse ISM due to the relative lack of neutral carbon atoms . </Dd> </Dl>

The average density of hydrogen atoms in the interstellar medium is about