McKee & Ostriker (1977) added a dynamic third phase that represented the very hot (T ~ 106 K) gas which had been shock heated by supernovae and constituted most of the volume of the ISM.
McKee e Ostriker (1977) engadiron unha terceira fase dinámica que representaba o gas moi quente (T ~ 106 K), que fora quentado pola sacudida de supernovas e constituía a maior parte do volume do MIE.