INTERSTELLAR & INTERPLANETARY MATTER A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z                    HOME ABOUT CATEGORIES SITE MAP COPYRIGHT ADVERTISE CONTACT entire Web this site HI region An interstellar cloud composed primarily of atomic hydrogen, detectable by its radio wave emission in the 21-centimeter line. HI regions have a density of 1 to 10 atoms/cm3 and a temperature of about 125 K (the spin temperature of neutral hydrogen). At least 95% of interstellar hydrogen is HI. Related category    • INTERSTELLAR AND INTERPLANETARY MATTER Also on this site: Encyclopedia of Alternative Energy & Sustainable Living Encyclopedia of History Transport Concepts & Designs (partner site) BACK TO TOP