The Physical State of Interstellar Hydrogen.
Abstract
The discovery, by Struve and Elvey, of extended areas in the Milky Way in which the Balmer lines are observed in emission suggests that hydrogen exists, in the ionized state, in large regions of space. The problem of the ionization and excitation of hydro- gen is first considered in a general way. An attempt is then made to arrive at a picture of the actual physical state of interstellar hydrogen. It is found that the Balmer-line emission should be limited to certain rather sharply bounded regions in space surround- ing 0-type stars or clusters of 0-type stars. Such regions may have diameters of about 200 parsecs, which is in general agreement with the observations. Certain aspects of the problem of the ionization of other elements and of the problem of the relative abundance of the elements in interstellar space are briefly discussed. The interstellar density of hydrogen is of the order of N = ~ cm-3. The extent of the emission regions at right angles to the galactic plane is discussed and is found to be small
- Publication:
-
The Astrophysical Journal
- Pub Date:
- May 1939
- DOI:
- 10.1086/144074
- Bibcode:
- 1939ApJ....89..526S