Abstract
Although insight into the structure of our Galaxy can be sought in various ways, study of the radio emission from the neutral hydrogen component is particularly suitable. Neutral atomic hydrogen, HI, is the main observed constituent of the interstellar medium; its physical properties are closely related to the properties of other Galactic constituents, both stellar and interstellar. The interstellar medium is transparent enough to hydrogen emission at the 21-cm radio wavelength that investigation of the entire Galaxy is possible, with the exception of a few directions along the Galactic equator. This transparency allows investigation of regions of the Galaxy which are too distant to be studied optically. HI is particularly important for the information it provides on the form and overall mass in the outer Galaxy, where it is the only directly observable constituent. Interstellar neutral hydrogen is so abundant and is distributed in such a general fashion throughout the Galaxy that the 21-cm hyperfine transition line has been detected in emission in every direction in the sky at which a suitably equipped radio telescope has been pointed. No time variation of a neutral hydrogen line has been found.
Published Version
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