Neutral Hydrogen in the Galaxy
•Hyperfine
transition
–1420.4058 MHz
/ 21 centimeters
–Occurs from
change in orientation of the magnetic dipole
•HI is observed
throughout the Milky Way galaxy and is an important tool in studying other galaxies
•Predicted in
the 1940’s by Hendrick van de Hulst
(Van De Hulst, H. C., 1945, Ned. Tijdschr. Natuurk,,
11, 201)
and
subsequently discovered by observation in 1951
(Ewen, H. I. & Purcell, E. M., 1951, Nature, 168,
350; Muller, C. A. & Oort,
J. H., 1951, Nature, 168, 357)