What we learn in this chapter

The gaseous matter between the stars, the interstellar medium (ISM), is a complex mix of gases (atomic and molecular), dust, radiation, magnetic fields, and cosmic rays. The dominant component is hydrogen. In its atomic form in the ISM, it emits radiation at the radio frequency of 1420 MHz. The transition is between the two hyperfine states of the ground state. Their energy difference, and hence the frequency of the transition, arises from the interaction of the magnetic moments of the proton and electron constituting the atom.

The orbital motions of clouds of gas about the center of the Galaxy may be tracked through studies of the 1420-MHz radiation. Multiple clouds at a given galactic longitude are distinguished by their different Doppler shifts. Application of the tangent-point method yields a rotation curve (orbital speed versus galactocentric radius) for the Galaxy. The curve is quite flat, indicating large amounts of unseen mass (dark matter). This is typical of other galaxies also. A flat rotation curve is expected for a spherical (or spheroidal) mass distribution with density that decreases as r−2. The differential motion of matter in the solar region of the Galaxy may be described with the Oort constants, which represent the shear and vorticity of the galactic material. […]