Spiral and irregular galaxies look so much more complicated than do ellipticals, that it is reasonable to expect their dynamics to be vastly more complex too. Consider firstly the motions of the stars Since spirals are so highly flattened, the method of velocity dispersion can fruitfully be used only in the nuclei and central bulges which are sufficiently nearly spherical for the velocity dispersion to reflect the total mass. Typical velocity spreads are of the order of a hundred kilometres per second. Using the estimate outlined above (page 339) it follows for example that the inner 20 pc (5 arc sec as seen from Earth) of M 31 contains 2 x 108M0, and the inner 2kpc of the central bulge about 1010M0. As in the case of elliptical galaxies, the systematic motions in the stellar component are very difficult to observe. Measurements of the central parts of M 31 have been made, with some puzzling results, but their reliability is doubtful.

Observations of the systematic motions of stars are essential for the study of the dynamics of galaxies like lenticulars that contain practically no gas. Because observations of the stellar velocities are extremely
difficult, most of our knowledge of dynamic is restricted to spiral types, where the spectral lines emitted by the gas can be used to determine the velocity fields. In the optical part of the spectrum, the emission lines most frequently used are the balmer series lines due to atomic hydrogen and a nearby line of singly-ionized atomic nitrogen. In the few cases where comparison is possible, the velocity of the gas corresponds moderately well to the velocities of the stars. Within about one kiloparsec of the centre of an ordinary spiral, the orbital velocity in the plane of the galaxy typically increases in direct proportion to the distance from the centre. This implies that the swirling matter in this region rotates as if it were a solid body. Beyond this radius, the velocity increases less rapidly, and after reaching a maximum at a radius usually between three and eight kiloparsecs, slowly decreases outwards.

Such ROTATION CURVES can also be observed by using the 21-cm hydrogen line. The radio line spectrum of a whole galaxy can be obtained at once. At optical wavelengths only the surfaces of the interstellar clouds are observed, whereas the radio waves can be seen from practically all the hydrogen gas in the disc.

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