Spiral Arms

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Abstract
Spiral arms are ribbon shaped. The Local Arm has a thickness of about 100 pc, a width of about 700 pc, and exhibits a wave-like appearance when viewed edge-on. This information was obtained through a study of the positions of B0–B2 stars. A study of the positions and motions of B0–B5 stars confirmed the view (Dixon 1967) that gases in spiral arms move under the influence of non-gravitational as well as gravitational forces. The non-gravitational forces tend to oppose the gravitational forces of the Galaxy. They cause the interstellar gases to orbit at less than gravitational circular velocity and they force the gases away from the mean galactic plane. The non-gravitational forces cease to act on material that has condensed to form stars and, as a consequence, newly formed stars fall away from the gases in which they were produced. The stars fall towards the centre of the Galaxy and towards the mean galactic plane. The gases of the Scorpius–Centaurus aggregate became detached from the inner edge of the local arm, and from the non-gravitational forces supporting the arm, about 2 × 10 7 yr ago. The gases are now falling inwards, towards the centre of the Galaxy, and having fallen towards the mean galactic plane from below, they are now moving upwards, away from the plane.

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