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Matrix Software > Learng Astrology > Astrophysical Directions > The Galaxy > Spiral Arm

Learn Astrology

Back to The Galaxy   |   Back to Astrophysical Direction 

General Features of Galactic Spiral Arm Structure

Many distant galaxies appear to us through telescopes to have a lovely vortical or spiral structure. For over a century, astronomers have assumed that our Milky Way is also a spiral galaxy, although this has been difficult to prove. We are embedded within our galactic plane and surrounded in all directions by an apparently chaotic distribution of stars, clusters, nebulae, and dust clouds. It has been only since the 1950's that we have understood what the spiral arm structure of our galaxy looks like.

General Features of Galactic Spiral Arm Structure

At this point in time astronomers have distinguished three major spiral arms this side of the galactic center. The Sun seems to be about 1000 light years from the central part of a spiral arm that includes the Orion Nebula, the Coal Sack, and the North American nebula -- the Carina-Cygnus arm. An outer spiral arm (including the double star cluster in Perseus) passes about 6000 light years beyond us (the Perseus arm) and an inner arm (Sagittarius arm) has been discovered between the Galactic Center and us. Much of this research has been made possible through radio astronomy and in particular studies of the 21-cm line of emission in interstellar hydrogen. A rough idea as to the probable spiral arm structure as astronomers see it today is given in Figure A. Keep in mind that this structure is rotating toward the right (clockwise), so that the tilt of the arms to the radius vector from the center indicates that the spiral arms are trailing in the rotation as in a vortex. The spiral arms that exist today were probably formed not too long ago and in cosmic time may be rather short-lived.

The spiral arms are gaseous envelopes filled with stars and dust held together by gravitational and magnetic forces. These arms will cohere until the gas and dust has condensed into individual stars and these stars are dispersed throughout the galactic plane. In fact the whole sequence, if we could speed up the time process, would appear as a spinning pinwheel shedding sparks or stars as it whirls. The new-formed stars stay embedded within the spiral arm where they were born until galactic rotation forces them to migrate and be scattered through space, dissolving the spiral arms. New arms are continually being formed.

Copyright (c) 1997-99 Michael Erlewine


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