The usual definition of what a galaxy is: 'An island universe', or 'A vast collection of stars' is not wrong, but it is misleading. Because of the vast distances that separate stars even in the most densely packed regions of most galaxies, a better definition is: 'A vast formation of plasma clouds that contain electrical currents and occasional, widely distributed tiny lumped points of matter called nebulae, stars and planets.'
The shape that characterizes most galaxies was first described by Hannes Alfven in 1981.
The shape he proposed is shown here. This diagram is a vertical cross section of a three dimensional figure. The horizontal line at the center of the diagram is actually a circular disk lying in the horizontal plane. It is this disk, when viewed not from the edge as it is here, but more face on, that is the familiar spiral shape picture of a galaxy that we are used to seeing. The parallel vertical lines coming out from the center of the galaxy (along its axis of rotation) represent a strong electrical current in a plasma that sometimes is visible as a "jet".
The two pairs of letters - DL - in the sketch are Double Layers within the jet plasma. Recall that DLs contain strong E-fields and are the source of radio frequency emissions. The two amorphous shapes at the left of the diagram show the resulting typical "double radio source" that is observed in many galaxies. These are due to the presence of DLs. In some galaxies the jet plasma is in the dark current mode; in others it is clearly visible.
This is Centaurus A. The jet is clearly visible in this image. Halton Arp's contention that quasars are emitted from the centers of Seyfert galaxies along their 'secondary axes' (axes of rotation) is supported by these pictures. Recall that DLs are also the locations of strong electromagnetic z-pinches which can compress dispersed material into denser objects.
The general shape of a rotating disk carrying electrical currents in the shape shown by Alfven defines what is called the homopolar motor - generator. Note that the horizontal disk (the arms of the galaxy) is where the current, I, is least spread out - the current density is greatest. This is where Population I stars are usually found.
In many galaxies the jet structure cannot be seen in visible light. So until the development of infrared and x-ray orbiting satellite telescopes, most of these features remained undiscovered. There are now many images of galaxies that show the Alfven structure. The image at the right was taken by the Subaru orbiting IR telescope of galaxy M 82.
Even our neighboring Andromeda galaxy, M 31, shows the disk like structure of the homopolar motor-generator morphology. Below on the right is a normal, visible light photograph of M 31. Below on the left is an image of that same object obtained by the Infrared Space Observatory (ISO) operated by ESA.
But Alfven also proposed that individual stars themselves had similar morphologies. He proposed an almost identical diagram for the operation of a typical star. Once again the plasmas involved may or may not all be in one of the visible modes of operation. So not all images of stars show this structure - but many do. Alfven's heliospheric circuit is shown here. Because our Sun is a typical star, this diagram would apply equally well to it.
This proposal remained in the realm of conjecture until the spring of the year 2001 when the spacecraft Ulysses discovered long plasma 'tubes' emanating from the bottom pole of our Sun. These tubes are long enough to extend out farther than the distance of the orbit of Mars from the Sun. There are also many images available now of individual 'jetted stars'. Of course, Alfven believed all stars were jetted - but some less visibly than others. Below are a pair of images of stars that show the plasma jets and characteristic disk shape clearly.
Notice in the right hand image (Goddard Space Flight Center) that the 'planets' (Herbig-Haro objects) are formed in a collinear array along the jet axis of the parent star. These clumpings are probably formed by DLs at those locations. In the center image (above) the tell-tale twisting shape of a large Birkeland current containing DLs is clearly visible. Dr. Anthony Peratt points out that the number of objects typically formed by the z-pinch effect is usually around nine.
Whether we choose to call it the homopolar motor - generator, the jetted disk, or the Alfven circuit, this shape is being seen with increasing frequency as more accurate and broader bandwidth instruments are developed. (Notice, for example, the shape of the "Crab pulsar" shown on the Main page of this site.) All the images shown below are of stars. All exhibit the homopolar disk shape.
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The Electric Sky (Mikamar Pub.)