Ronn Blankenship wrote: > >> Are there any observations of the "end" of a Spiral Arm? > > I don't think you can define the "end" of a spiral arm. Or the "edge" of > the Galaxy. > What do you mean by "you can't define"? I guess that it's possible to define it - the problem would be if it was possible to apply any such definition :-)
> The arms of a spiral are pretty well defined by the bright OB associations > because such stars form there and have such short lives that they do not > move far from their birthplace before they die. > So, here we go with a definition: the Spiral Arm's terminus is at the farthest OB star! :-) >> and that those bright stars in the Spiral >> Arm would be clumped together. > > Not necessarily. The spiral arms can be several hundred parsecs wide in > the plane of a spiral galaxy. > But would they be so wide in the "terminus"? >> This would mean something like a geometry of about >> 90 degrees [or a little less] for the angle >> (Center of the Milky Way) - Terminus - (bright stars of >> the Spiral Arm) > > Perhaps, but you wouldn't be able to actually see the central > bulge from there unless you were located significantly > (a kiloparsec?) above/below the plane of the galaxy, > because of extinction by the intervening ISM, clouds > of which concentrate close to the galactic plane. > Hmmm... > In reality, the sky wouldn't look a whole lot different from there > than it does from Earth, unless Terminus were actually some > distance beyond where the spiral arm appeared to end in a > visible-light photo taken from outside, > in which case there might be fewer naked-eye stars all around > the sky and a "Milky Way" limited to one side of the sky. > In the Foundation books, the sky from Terminus City is described more or less this way: there is a clear image of the center of the galaxy, and the galaxy usually spans from horizon to horizon. The galaxy is seen as if from "above". There are _no_ bright stars in the sky at all, except when a group of stars called "The Diamonds" is visible, and they are grouped within 20 degrees. Does this description make any sense? Can you locate Terminus [how far above the plane, where can be the closest Spiral Arm stars] with this data? :-) Alberto Monteiro _______________________________________________ http://www.mccmedia.com/mailman/listinfo/brin-l
