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 :-)
You are correct. You can certainly "define" it as pretty much anywhere you want it to be. How useful a particular definition is, or how much a given definition corresponds to "reality" (whatever that may mean) is another thing. How about: "It's no more meaningful to talk about "the end of a spiral arm" or "the edge of a spiral galaxy" than it is to talk about where the Earth's atmosphere ends. In the latter example, one can give many answers, such as where the atmosphere is too thin to breathe, or too thin to support combustion of fuel in an aircraft engine, or too thin to provide enough lift for an airplane to fly aerodynamically, or thin enough you can see stars in the daytime, or thin enough that a satellite can remain in orbit for a few weeks, or . . .
> 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! :-)
Umm . . . there would likely be a number of A-G stars (Sun-like or brighter, but too faint to show up in photographs of other spiral galaxies) out beyond the last bunch of O and B stars . . .
>> 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? :-)
Precisely? No. That description would put it out close to the same distance from the center as the "edge", but in the halo a few kiloparsecs "above" the central plane of the Galaxy, where the density of stars has fallen to a level where there are no stars close enough to be "bright" except for one of those clusters of A stars I mentioned earlier.
FWIW, while I never tried to work out the location of Terminus, I did consider the problem of an Earth-like planet orbiting a solar-type star in a part of the galactic outskirts where location would make the sky look quite different from ours, e.g., starry on only one side and dark on the other, for a story I was working on some time back (put on hold for other reasons), and I couldn't make it work at "the edge of the galaxy" or "the end of a spiral arm" . . .
-- Ronn! :)
_______________________________________________ http://www.mccmedia.com/mailman/listinfo/brin-l
