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jupiterzkool
Pooh-Bah
Reged: 05/08/06
Posts: 1341
Loc: Pasadena, CA
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Apparently, the milky ways is missing two appendages.
-S
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Scott G. Edgington, Planetary Scientist
Cassini-Huygens: Mission to Saturn & Titan
Yes, Asia, John Wetton Fan
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 6782
Loc: Beatrice, Nebraska
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You know, it is funny. The overall "perceived" form of the Milky Way has changed a bit over the years, but it never really seemed to "gel" much until very recently. The two spiral indicators used for so much of the time were the 21 cm Hydrogen line scans and the galactic Synchrotron radiation emission "peaks" which showed where the arms were supposedly edge-on to our line of sight. Although they showed that the Milky Way is probably a spiral galaxy, neither method produced any really good "grand design" spiral structure, although many people tried to interpret these indicators to force a structure on it. There were a number of studies of open clusters and HII regions which did show the "local" structure of the galaxy (within 3000 parsecs of the sun), but they only showed a hint of the arms and local spurs in our area. This local structure suggested a 2-arm spiral with multiple spurs, but little else. Thus, I don't feel any "four arms" were firmly supported by the data. They were more constructs based on someone's interpretation rather than on hard data. If you look at other spiral galaxies, you rarely find any with four well-separated distinct and dominant arms. At best, you may find some Sa spirals that have multiple tightly-wound spiral arcs. At least now, maybe this picture will be a little more accurate, although I doubt that the Milky Way has arms that are as smooth and orderly as those shown in the artist's conception. Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 10473
Loc: Amargosa Valley, NV, USA
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Once the core of the Milky Way was shown to be a bar rather than a lenticular disk, it became harder to visualize anything but two major arms -- but this is the first time I've seen that confirmed.
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"S.O.E." (Sauron's Other Eye) 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Next Project: The "Eye of Sauron" Observatory!
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PhilCo126
Pooh-Bah
Reged: 01/14/05
Posts: 1300
Loc: coastline of Belgium
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New findings about our home Galaxy, the Milky Way:
http://journals.aol.com/stuartatk/Cumbrian-Sky/entries/2008/06/04/another-new-milky-way.../3712
Indeed, less arms and more of a barred spiral !
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TS 152/1200 ( f 7.90 ) Achromatic Refractor
Saving/Looking for 180/1620 ( f 9 ) Refractor
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dgs©
Postmaster
Reged: 03/29/04
Posts: 13903
Loc: West Monroe, Louisiana
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Even APOD is getting into the Two Arms thing...
Oddly, if you hover your mouse over the image, labels show up for:
1. Scutum-Centaurus Arm
2. Perseus Arm
3. Norma Arm
4. Outer Arm
5. Sagittarius Arm
6. Near 3kpc Arm
7. Far 3kpc Arm
Must be some other base numeric system that I'm not familiar with. 
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- david
8"Ø Newtonian on SVP, Moonlite CR2, Telrad
PST Oberwerk Ultra 15x70 Orion Ultraview 10×50
Hand-me-down Sears Refractor (Discoverer) 60mm×900mm
"What we have done for ourselves alone dies with us; what we have done for others and the world, remains and is immortal." --Albert Pike
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