Astronomers Unravel Galactic Mystery

A new picture of the Milky Way emerges which reveals our galaxy to be a tightly wound “grand design” two‐armed spiral – not a four‐armed spiral as has previously been supposed.
New portrait of the Milky Way revealed
New portrait of the Milky Way revealed
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Oregon - US

Dec. 28, 2009 - PRLog -- Spiral galaxies are among the most beautiful and familiar objects in the heavens, but the reason why galaxies assume spiral forms has remained an elusive mystery for decades.  Now, two independent researchers have published a compelling solution to this long-standing problem in the scientific journal Proceedings of the Royal Society A. The new picture of the Milky Way which emerges presents our galaxy as a tightly wound “grand design” two‐armed spiral – not a four‐armed spiral as has previously been supposed.

Charles Francis of Hastings, U.K., and Erik Anderson of Ashland, Oregon, had been working on a quite different problem when the discovery was made. Anderson had compiled data from existing sky surveys on more than 20,000 Milky Way stars with accurately known positions and velocities. By treating orbits as precessing ellipses, Francis found that mutual gravitation naturally leads to orbital alignments that generate spiral patterns.

Francis likens the gravitational field of a spiral galaxy to a giant funnel with spiral grooves. Stars, like rolling marbles, are channeled down the grooves until they build enough momentum to escape. Escaping stars migrate away from the galactic center, crossing over the next highest groove, and eventually falling back into the same groove they came from.

Rainer Klement, at the Max Planck Institute for Astronomy in Heidelberg, Germany, praises the model’s elegance. “It is a very nice paper with very good ideas and explanations for the kinematic structures we observe,” Klement remarks. “It comes up with an elegant way of explaining the velocity distribution in the solar neighborhood.” Klement believes that future observations, covering even broader regions of the galaxy with even greater precision, will support the paper’s conclusions.

The introduction of a working model for spiral galaxies may also reform the field of galactic dynamics.  “It comes as a surprise to most people that galactic orbits of stars are treated in textbooks using a model of epicycles,” Francis notes.  Epicycles, arbitrary “loops” imposed upon orbital motions, were made notorious by ancient Greek astronomers.  Popularized by Ptolemy in the 2nd century A.D., epicycles fitted the motions of planets to an Earth-centered universe.  Epicycles were reintroduced to astronomy in the 1920’s to fit the galactic orbits of stars – a development which Francis believes has distracted astrophysicists ever since with unnecessary complications.  “In popular culture, ‘adding epicycles’ refers to the process of introducing fudges to make a theory fit data, when actually the theory needs to be replaced in its entirety.”

Interviews on request:
Charles Francis,, Tel: +44 (0)1424 713467 (UK)
Erik Anderson,, Tel: +1 530-415-3982 (USA)

Supporting Documents:
Peer-reviewed article, “Galactic Spiral Structure,” @
Web-tutorial with animations @

Supporting Artwork:
Portrait of the Milky Way @
Spiral funnel diagram @

See also:
Epicycles in modern astronomy @
Source:Erik Anderson
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Tags:Space, Astronomy, Galaxies, Milky Way, Physics
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Location:Oregon - United States
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