Galactic Spiral Structure
Charles Francis, Erik Anderson

TL;DR
This paper investigates the structure and dynamics of the Milky Way's spiral arms, proposing a symmetric two-armed model with stable stellar orbits, and suggests the galaxy evolved into this form approximately 9 billion years ago.
Contribution
It introduces a new model of spiral arm motions based on perturbed elliptical stellar orbits and re-examines the Milky Way's spiral structure, challenging previous assumptions.
Findings
Almost all local stars belong to six major streams.
The Milky Way likely has a symmetric two-armed spiral with a smaller pitch angle.
The galaxy evolved into a grand design two-armed spiral about 9 billion years ago.
Abstract
We describe the structure and composition of six major stellar streams in a population of 20 574 local stars in the New Hipparcos Reduction with known radial velocities. We find that, once fast moving stars are excluded, almost all stars belong to one of these streams. The results of our investigation have lead us to re-examine the hydrogen maps of the Milky Way, from which we identify the possibility of a symmetric two-armed spiral with half the conventionally accepted pitch angle. We describe a model of spiral arm motions which matches the observed velocities and composition of the six major streams, as well as the observed velocities of the Hyades and Praesepe clusters at the extreme of the Hyades stream. We model stellar orbits as perturbed ellipses aligned at a focus in coordinates rotating at the rate of precession of apocentre. Stars join a spiral arm just before apocentre,…
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