Galaxies are made up of stars. As simple as this statement seems, it is
important to realize the meaning of this: to understand how galaxies work,
we often need to understand the stellar orbits which populate galaxies.
Because the mass of a galaxy is spatially distributed (i.e. in a disk,
or a spheroid, etc) rather than concentrated in a mass at the center,
stellar orbits do not follow the familiar closed ellipses associated
with Keplerian motion. Instead, they take on a variety of shapes, from
rosette-shaped loop orbits to rectangular box orbits, and including
families of resonant orbits known as boxlets. The shapes of galaxies,
their density distribution, and their dynamical evolution are all closely
tied to the relative fraction of stars on these different types of orbits.
Visualizing the orbital content of a galaxy can be a cumbersome task --
characterizing 6-dimensional phase space (x, y, z, vx, vy, vz) is difficult.
Astronomers use a conceptual tool known as a "surface of section" to
characterize the orbital content of a given galaxy model. In this applet
(aptly named "SOS") we explore how stellar orbits depend on the density
distribution of a galaxy, and how we can use surfaces of section to
describe stellar orbits.
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