  Usual explanations of spiral galaxy rotation curves assume circular orbits of
stars. The consequences of giving up this assumption are investigated here. In
particular, hyperbolic stellar trajectories are found to be interesting. The
two suggested models for the production of such trajectories will also explain
the observed flat rotation curves without the postulation of dark matter or
MOND. It is suggested that spiral galaxies may have started as compact objects
with significant angular momenta and then disintegrated. The first model
conjectures the existence of a spinning hot disk around a spherical galactic
core. The disk is held together by local gravity and electromagnetic scattering
forces. However, it disintegrates at the edge producing fragments that form
stars. Once separated from the disk, the stars experience only the centrally
directed gravitational force due to the massive core and remaining disk. A
numerical simulation shows that a high enough angular velocity of the disk
produces hyperbolic stellar trajectories that agree with the observed rotation
curves. The second model conjectures a significant initial thermonuclear event
that produces a dust plume along with large stars. This dust plume is made of
ordinary matter. However, it acts like the postulated dark matter in producing
initial circular trajectories. Unlike dark matter, the plume can be shown to
escape the galaxy rapidly causing the star trajectories to evolve to hyperbolic
shapes. This process can be seen to produce the observed rotation curves due to
the initial circular orbits. Also, as the plume dissipates rapidly it does not
obfuscate the stars from view. Both models have weaknesses as do the currently
known models using dark matter or MOND.
