EU-Fic/Science/Physical Astrography

The Galaxy

The general term by which the denizens of the Star Wars civilization refer to its self-contained home. Actually consists of a single grand-design or semi-grand design (2-4 spiral arms) spiral galaxy, its globular clusters, and satellite dwarf galaxies, and other gravitationally bound structures.

The primary

The primary consists of the spiral galaxy itself. The spiral galaxy consists of three distinct morphological components characterized by orbits and stellar populations. The bulge, the disk, and the halo.

The bulge

The bulge is the superficial "hub" of the galaxy. At its heart is a supermassive black hole massing in the billions of main-sequence stellar masses, surrounded by a very dense collection of stars and stellar corpses and an accretion disk that produces high-intensity radiation and shreds star systems. The bulge is characterized by very old, dim red stars and stellar corpses, orbiting in near-circular orbits around the black hole at every orientation (hence why it appears as a sphere). The bulge is very low in post-helium elements and lacks warm main-sequence stars. The handful which do exist will be migrants from the disk where gravitational disturbances sent them spiraling inwards. Density increases directly proportionally with distance from the core. Near the edge of the bulge, it is intersected by the disk.

The disk

The most interesting portion of the galactic system, the disk is a population of younger stars in elliptical orbits confined to a single plane. The metallicity of the disk is higher than the other components, and the distance between high-metallicity stars is much lower, and also the rate of star formation. The spiral arms are not actual discrete phenomena but the product of density waves caused by "bunching up" of stars along their orbits; their close proximity makes the arms more dense, brighter, which in turn causes bouts of star formation, further brightening them with short-lived young star populations. The density of the disk increases inversely proportionally with radial distance from the core, with latitudinal distance from the plane of rotation, and directly proportionally with the spiral structure. The disk gradually diffuses outward and exists beyond its visible components further out.

The halo

The halo exists outside the disk and bulge. It consists of a sparse population of old stars in elliptical orbits at random inclinations. Most of these stars are old and dim, but a handful will be former disk stars ejected by gravitational perturbations. Many of the stars may be remnants from collision events.

The satellites

Globular clusters

These congolmerations of tightly-packed stars orbit the center of the galaxy and are mostly very old, red stars that formed at the beginning of the galaxy's formation. Their mellaticity is very low. Any high-metallicity stars were likely captured from the disk. The higher-metallicity clusters are generally associated with bulge (orbit closer in), and the lower ones with the halo (orbit further out). Their cores may be dominated by intermediate-mass black holes in the many thousands of stellar masses.

Satellite dwarf galaxies

These galaxies are similar to normal ones but they are small and have low stellar populations, and orbit close to the primary spiral. They have their own central black holes. Spheroidal, elliptical, and irregular dwarfs are more common than well-defined dwarf spirals.

Galactic remnants

These are semi-defined whisps of stars and gas and other material looping to and from the galaxy and through the halo, relics of collision events between the primary and its own satellites or interloping galaxies.

The dark halo

A massive structure surrounding the galaxy, the dark halo is a nebulous and nearly-unobservable body of matter that contains 95% of the mass of the galactic system. The halo mostly likely consists of a minority of dark, compact objects which are difficult to observe (MACHOs, like starless planets) and a great majority of nebulous particles which do not interact readily with normal matter, confounding detection and observability (WIMPs).