WB Chapter 1 - Galaxies III - Structure

The structures of different galaxies can vary in size and detail, so in this article we are just going to discuss the different locations in spiral type galaxies

Spiral galaxies consist of four distinct components:

• A flat, rotating disc of (mostly newly created) stars and interstellar matter
• A central stellar bulge of mainly older stars, which resembles an elliptical galaxy
• A near-spherical halo of stars, including many in globular clusters
• A supermassive black hole at the very center of the central bulge

 The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy.

Spiral arms
Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name. Naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very "loose" arms, whereas Sa and SBa galaxies tightly wrapped arms (with reference to the Hubble sequence). Either way, spiral arms contain a great many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so remarkable.

Galactic bulge
A bulge is a huge, tightly packed group of stars. The term commonly refers to the central group of stars found in most spiral galaxies.

Using the Hubble classification, the bulge of Sa galaxies is usually composed of population II stars, that is old, red stars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are a great deal lesser, and are composed of young, blue, Population I stars. Some bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity), and others simply appear as higher density centers of disks, with properties similar to disk galaxies.

Many bulges are thought to host a supermassive black hole at their center. Such black holes have never been directly observed, but many indirect proofs exist. In our own galaxy, for instance, the object called Sagittarius A is believed to be a supermassive black hole.

Galactic spheroid
However, some stars inhabit a spheroidal halo or galactic spheroid. The orbital behaviour of these stars is disputed, but they may describe retrograde and/or highly inclined orbits, or not move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example, the Sagittarius Dwarf Elliptical Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it.The bulk of the stars in a spiral galaxy are located either close to a single plane (the Galactic plane) in more or less conventional circular orbits around the center of the galaxy (the galactic centre), or in a spheroidal galactic bulge around the galactic core.

Unlike the galactic disc, the halo seems to be free of dust, and in further contrast, stars in the galactic halo are of Population II, much older and with much lower metallicity than their Population I cousins in the galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many globular clusters.

The motion of halo stars does bring them through the disc on occasion, and a number of small red dwarf stars close to the Sun are thought to belong to the galactic halo.

Quoted from Wikipedia http://en.wikipedia.org/wiki/Spiral_galaxy#Structure