Galactic Types

The Milky Way galaxy is a spiral shaped galaxy consisting of over 400 billion stars. Gas and dust are arranged into three general components as shown by the picture below. The size of our galaxy is the order of magnitude 100 000 ly and it rotates around its centre in 200 - 300 million years.

The halo is a roughly spherical distribution which contains the oldest stars in the Galaxy. The Halo consists of the oldest stars known, including about 146 Globular Clusters, believed to have been formed during the early formation of the Galaxy with ages of 10-15 billion years from their H-R Diagrams. The halo is filled with a very diffuse, hot, highly-ionized gas. The very hot gas in the halo produces a gamma-ray halo. Investigations of the gaseous halos of other spiral galaxies show that the gas in the halo extends out to hundreds of thousands of light years. Studies of the rotation of the Milky Way show that the halo dominates the mass of the galaxy, but the material is not visible, now called dark matter.
The nuclear bulge and Galactic Center. The nuclear bulge is the central, spherical part of a spiral galaxy. It is surrounded by a disk-shaped mass of stars with spiral arms. The Nuclear Bulge appears as a distinct, massive disk-like complex of stars and molecular clouds which is, on a large scale, symmetric with respect to the Galactic Centre. It is distinguished from the Galactic Bulge by its flat disk-like morphology, very high density of stars and molecular gas, and ongoing star formation.

The disk, which contains the majority of the stars, including the sun, and virtually all of the gas and dust The disk of the Galaxy is a flattened, rotating system which contains the Sun and other intermediate-to-young stars. The sun sits about 2/3 of the way from the center to the edge of the disk (about 25,000l.y.). The sun revolves around the center of the galaxy about once every 250 million years.