The Milky Way

The Milky Way system is a spiral galaxy consisting of over
400 billion stars , plus gas and dust arranged into three general components as shown to the left:

The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias) sometimes referred to simply as "the Galaxy"), is a barred spiral galaxy that is part of the Local Group of galaxies. Although the Milky Way is one of billions of galaxies in the observable universe,[4] the Galaxy has special significance to humanity as it is the home galaxy of the planet Earth. The plane of the Milky Way galaxy is visible from Earth as a band of light in the night sky, and it is the appearance of this band of light which has inspired the name for our galaxy.

• The halo - a roughly spherical distribution which contains the oldest stars in the Galaxy,

• The nuclear bulge and Galactic Center.

• The disk, which contains the majority of the stars, including the sun, and virtually all of the gas and dust

The Halo

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 also filled with a very diffuse, hot, highly-ionized gas. The very hot gas in the halo produces a gamma-ray halo.

Neither the full extent nor the mass of the halo is well known. Investigations of the gaseous halos of other spiral galaxies show that the gas in the halo extends much further than previously thought, 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.