Milky Way
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The Milky Way

MilkyWay.jpg (499323 bytes) MilkyWaySide.jpg (102917 bytes)

The Milky Way is a gravitationally bound collection of roughly a hundred billion stars. Our Sun is one of these stars and is located roughly 24,000 light years (or 8000 parsecs) from the center of our the Milky Way.

The Galaxy has three major components:

bulletA thin disk consisting of young and intermediate age stars — this disk also contains gas and is actively forming new stars. Dust in the disk makes it appear orange in the picture. Dust absorbs blue light more than red light and thus makes stars appear reddish. Our Galaxy has spiral arms in its disk — these spiral arms are regions of active star formation.
bulletA bar of older stars (white in the COBE picture).
bulletAn extended dark halo whose composition is unknown. Since the matter in the halo does not consist of luminous stars, it does not show up in the COBE image. The existence of the dark halo is inferred from its gravitational pull on the visible matter.

 
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:

 
bullet The halo — a roughly spherical distribution which contains the oldest stars in the Galaxy,
bullet The nuclear bulge and Galactic Center.
bullet 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.

The Disk

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. by the most modern estimates). The sun revolves around the center of the galaxy about once every 250 million years. The disk also the galaxy about contains atomic (HI) and molecular (H2) gas and dust.

 

Copyright © 2010 Tim Stouse
Last modified: December 10, 2010
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