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Spiral Galaxy

General Information

  1. Spiral galaxies are plate shaped with a central bulge. The whole galaxy is surrounded by an invisible spherical halo of dark matter.

  2. The disk of a typical spiral galaxy is about 100,000 light years across and 2000 light years thick. The spherical central bulge is about 10,000 light years in diameter and the halo is over 100,000 light years across.

  3. The average spiral galaxy contains about 100 billion stars.

  4. Different generations of stars can be found in each of these three regions: first generation – old – stars are found in the halo; second generation – newly formed – stars are found in the disk; both generations are found in the central bulge.

  5. Most of the dust and gas in the galaxy is found in the disk. Almost none is seen in the halo.

  6. The spiral pattern traces the location of current star formation. It may be caused by spiral ripples in the gas of the disk or by the effects of exploding stars (supernovae) on the gas.

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References
Item Updated Notes
Last reviewed 2003-01-13 All looks OK