Expert: Tom Whiting - 12/20/2010

Question
what are some caracteristics of sprial galaxies with some explanation.

Answer
Spiral galaxies are sort of pancake-shaped (due to rotation) similar to our own Solar system.
Local examples are our own Milky Way, M-31 (Andromeda) and M-33 (Pinwheel Galaxy) in Triangulum.
They contain older Population II stars in toward the core, with younger Population I type stars in the spiral arms. They also contain vast amounts of dust and gas (like the Cygnus Rift) which when colliding together, start off a new round of star formation.  It seems all large spiral galaxies have a massive black hole at their core which may have started the entire formation process at the beginning, several million years after the Big Bang.  All this is opposed to elliptical galaxies which are all spherical in nature, probably slow rotators, and are mostly composed of all old Population II stars with little dust and gas for new star formation.  However, in common is a massive black hole in the core of elliptical galaxies also. Examples in our Virgo Supercluster of elliptical galaxies are M-84, 86, and the central galaxy M-87.
Both types of galaxies are surrounded by a 'ring' of old globular star clusters, made up of all old Population II stars. But it's only the spiral galaxies that have open star clusters, relatively new, young Population I stars like the sun.  (lots of heavy elements).
Examples of open star clusters are the Pleiades (M-45), the Beehive in Cancer (M-44), and the double open cluster in Perseus. (NGC 884 and 869).  Notice most of the open star clusters run up and down the plane of the Milky Way, whereas globular star clusters are centered on the "center"
of the Milky Way in Sagittarius, and formed up billions of years ago as the spiral galaxies developed their pancake spiral structure.
Clear Skies,
Tom Whiting
Erie, PA USA