Introductory Astronomy: Spiral Galaxies

Spiral galaxies are strikingly beautiful objects. Although they are not the most common type of galaxy, they are brighter than the more common small elliptical galaxies so they are easy to observe. Spiral galaxies can be 10,000 to 30,000 parsecs across. They contain 1-100 billion stars (and therefore have masses of 1-100 billion solar masses). Spiral galaxies also contain large amounts of gas and dust, which tells us that star formation is taking place inside them.

Spirals are easily identified by their main components. They are in the shape of a flat disk with a bright central nucleus and spiral arms that extend out from the nucleus. (The spiral arms are regions where stars are being born.) A spiral galaxy that is observed "face-on" will therefore look like a pinwheel ; one that is observed "edge-on" will look somewhat like a flying saucer (or frisbee), but with dark streaks across the center (these streaks occur when the gas and dust of the galaxy block the light from the stars). Spiral galaxies can also have a central bar in their nucleus. When this happens, the arms of the galaxy extend outward from the ends of the bar (instead of from the actual center of the galaxy).

Hubble classified spiral galaxies further by comparing their basic characteristics. He defined the S0 galaxies ("S" for Siral) as those galaxies which have a very bright nucleus with lots of gas and dust but no visible spiral arm structure, Sa galaxies as those galaxies which have a very bright nucleus and tightly wound arms, Sc galaxies as those which have a small nucleus and open arms, and Sb galaxies as those which are intermediate between the Sa and Sc galaxies. Spiral galaxies with bars have the same classification, but are given the designation B ("B" stands for Barred) as well. So, for example, a barred spiral galaxy with a small nucleus and very open arms would be classed as an SBc galaxy.

The Milky Way Galaxy is a spiral galaxy. It is classed as a Sbc galaxy, which means that it is somewhere between types Sb and Sc. Many scientists also believe that the Milky Way has a bar at its center; this would make it a type SBbc galaxy.