Page 5, External Galaxies: [Previous Page] [Next Page] [Class Home Page] Text Pages 376-377

Spirals galaxies have extended disks or arms. They progress from very bulge-dominated spirals (Sa), to arm-dominated spirals (Sc). Below are examples of the three kinds of spiral galaxies we see. Their designations are, respectively Sa, Sb, and Sc.

M94 is an Sa galaxy. Its arms are dwarfed by the immense central bulge. What you can clearly see in this photo is the dust lanes. This tells us that gas and dust exist in the outer disk region of the galaxy. Sa galaxies have some star formation occurring.

M81 is an Sb galaxy 12 million light years away. Our Milky Way galaxy probably looks like this Sb galaxy. Sb's show fairly extensive arms, yet a reasonably large and luminous central bulge. About 15% of the visible mass is in cold gas and dust. Star formation is seen along the arms as very bright knots of HII regions. Sb galaxies have a fairly massive Pop II stellar bulge component and an extended halo of Globular Clusters, similar to what is seen in our own galaxy.

M74 is an Sc galaxy. "Late" Spirals are designated as Sc They have extensive arms showing vigorous star formation. Their bulges are nearly non-existent. Typical gas and dust mass fraction is greater than 20-30%. All galaxies contain some old stars, but late-type Spiral galaxies have the highest ratio of new, young stars. M74 is the largest galaxy among a small group of galaxies 35 million light years away. The spiral arms are dotted with enormous HII regions, which trace the gas inward all the way to the small central bulge.