Spirals Catch a Density Wave

In the preceding chapter, we took a prolonged look at Way. Hubble labeled all spiral galaxies with the letter " pending on the size of the galactic bulge. Sa galaxies have large bulges, Sb medium-sized bulges, and Sc the smallest bulges. Very clearly defined, tightly wrapped spiral arms are also associated with Sa galaxies, whereas Sb galaxies exhibit more diffuse arms, and Sc galaxies have more loosely wrapped, less defined spiral arms.

Another subtype of the spiral galaxy is the barredspiral galaxy, which exhibits a linear bar of stars running through the galactic disk and bulge out to some radius. In these barred galaxies, the spiral arm structure begins at the ends of the bar. This spiral subgroup, designated SB, also includes the a, b, and c classifications based on the size of the galactic bulge and the winding of the arms. Some astronomers believe that the Milky Way is a barredspiral galaxy, though our viewpoint (stuck in the disk!) makes it difficult to tell.

one spiral galaxy, the Milky 'S" and added an a, b, or c, de-

A barred-spiral galaxy is a spiral galaxy that has a linear feature, or bar of stars running through the galaxy's center. The bar lies in the plane of the spiral galaxy's disk, and the spiral arms start at the end of the bar.

Star Words

A barred-spiral galaxy is a spiral galaxy that has a linear feature, or bar of stars running through the galaxy's center. The bar lies in the plane of the spiral galaxy's disk, and the spiral arms start at the end of the bar.

NASA and M Carofe {Cctumba Univcrstfy)' STScl-PRC99-34a

A region in NGC 1365, a barred-spiral galaxy located in a cluster of galaxies called Fornax. The central image was photographed through a ground-based telescope. The images to the left and right show the central bulge as seen at optical and infrared wavelengths with the Hubble Space Telescope.

(Image from NASA and M. Carollo)

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