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The Spring 2011 issue of The Earth Scientist is focused on modernizing seismology education. Thanks to IRIS, you can download this issue for free as a pdf. Print copies are available in our online store.
Our neighbor, the Andromeda spiral galaxy.
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Image provided by Jason Ware

Spiral Galaxies

Spiral galaxies may remind you of pinwheels turning slowly as though in some intergalactic breeze. They are rotating disks of gas, dust and stars. Through a telescope or binoculars, the bright nucleus of the galaxy may be visible but the spiral arms which are more diffuse can be difficult to discern.

Spiral galaxies are complex objects and have several components: a disk, a bulge, and a halo. The disk contains gas, dust, and young stars in its spiral arms. The dense bulge or nucleus in the center of the disk contains mostly old stars and no gas or dust. The halo is the home of a very few scattered stars and globular clusters. While mostly empty of visible matter, the halo is also the realm of dark matter in spiral galaxies.

Spirals are further subdivided based on the appearance of the arms and the nucleus. Sa types have large nuclei and tightly wound arms, while Sc types have small nuclei and sprawling arms. Sb types are somewhere in between. Spiral galaxies can also have bar-like structures through them. These galaxies are classified as SB, and are further subdivided a-c in the same way as regular spirals.

Spiral galaxies don't come in a dwarf variety, and there are not usually many of them in clusters compared to the number of ellipticals. But they are more common than ellipticals in the regions between clusters.

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The Summer 2010 issue of The Earth Scientist, available in our online store, includes articles on rivers and snow, classroom planetariums, satellites and oceanography, hands-on astronomy, and global warming.

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