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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.
Click on image for full size
Image provided by Jason Ware

Spiral Galaxies

Spiral galaxies may remind you of a pinwheel. They are rotating disks of mostly hydrogen gas, dust and stars. Through a telescope or binoculars, the bright nucleus of the galaxy may be visible but the spiral arms are dimmer and difficult to see.

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 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. The halo is also the home of dark matter in spiral galaxies.

Spirals are subdivided based on the appearance of the arms and the central region. Sa types have a large, bright central region and tightly wound arms, while Sc types have a smaller central region and loosely wound arms. Sb types are somewhere in between. Spiral galaxies can also have bar-like structures through them. These galaxies are classified as SB.

Galaxies like to live together in groups called clusters. There are not many of spirals in a cluster usually, 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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