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An old friend in a new light.

OUR KNOWLEDGE of the ultraviolet universe remained meager until telescopes could be launched into space, since these short wavelengths of light are largely absorbed by Earth's atmosphere. Ultraviolet astronomy now has been pursued successfully for decades. However, the GALEX space telescope, launched in April 2003, is imaging objects with a level of detail that is unprecedented in ultraviolet astronomy. One of the most beautiful and interesting cases is this mosaic covering the Andromeda Galaxy, M31.

One is immediately struck by the conspicuously bright spiral arms that wind around the center of M31. These arms stand out in this picture because they contain large numbers of luminous blue stars--massive young objects that can be found only near their birthplaces. The most brilliant of these stellar nurseries is the huge cluster of young stars toward the lower-right (southwestern) limb of the great galaxy. Known as NGC 206, this giant complex of young stars is bright enough to be seen visually with amateur telescopes.

Near the opposite (northeastern) end of the image is another highly luminous object, the star-forming complex known as OB 102. Unlike NGC 206, OB 102 is rich in ionized gas and peppered with young open clusters. Why two ostensibly similar objects should differ to GALEX's ultraviolet eyes is an intriguing question that may find an answer when the data are analyzed.

Earlier space-based ultraviolet imagers showed that, at these wavelengths, spiral galaxies look quite similar to views made in the light of hydrogen alpha (H[alpha]), which comes from regions of hot gas (the inset shows M31 in H[alpha] light). This is reasonable, because copious ultraviolet light from massive, young, hot stars energizes any nearby interstellar gas. However, there is not a perfect one-to-one correspondence between M31's ultraviolet and H[alpha] light. For instance, NGC 206 is faint in H[alpha], but brilliant in the ultraviolet. Also, M31's inner spiral arms are more noticeable in the ultraviolet than in H[alpha]. These cases suggest that the gas content of these star-forming regions is low, probably because much of their gas has been consumed making stars. On the other hand, the arms that are to the upper right (west) of M31's central bulge seem unusually faint in the GALEX mosaic when compared to existing H[alpha] (and red-light) images. This is the result of a layer of dust, which obscures ultraviolet rays more than red light.

The author of 18 books for both active researchers and the general public, University of Washington astronomy professor PAUL HODGE edits the Astronomical Journal.

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Title Annotation:GALEX, a space telescope
Author:Hodge, Paul
Publication:Sky & Telescope
Geographic Code:1USA
Date:Aug 1, 2004
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