The most detailed ultraviolet light surveys ever made of the two major galaxies that are closest to our own Milky Way, the Large and Small Magellanic clouds, have been created by astronomers at NASA and Penn State.

"Prior to these images, there were relatively few UV observations of these galaxies and none at high resolution across such wide areas, so this project fills in a major missing piece of the scientific puzzle," said Penn State astronomer Michael Siegel.

Siegel is the lead scientist for the Ultraviolet/Optical Telescope (UVOT) on NASA's Swift observatory, a satellite with three telescopes in Earth's orbit that is controlled by Penn State from the Mission Operations Center on the University Park campus.

No telescope other than Swift's Ultraviolet/Optical Telescope can produce such high-resolution, wide-field, multicolor surveys in the ultraviolet. Ultraviolet light provides a clearer picture of the hottest stars and star-formation regions. Viewing

in the ultraviolet also allows astronomers to suppress the light of normal stars like the sun, which are not very bright at such higher energies. Swift's wide-field imaging capabilities provide a powerful complement to the deeper, but much narrower-field imaging power of NASA's Hubble Space Telescope.

"We took thousands of images and assembled them into seamless portraits of the main body of each galaxy, resulting in the highest-resolution surveys of the Magellanic Clouds at ultraviolet wavelengths," said Stefan Immler, who proposed the survey program and led NASA's contribution from the agency's Goddard Space Flight Center.

Immler presented a 160-megapixel mosaic image of the Large Magellanic Cloud (LMC) and a 57-megapixel mosaic image of the Small Magellanic Cloud (SMC) at the 222nd American Astronomical Society meeting in Indianapolis.

"With these mosaics, we can study how stars are born and evolve across each galaxy in a single view, something that's very difficult to accomplish for our own galaxy because of our location inside it," Immler said.

The new images reveal about 1 million ultraviolet sources in the LMC and about 250,000 in the SMC. The images were made by light in a range of UV wavelengths that largely are blocked by Earth's atmosphere - from 1,600 to 3,300 angstroms.

The LMC and SMC are about 163,000 light-years and 200,000 light-years from Earth, respectively, and orbit each other as well as our own Milky Way galaxy. The LMC is about one-10th the size of the Milky Way and contains only 1 percent of the Milky Way's mass. The SMC is half the size of the LMC and contains about two-thirds of its mass.

Despite their modest sizes, the galaxies loom large in the sky because they are so close to us. Both extend far beyond the field of view of Swift's Ultraviolet/Optical Telescope so thousands of images were needed in order to cover both galaxies in three ultraviolet colors centered at wavelengths of 1,928 angstroms, 2,246 angstroms and 2,600 angstroms.

To produce the 160-megapixel LMC mosaic, Swift's UVOT acquired 2,200 snapshots for a cumulative exposure of 5.4 days. The 57-megapixel SMC image comprises 656 individual images with a total exposure of 1.8 days. Both images have an angular resolution of 2.5 arcseconds, which is a measure of their sharpness. Sources separated by this angle, which is equivalent to the size of a dime seen from 1 mile away, are visible as distinct objects.

The Large and Small Magellanic clouds are readily visible from the Southern Hemisphere as faint, glowing patches in the night sky. The galaxies are named after Ferdinand Magellan, the Portuguese explorer who in 1519 led an expedition to sail around the world. He and his crew were among the first Europeans to sight the objects.