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Stellar Loops


Galaxy NGC 55

This spectacular image of a nearby galaxy, NGC 55, captures huge filaments, loops and "bubbles" of ionized hydrogen gas thousands of light years across. These structures probably result from material blown outward by many generations of massive stars in galaxy. NGC 55 is an irregular galaxy that has hints of spiral structure; it is similar to the Large Magellanic Cloud, a companion to the Milky Way that is visible in the southern sky. NGC 55 resides in a relatively nearby gathering of galaxies, the Sculptor Group, which is located about 5 million light years from the earth.

Annette Ferguson of Johns Hopkins University presented this new view of NGC 55 on June 11 at a meeting of the American Astronomical Society in Madison, Wisc. She and a number of colleagues created the image using the 1.5-meter optical telescope at the Cerro Tololo Inter-American Observatory in Chile. To capture the many faint details of the galaxy's structure, Ferguson's team made a composite of observations from September, 1994 and from September, 1995.

Astronomers theorize that the tangled protrusions from NGC 55 resulted from disruptions caused by a class of hot, rapidly evolving stars within the galaxy. These stars, which have at least ten times the mass of the sun, shine brilliantly and eject material in powerful stellar winds. When such massive stars exhaust their nuclear fuel, they undergo cataclysmic detonations--supernova explosions--which blast away the star's outer layers. The combination of stellar winds and supernova explosions sculpts the interstellar hydrogen gas into the bubbles and filaments seen in this image.

Similar formations are seen in some other galaxies, most notably M82, an object so roiled by stellar evolution that astronomers once mistakenly considered it an exploding galaxy.

Energetic photons from the hot stars ionize the hydrogen, removing each atom's single electron. These loose electrons soon "recombine" with bare hydrogen nuclei (protons), releasing energy in the process. That energy appears in the form of light of a specific wavelength--6,563 Angstroms (an Angstrom is one ten billionth of a meter), which falls toward the red end of the visible spectrum. Ferguson and her colleagues used a special filter, limiting their observations to this narrow band of the spectrum. For comparison, take a look at unfiltered color and black-and-white images of the galaxy.

Observations such as this one demonstrate that, despite the successes of the orbiting Hubble Space Telescope, breathtaking research can still be done from the ground. If anything, there has been a resurgence in the building of new terrestrial instruments that deliver unprecedented clarity at a variety of wavelengths.

Until recently, the 10-meter Keck Telescope on Mauna Kea, Hawaii, was the world's largest optical telescope. Now it has a twin: on May 8, scientists formally dedicated the Keck II Telescope, located next to Keck I atop the 13,796-foot dormant volcano. Keck II will incorporate three specialized spectrographs--instruments for recording an object's spectrum-- that will complement the capabilities of the five-year-old Keck I.

Even more exciting, the two telescopes are designed to work together as an interferometer--a system in which light from two or more telescope is brought together so that the light waves combine and interfere with each other. This new technique of optical interferometry can yield extremely high-resolution images, essentially allowing the two telescopes to function as a single, enormous instrument. Because Keck I and Keck II are some 85 meters (nearly 280 feet) apart, they will have a resolving power equivalent to that of a telescope with an 85-meter mirror, or about 0.005 arc seconds at infrared wavelengths. That is about ten times the precision of the Hubble Space Telescope.

Also at the June meeting of the American Astronomical Society, astronomers from the University of Massachusetts, Amherst and the Instituto Nacional de Astrofosica, Optica y Electronica in Puebla, Mexico revealed plans to build one of the world's largest high-precision radiotelescopes. Dubbed the Large Millimeter Wave Telescope (LMT), the 50-meter instrument will be constructed over the next five years on a yet-to-be selected mountain site in Mexico.

Millimeter waves (shorter than the waves observed by conventional radio telescopes) can penetrate through dust clouds and interstellar haze, revealing the hidden details of star birth, planetary formation and other mysterious astronomical processes. When completed, the LMT's huge dish-shaped antenna will stand taller than a 16-story building, and be able to point to a cosmic source with an accuracy of 1/1,000th of a degree (about four arc- seconds). It will also provide a view of the southern sky that is relatively unexplored, and which includes the center of our Milky Way galaxy.

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