Astronomers Get a Look Inside Galaxies


Edwin Hubble, whose 1936 masterwork, "The Realm of the Nebulae," explained that certain fuzzy objects in the night sky were actually galaxies far beyond our own Milky Way, would have been impressed with the research of his present-day followers.

This new generation of astronomers is actually looking into the cores of certain rare galaxies to identify and map their many parts -- clouds of gas arranged in circular patterns around a brilliantly luminous center.

Indeed, studying a galaxy's innards is a "hot" branch of astrophysics today, as one scientist put it.

Results of the latest research in this field have been published in the April 20 issue of The Astrophysical Journal.

The article describes the work of a group of scientists at the Johns Hopkins University, the National Aeronautics and Space Administration's Space Telescope Science Institute on the university's Homewood campus, NASA's Goddard Space Flight Center and scientists at the University of California and the University of Colorado.

The group has been able to locate and map a galaxy's core features, a merry-go-round of gaseous clouds revolving around the bright center of the galaxy. These features are 10,000 times smaller than anything that could be effectively resolved by the largest telescopes on Earth, the astronomers said.

Their mapping instrument was the International Ultraviolet Explorer (IUE), a scientific satellite operated by NASA and the European Space Agency that has yielded a Niagara of astronomical data since its launching in 1978.

The goal of these studies was to determine "how much stuff is at each [cloud's] location and what's its character," said Dr. Julian Krolik who earned his Ph.D. in physics at the University of California (Berkeley) in 1977 and has been at Hopkins since 1984.

By "character" he meant such information as the chemical composition, density, temperature and pressure of the gas clouds.

The clouds are illuminated by absorbing ultraviolet light from the hot core, or nucleus, of the galaxy. The clouds then re-emit light in wavelengths corresponding to their chemical content. (Every chemical element has its own "signature" wavelength.)

The specific target of this study was a so-called active galaxy with the astronomical designation of NGC 5548, according to Dr. Krolik and his Hopkins associate, Dr. Gerard Kriss.

Their analysis of the clouds was achieved by monitoring variations in the intensity of light emitted by the clouds and comparing it to the intensity of the source of the light originating in the luminous core.

"These clouds emit light in a number of wavelengths and the strongest wavelengths are in the ultraviolet," said Dr. Kriss who received his Ph.D. in physics from the Massachusetts Institute of Technology in 1982 and has been at Hopkins since 1985.

"The ultraviolet observations were critical to the success of this project," Dr. Krolik pointed out. "Using a telescope in space allowed us to make observations at the times we chose. On the ground, you have to deal with clouds [in the Earth's atmosphere], or the bright full moon, and critical observations can be missed."

Vera Rubin, an astronomer at the Carnegie Institution of Washington and a world authority on galaxies, characterized the work of Dr. Krolik and Dr. Kriss and their many associates as "spectacular."

Helmut Abt, managing editor of The Astrophysical Journal, said the subject of these galactic studies is "a hot topic in physics right now."

"They did a superb job with a huge amount of data," Dr. Abt declared.

Their year-long work required 60 observations with the IUE, Dr. Kriss said. Given the fact that observing time on this remarkably durable and useful satellite is at a premium in the astronomical community, the researchers felt fortunate to be able to make so many observations.

Galaxies, such as the Milky Way in which the Earth resides, are described by astronomers as the major building blocks of the universe and are, therefore, of immense importance in helping these scientists along the tedious but intriguing path to a better understanding of the birth, evolution and future life of the universe.

Active galaxies, astronomers explain, emit far more powerful energy, in the form of electromagnetic radiation (X-rays, ultraviolet light, visible light, etc.), than can be accounted for by the billions of stars that make up the galactic whole.

Stars, on the other hand, only emit light in the visible and ultraviolet bands of the spectrum, astronomers note.

The central region of these galaxies contains a solar-system-sized object capable of outshining the rest of the galaxy by factors of 100 to 1,000. The ultimate energy source for active galaxies may be the accretion, or falling inward, of matter onto a black hole, which has a total mass perhaps a billion times greater than that of the Earth's sun.

All these data would certainly be impressive to Hubble, who died in 1953, but hardly surprising.

In his "Realm of the Nebulae," which was republished by the Yale University Press in 1982, he remarked:

"The history of astronomy is a history of receding horizons. Knowledge has spread in successive waves, each wave representing the exploitation of some new clue to the interpretation of observational data."

Albert Sehlstedt is a retired science writer for The Sun.

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