Originally created 01/10/01

Astronomers make large finding



SAN DIEGO -- A supercluster of quasars and galaxies massed together across 600 million light years of space is the largest structure in the observable universe, astronomers say.

In a study presented Monday at the national meeting of the American Astronomical Society, researchers reported that the structure, which includes billions upon billions of stars, is believed to be 6.5 billion light years away.

"We have found nothing bigger in the (astronomy) literature and nobody has brought to our attention anything bigger," said Gerard Williger, a researcher at the National Optical Astronomy Observatories now working at NASA's Goddard Space Flight Center in Greenbelt, Md.

When viewed from Earth, the structure is just below the center of the constellation Leo the Lion. It stretches across an expanse of the sky of two degrees by five degrees, or an area about forty times that of the full moon as seen from Earth.

Williger said it is not known if the gathering of quasars and galaxies is bound together gravitationally or if it is a chance cluster formed by a ripple in the smooth expansion of the universe that followed the Big Bang, which is thought to have set off the formation of the universe.

"This may be an artifact of the Big Bang," he said, speculating that conditions at that point in space may have been uniquely ripe for the quick formation of stars, galaxies and quasars.

That such a large structure could form so quickly after the Big Bang calls into question some of the traditional theories of how the universe evolved, Williger said, since it is difficult to explain how gravity could pull together such an immense cluster in a relatively short time. Further study, which would include calculations of the mass in the structure, may yield new understanding.

"A successful theory has to explain the extremes," said Williger.

Light from the galaxies began its long journey about 6.5 billion years ago when the universe was just a third of its present age and the solar system, including the Earth, had not yet been formed, he said.

The structure includes at least 11 galaxies and 18 quasars in an area where the expected density of objects would be expected to include only two or three quasars and perhaps four galaxies, he said.

Quasars are galaxies with very active and bright center objects, thought to be powered by black holes. Quasars can shine with the brilliance of a trillion suns and astronomers can use this light to silhouette objects nearer Earth. Average galaxies, such as the sun's home, the Milky Way, can contain 100 billion stars. A light year is the distance light travels in a year in space, about 6 trillion miles.

Williger and his colleagues, using the four-meter telescope at the Cerro Tololo Inter-American Observatory in Chile, detected the supercluster's non-quasar galaxies indirectly, by analyzing the light received from quasars that are even farther away. This light is absorbed by the halos of gas surrounding the galaxies, producing shadows that reveal the presence of the galaxies.

There are other clusters of quasars in distant space, but none are as densely grouped as the supercluster in Leo, said Williger.

Williger said the Leo supercluster is more than twice the size of The Great Wall, a gathering of galaxies much closer to the Earth. The Great Wall is about 250 million light years across.

On the Net:

Astronomy information: http:www.gsfc.nasa.gov/

Images: http://www.gsfc.nasa.gov/GSFC/SpaceSci/origins/largecluster.htm