Seeing green: color of the cosmos. (Cosmology).

We live in a pale-green universe. That's the conclusion of researchers who analyzed the colors of some 200,000 galaxies as part of the largest galaxy survey completed to date.

The survey mapped the brightness and distances of galaxies in two giant swaths that together cover 5 percent of the sky. From those data, Karl Glazebrook and Ivan Baldry of Johns Hopkins University in Baltimore and an international group of colleagues constructed what they call the cosmic spectrum.

The result is a record of the intensity of radiation that the many galaxies in nearby regions of the cosmos emit at various visible-light wavelengths. The average color, which is what the human eye would see if an observer could view from afar all the light sources in the universe together, is a few percent greener than turquoise.

Glazebrook and Baldry note that although the universe doesn't have any green stars, the large number of old, red stars and young, blue stars combine to give the overall pale-green color.

The researchers assert that the survey, known as 2dF, or 2-degree field, is large enough to make it a representative sample of the local universe.

Completed this month, 2dF used a fiber-optics spectrograph on the Anglo-Australian Telescope in Coonabarabran, Australia that can measure distances up to several billion light-years of 400 galaxies each night.

The cosmic spectrum makes for more than a pretty picture, Glazebrook adds. By analyzing it, researchers can determine the proportions of older, cooler stars and hotter, younger stars in today's universe. Comparing the spectrum to cosmological models, the scientists can then extrapolate what the rate of star formation was in the past.

The models suggest that the majority of stars formed more than 5 billion years ago. Young, hot stars tend to emit more of their energy at bluer wavelengths, while older, cooler stars emit most of their light at redder wavelengths. The data therefore suggest that early in its history the universe was in a blue period, dominated by the light of young stars. It since has moved into a middle-aged green period and will ultimately enter a final red period, in which few stars are born and the elderly survivors color the cosmic canvas. --R.C.