Measure of universe's age reveals green hue

Astrophysicists study color of stars to track evolution of universe

January 11, 2002|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Forget the fact there are no green stars. The universe, it turns out, is pale green. Hospital green, you could say.

And in a few billion years, it will blush to a redder hue.

Astrophysicist Ivan K. Baldry, a post-doctoral fellow at the John Hopkins University, calculated the cosmic color by merging the visible light spectra from 200,000 galaxies drifting 1 billion to 3 billion light-years from the Earth. He described the results as somewhere between "medium aquamarine," and "pale turquoise."

But then again, speaking by cell phone from Washington, where he presented his findings yesterday to the annual meeting of the American Astronomical Society, he pulled some money from his pocket and allowed "it could be quite close to the greenback." Specifically, the back side of U.S. currency. The color of money.

Or mint. Whatever. He's left a sample online at www.pha.jhu.edu/~kgb/cosspec/.

Baldry and fellow Hopkins astrophysicist Karl Glazebrook tackled the color question as part of a weightier research quest.

They wanted to use the combined light from thousands of galaxies to take a measure of the relative presence of young stars and old stars in the nearby galaxies, which represent the universe at a more recent stage in evolution. (Light travels at a fixed speed, so light from more distant objects has taken longer to get here, revealing them as they appeared a longer time ago.)

Young stars, it seems, are mostly bluish. They get redder in their dotage.

Scientists reason that there must have been more young, blue stars long ago, early in the history of the universe. More recently, Baldry said, after the passage of 10 billion or 12 billion years, "the universe is running out of gas to form stars. As the universe goes on, more and more of the stars that are left are the older stars, the redder stars."

And that's also the pattern suggested by direct observations. Nearby regions have fewer young, blue stars than the most distant regions visible to the Hubble Space Telescope, which reveal the universe as it was early in its evolution.

For a larger sample of the universe, Baldry and Glazebrook borrowed light data from a survey of 200,000 galaxies, as far as 3 billion light-years away. It was gathered as part of the Australian 2dF Galaxy Redshift Survey - an effort to map the filaments and clusters of galaxies that form the large-scale structure of the universe.

The Hopkins scientists broke down the galaxies' combined light into rainbow-like spectra to reveal the representative chemistry of their constituent stars.

Older stars are cooler, with different chemical profiles than younger stars, Baldry said. "By comparing our data with models of past star formation history, we can constrain the number of young stars there are."

The results suggest that the formation of young, blue stars has fallen off sharply. "Six [billion] to 12 billion years ago," Baldry said, "star formation was taking place at a rate five times larger than it is today. In the future, there will be more red stars, so the universe should become redder."

But Baldry and Glazebrook weren't finished. For "a bit of fun," as Glazebrook put it, they decided to figure out what the universe would look like to the human eye if the visible light from all 200,000 galaxies in the survey were merged.

They calculated the intensity, or brightness for each wavelength, or color, and used standard ophthalmological methods to translate the combined results into the primary colors the eye would see.

The result, Baldry said, was "a mixture of red, green and blue. More green than blue, and more blue than red. The predominant color is green."

Call it hospital green.

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