Color of the universe

Posted: January 11, 2002

If the universe has a distinctive color, it would be a shade of bluish green, astronomers announced Thursday.

Astronomers from Johns Hopkins University reached this conclusion after studying the light from over 200,000 galaxies. They reported their findings -- and their color -- at a meeting of the American Astronomical Society in Washington, DC.

Color change over time. Credit: Johns Hopkins University
"The color is quite close to the standard shade of pale turquoise, although itıs a few percent greener," said Karl Glazebrook, an assistant professor of astronomy at Johns Hopkins. The exact RGB values of the color, which doesnıt have a name, are (0.269, 0.388, 0.342).

Glazebrook and colleague Ivan Baldry found this "universal color" by studying spectra of 200,000 galaxies taken by the 2dF Galaxy Redshift Survey in Australia. This survey uses an instrument that can take the spectra of up to 400 galaxies simultaneously over a two-degree field of view, hence the name.

The astronomers initially combined the spectral data from all the 2dF galaxies to compute a single "cosmic spectrum." This spectrum shows, among other things, the absorption lines from the various elements in the universe. This spectrum can also be used to compare models of star formation in the history of the universe.

For "a bit of fun", according to Glazebrook, they combined the light in the cosmic spectrum into a single color. This color would be what a person would see if all the light in the universe was somehow combined into a single source. The result was the bluish green color released Thursday.

"From one perspective, itıs surprising that it turns out to be greenish, because there are no green stars," said Glazebrook. "But itıs the large numbers of old red stars and young blue stars in the universe that gives us the green."

The result is not entirely frivolous, however, since the color serves as a measure of the age of the galaxies. The fact that the color is greenish indicates that the stars in the galaxies surveyed are, on average, middle-aged. Had this survey been performed earlier in the universeıs history, the color would have been bluer, since brighter but shorter-lived blue giant stars would have dominated. As the universe ages, the color will gradually redden as galaxies are dominated by long-lived red dwarfs.