Milky Way is ‘white as snow’

U. PITTSBURGH (US) — The most accurate determination yet of the color of the Milky Way galaxy finds it is pure white—”almost mirroring a fresh spring snowfall,” astrophysicists say.

While color is one of the most important properties of galaxies that astronomers study, it has been difficult to get that measurement for the Milky Way because our solar system is located well within the galaxy so clouds of gas and dust obscure all but the closest regions from view.

“The problem is similar to determining the overall color of the Earth, when you’re only able to tell what Pennsylvania looks like,” says Jeffrey Newman, professor of physics and astronomy at the University of Pittsburgh.

Images of 25 Milky Way analog galaxies found by Licquia and Newman. These objects are shown in order from bluest (top left) to reddest (bottom right) overall color. They are relatively close to the Milky Way—about 500 million light years away. (Credit: Sloan Digital Sky Survey)

To circumvent this problem, Newman and PhD student Timothy Licquia set out to determine the Milky Way’s color by using images from other, more distant galaxies that can be viewed more clearly that were observed by the Sloan Digital Sky Survey (SDSS), a project that has measured the detailed properties of nearly a million galaxies and has obtained color images of roughly a quarter of the sky.

Findings were presented at the 219th American Astronomical Society (AAS) Meeting in Austin, Texas.

“The problem we faced was similar to determining the outside climate when you are in a room with no windows,” says Newman. “You can’t see what’s happening, but you can look online and find current weather conditions someplace where they should be about the same—the local airport, for example.”

The team identified galaxies similar to the Milky Way in properties that were able to be determined—specifically, their total amount of stars and the rate they are creating new stars—both related to the brightness and color of a galaxy. They realized the Milky Way Galaxy should then fall somewhere within the range of colors of these matching objects.

“Thanks to SDSS, the large, uniform sample needed to select Milky Way analogs already existed. We just needed to think of the idea for the project, and it was possible,” Newman says. “Although it is a relatively small telescope, only 2.5 meters (100 inches) in diameter, SDSS has been one of the most scientifically productive in history, enabling thousands of new projects like this one.”

The overall spectrum of light from the Milky Way is very close to the light seen when looking at spring snow in the early morning, shortly after dawn.

Michael Ramsey, associate professor of geology, notes that new spring snow is the whitest (natural) thing on Earth. Many cultures around the world have given the Milky Way names associated with milk—human vision is not sensitive to colors seen in faint light, so the diffuse glow of the Galaxy at night appears white. That association has proven to be very appropriate, given the Milky Way’s true color.

Most galaxies are divided into two broad categories based on their colors– relatively red galaxies that rarely form new stars and blue galaxies where stars are still being born. (The brightest stars are generally blue, but they are very short-lived on cosmic scales and die out quickly.) The new measurements place the Milky Way near the division between the two classes.

This adds to the evidence that although the Milky Way is still producing stars, it is “on its way out,” Newman says.

“A few billion years from now, our Galaxy will be a much more boring place, full of middle-aged stars slowly using up their fuel and dying off, but without any new ones to take their place. It will be less interesting for astronomers in other galaxies to look at, too: The Milky Way’s spiral arms will fade into obscurity when there are no more blue stars left.”

The Milky Way’s color is exceedingly close to the “cosmic color” measured by Ivan Baldry, a professor of astrophysics at Liverpool John Moores University in England, and his collaborators in 2002.

“This close match shows that in many ways the Milky Way is a pretty typical galaxy. This also agrees well with the ‘Copernican Principle’ embraced by the field of cosmology—that, just as the Earth is not in a special place in the solar system, we should not expect to live in an unusual place in the Universe,” Newman says.

The light from the Milky Way closely matches the light from a D48.4 standard illuminant, or a light bulb with a color temperature of 4700-5000K.

“It is well within the range our eye can perceive as white—roughly halfway between the light from old-style incandescent light bulbs and the standard spectrum of white on a television.”

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