What do you suppose the Milky Way’s color is based on what you’ve seen with your eyes? If you guessed white, you’re dead on. A team of astronomers in Pitt’s Kenneth P. Dietrich School of Arts and Sciences announced last week the most accurate determination yet of the color of the Milky Way Galaxy: “a very pure white, almost mirroring a fresh spring snowfall.”
It seems obvious, but it’s really not easy to figure out the overall color of something you’re inside of or surrounded by. Looking out my window here in northern Minnesota, I might be tempted to call the overall color of the Earth gray, but I know better. What the team did instead was flip through the 930,000 galaxies images taken by the Sloan Digital Sky Survey and identify those that had qualities very similar to the Milky Way. In particular, they examined the total numbers of stars and the rate at which the galaxies were creating new stars, which are both related to the brightness and color of a galaxy. They figured the Milky Way should then fall somewhere within the range of colors of these matching objects.
Jeffrey Newman, Pitt professor of physics and astronomy, described the overall spectrum of light from the Milky Way galaxy as being very close to the light seen when looking at spring snow in the early morning, shortly after dawn. You might also liken the color to the word that’s always been staring at us in the galaxy’s name: milk.
Astronomers classify galaxy colors broadly into two types – young ones that have a bluish color because they’re hatching lots of hot blue stars thanks to their abundant supply of dust and older red galaxies with more evolved stars. Big blue stars burn their fuel quickly and flame out while relatively young, while many of the smaller stars are either reddish or evolve into red-colored stars.
The Milky Way falls comfortably between both categories. Although it’s still producing new stars, it’s headed for middle age.
“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,” according to Newman.
In case you have no freshly-driven snow out your front door, our galaxy’s color is roughly halfway between the light from old-style incandescent light bulb and the standard white on a television. The next time you have a chance to see that band of milky light cascading from Gemini’s feet across Orion’s shoulder and into Canis Major, consider its individual stars as snowflakes piling up on your walkway after a winter storm.