An image of one of the Milky Way analogs found by Timothy Licquia and Jeffrey Newman. Image: University of Pittsburgh |
A team of astronomers in the University of Pittsburgh’s
Kenneth P. Dietrich School of Arts and Sciences announced the most accurate
determination yet of the color of the (aptly named) Milky Way Galaxy: “A very
pure white, almost mirroring a fresh spring snowfall.” Jeffrey Newman, Pitt
professor of physics and astronomy, and Timothy Licquia, a PhD student in
physics at Pitt, reported their findings at the 219th American Astronomical
Society (AAS) Meeting in Austin,
Texas.
While color is one of the most important properties of
galaxies that astronomers study, it has been difficult to make the measurement
for the Milky Way, as our solar system is located well within the galaxy.
Because of this, clouds of gas and dust obscure all but the closest regions of
the galaxy from view, preventing researchers from getting the “big picture”.
“The problem is similar to determining the overall color
of the Earth, when you’re only able to tell what Pennsylvania looks like,” Newman noted.
To circumvent this problem, Newman and Licquia set out to
determine the Milky Way’s color by using images from other, more distant
galaxies that can be viewed more clearly. These galaxies were observed by the
Sloan Digital Sky Survey (SDSS), a project in which Pitt had an instrumental
role that measured the detailed properties of nearly a million galaxies and has
obtained color images of roughly a quarter of the sky. Without the large set of
galaxies studied by SDSS to compare to, an accurate color determination was not
possible. The new color measurement is allowing Pitt researchers to better
understand the development of the Milky Way Galaxy and how it is related to
other objects astronomers observe.
“The problem we faced was similar to determining the
outside climate when you are in a room with no windows.” said 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 Pitt 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 at which they are creating new stars, which are
both related to the brightness and color of a galaxy. The Milky Way Galaxy, the
Pitt researchers realized, 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,” said Newman. “Although it is a
relatively small telescope, only 2.5 m (100 in) in diameter, SDSS has been one
of the most scientifically productive in history, enabling thousands of new
projects like this one.”
Newman described the overall spectrum of light from the
Milky Way as being very close to the light seen when looking at spring snow in
the early morning, shortly after dawn. Michael Ramsey, Pitt 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.
Astronomers divide most galaxies 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 it’s way out,” according to Newman. “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; these researchers measured the average color of
galaxies in the local universe.
“This close match shows that in many ways the Milky Way
is a pretty typical galaxy,” said Newman. “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.”
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 4,700
to 5,000 K. “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,” said Newman.