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What colour is the
Milky Way?

by Phil Unsworth
for Astronomy Now
Posted: 16 January 2012


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Astronomers at The University of Pittsburgh have been working to find an accurate answer as to the colour of the Milky Way Galaxy, finding it to be “a very pure white, almost mirroring a fresh spring snowfall”.

The colour of a galaxy is one of its more important properties as it can reveal what activity is happening within it, however this can be problematic for our own Milky Way, due to our position within it where all but the closest regions are obscured by clouds of gas and dust, preventing astronomers from seeing the 'big picture'.


Twenty-five Milky Way analogue galaxies analysed by Licquia and Newman in order from bluest (top left) to reddest (bottom right). Image: Sloan Digital Sky Survey.

Given this hindrance, the University Of Pittsburgh team, including Jeffrey Newman and Timothy Licquia – Professor of physics and PhD student in physics, respectively – decided to find a way around the problem, using information that could be determined more easily.

“The problem is similar to determining the overall colour of the Earth, when you’re only able to tell what Pennsylvania looks like,” says Newman.

The team used data and images collected by the Sloan Digital Sky Survey (SDSS); a project in which the university had a significant role. The SDSS has measured various properties of close to a million galaxies, including colour. Armed with this information, and using only those galaxies which are comparable to the Milky Way in terms of number of stars and the rate at which new stars are produced, the team were able to pinpoint an accurate range of colours that the Galaxy could fall into.

Star numbers and production rates are both related to a galaxy's brightness and colour. “Usually it is the 'dead, red' ellipticals with no new stars that cluster into the red sequence, and blue star-forming spirals that cluster into the blue cloud, but that isn't the whole story," says Licquia. "Our results show that the Milky Way must be right near the division line between the two classifications. This means our Galaxy is one of the brightest of redder spiral galaxies. This also tells us that we are on our way out as a star forming galaxy. In a few billion years, the Milky Way will be a much more boring place with only our elder red stars remaining and our beautiful spiral arms (where star formation takes place) will fade away.”

From Earth's edge-on view of the Milky Way's disc it appears milky white, earning its name. At faint light levels, the human eye is insensitive to colour, but now the team's efforts show that the galaxy actually is white, positioning it on the borderline between the two divisions.

According to the scientists, the white of the Milky Way is close to the white of light on spring snow, just after dawn, the whitest naturally occurring thing on Earth.

The new knowledge of the Galaxy's colour can help to further our ideas of how it works and develops.“For instance, they [similar galaxies] span a broad range in morphology (shape), despite matching the Milky Way in some of its most fundamental properties," says Newman. "We hope to use this sample to find out how the galaxies that most resemble the Milky Way in all ways differ from those that match in only some ways.”

Licquia adds, “Our work has made major strides in more precisely knowing how our own Galaxy would appear from such a vast distance away. One of the inspirations for our project stems from a study trying to make such a comparison, but the previous uncertainties in the properties of our Galaxy were so high that no real conclusions could be drawn. Our results will be beneficial to a wide variety of galaxy evolution studies.”

The team announced their findings at the 219th American Astronomical Society Meeting in Austen, Texas last week.

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