Why Does Blue Light Travel Faster Than Red in the Milky Way?

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The discussion centers on the phenomenon of light traveling through the Milky Way, specifically why blue light travels faster than red light due to scattering. The University of Sydney's research indicates that longer wavelengths, like red, are slowed more than shorter wavelengths, such as blue, when interacting with the Warm Ionised Medium (WIM). Participants question whether scattering is the same as refraction, noting that blue light is typically refracted more. The analysis relies on data from pulsars to measure the lag of red light behind blue, which helps estimate the amount of WIM encountered. The conversation highlights the complexities of light behavior in space and the implications for astronomical measurements.
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I was reading an astronomy article about the size of the milky way

http://www.usyd.edu.au/news/84.html?newsstoryid=2163"

The article says that the longer wavelengths (red) travel more slowly than the shorter wavelengths (blue) due to scattering.

Is scattering the same as refraction?
If so I thought that blue light was refracted the most and therefore slower than red.
Can someone explain?

Quoting the relevant passage from the article;
The University of Sydney team's analysis differs from previous calculations because they were more discerning with their data selection. "We used data from pulsars: stars that flash with a regular pulse," Professor Gaensler explains. "As light from these pulsars travels to us, it interacts with electrons scattered between the stars (the Warm Ionised Medium, or WIM), which slows the light down.

"In particular, the longer (redder) wavelengths of the pulse slow down more than the shorter (bluer) wavelengths, so by seeing how far the red lags behind the blue we can calculate how much WIM the pulse has traveled through.
 
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I could be wrong, but maybe refractive index increases towards the red in a plasma.
 
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