Why is the Milky Way Milky?

This week I ran away from my life as a graduate student and went backpacking in Mt. Rainier National Park. The last night was clear and we spent some time staring at Mt. Rainier with a backdrop of the Milky Way galaxy. It looked like this:

Did you know that an estimated 80% of North Americans cannot see the Milky Way at night? This tidbit made me reflect on the fact that I'm incredibly lucky to have both the time and the funds to escape the city lights. (Image Credit: Michael Matti)

Did you know that an estimated 80% of North Americans cannot see the Milky Way at night? This tidbit made me reflect on the fact that I'm incredibly lucky to have both the time and the funds to escape the city lights. (Image Credit: Michael Matti)

While we were staring at the galaxy my friends were asking me why it looks, well, Milky. 

There are several reasons:

  1. Our location in the galaxy
  2. Low Surface Brightness
  3. Dust

Location is always everything. We live in the galactic suburbs, in a spiral arm. The human eye can only see so far into the galaxy, so we're only seeing a fraction of the visible light of the galaxy. This is part of the reason the Milky Way looks so faint.

Our galaxy is known to astronomers as a Low Surface Brightness galaxy, meaning that it is often fainter than the ambient light in the sky. This means it is inherently faint. Even when you're in a very dark location it can still be difficult to discern the galaxy from the background sky, which makes it appear very diffuse and cloudy to our eye.

Lastly, our galaxy may have billions of stars but it also has a lot of dust and gas. Visible light cannot go through dust. Part of the spilled milk appearance comes from the fact that we're seeing the effect of dust obscuring our line of sight towards the rest of the galaxy. If you want to look through dust, you have to use infrared light. I will leave you with this Spitzer (infrared) image looking towards the center of our galaxy:

The Spitzer space telescope is able to peek through the dust in our galaxy. This is a false color composite from Spitzer - red is hot dust, blue is cooler stars. The stars around the central black hole are shown by the white blob at the center. Astronomers rely upon a variety of different types of light to avoid some of the problems of visible light that I mentioned above. (Image Credit: NASA/JPL-Caltech/S. Stolovy (SSC/Caltech))

The Spitzer space telescope is able to peek through the dust in our galaxy. This is a false color composite from Spitzer - red is hot dust, blue is cooler stars. The stars around the central black hole are shown by the white blob at the center. Astronomers rely upon a variety of different types of light to avoid some of the problems of visible light that I mentioned above. (Image Credit: NASA/JPL-Caltech/S. Stolovy (SSC/Caltech))

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