Friday, January 17, 2014

It's the neighborhood! Why are stars brighter in the winter than the summer?

A couple of days ago, a young person asked me the question, "Why do stars seem so much brighter in the winter?"

I immediately answered by saying that it's mostly because the sky is clearer in this area with little dust and the like obscuring our view and that there was less moisture in the sky because it is so much colder.

Both these answers are right but after thinking about it, there is so much more to the answer so I figured I'd give it here.

Why are stars so much brighter in the winter than in the summer?

A part of the answer has to do with the fact that the lower blanket of air surrounding us, the troposphere, is not as deep as it is during the summer.  That means there is less moisture in the air to freeze and scatter light.  Huh?

Well think about it this way:  ice crystals, water vapor, and particles (pollen, dust, smoke, pollution) scatter light when light hits them.  The light they scatter is mostly red light (think about what a prism does when it it is hit by light--it breaks up white light and makes a rainbow of colors). The blue wavelengths of light remain for us to see so the sky looks blue.  The more water and particles that are in the air, the blue wave lengths of light are scattered as well.  So on a very humid day or hazy one, the sky looks pale blue or even almost white--sort of a reverse prism. In winter, the first layer of air is not as big so there is less stuff in that air to scatter and break up the white light into colors.Most of the time, the sky in winter during the day time is not as bright blue as the summer.  This goes for the night sky as well except the lack of particles in the air makes the seeing and transparency better in the sky which in turn, makes it brighter.

That's enough for theory.  Another (and probably bigger) reason the stars in the winter are brighter than the stars in the summer has a great deal to do with our neighborhood and which way we are looking.  If you stand in front of your house and look one way you may see more houses, trees, and people while when looking the other way, there might be fewer or no houses or trees, perhaps a road and a field across the road. The same thing happens from earth.

During the summer (as our planet takes its annual turn around the sun) the part of the earth we are standing on is facing the center of the Milky Way.  We can't see the center because of galactic dust but we are looking through some 25,000 light years of stars ( about 6 trillion miles{distance light travels in a year X 25,000} dusts and dark matter.  The center of just about any galaxy (including ours) is the most heavily populated (with stars) part of the galaxy.  We are also looking beyond the center of the galaxy to its extreme reaches and beyond. So, actually we're looking at at least 75,000 light years of stars. The light from those billions of stars brightens the night sky and makes all stars, even the closest ones, somewhat paler to our eyes.

Are your eyes glazing over yet?

In the winter months, we are looking in a different direction...sort of like across the road into the field.  Now we are looking across only (only??) 25,000 light years and out beyond our galaxy into unpopulated space.  The stars we see are generally fewer and much closer to us--neighbors, so to speak -- and most reside in our own private arm (the Orion Arm or the Orion Spur) of the Milky Way.  because the stars are closer and don't have to compete with as much background light, they appear to be brighter, crisper, and clearer to our eyes.

There, I think I answered the question but here's a graphic that may help you visualize this a little better thanks to Earth/Sky:

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