Step outside on a crisp, clear winter’s night and the stars seem to shine brighter and twinkle more vibrantly than at any other time of year. This leads folks to ask, “Are the stars brighter in the winter?” The short answer is yes and no. If however, the question were rephrased to, “Do the stars appear brighter in winter?”, then the answer would be a definite yes.
Now the long answer…
More Bright Stars In Winter
In the winter we are looking at the rich starfield of the winter Milky Way, and the spiral arm that is farther out than we are.
In the summer we’re looking toward the core of our galaxy. Ordinarily, you’d think we’d see more bright stars looking toward the core. That said, if you look at a list of the brightest stars in the sky visible from Earth, you’ll see the majority of them are actually in the winter sky.
It is merely coincidence that a majority of the brightest stars are in the winter, they could just have easily been on the summer, core-facing side. It is this coincidence, however that helps to create the impression the winter stars are brighter. It is not the primary reason though.
Humidity Makes the Difference!
The clarity of the atmosphere, and thus the apparent brightness of the stars, is primary based on the amount of humidity in the air. During the warm days of summer, the atmosphere can hold more moisture. This moisture creates hazy skies, thus dimming the stars. During the winter, the cold air can’t hold as much moisture. Little moisture means little to no haze, allowing the stars to be seen at their best. Thus while the stars of winter are not intrinsically brighter, the clarity of the atmosphere gives the appearance that they are.
Humidity also has an affect on the amount of twinkle we see. As starlight comes through the atmosphere it passes through pockets of different temperature. Different temperatures create different densities. As the light passes through different densities, it is slightly bent. The random movement of these density differences across the air column is what causes stars to twinkle.
Moisture helps to even out temperature differences in the atmosphere. Thus during the hazy skies of summer, stars are a little dimmer and they have less twinkle. In the winter, they shine brighter and twinkle more.