It seems like that to you too May the winter stars shine brighter? It’s partly an impression, but there is a grain of truth: actually in winter the light from the stars “stands out” much more than the darkness of the sky, which is greater when the air is colder due to the lower humidity and the sun being lower below the horizon. CIt is then a purely geometric question: on winter nights the northern hemisphere points towards an area of the Milky Way full of very bright stars, while in summer it points towards the galactic centre, significantly increasing the background brightness of the sky.
In winter the night sky is darker: atmospheric causes
Let’s start from a premise: hot air is able to retain quantities of water vapor (humidity) greater than cold air. Less humidity means less light dispersion by water vapor and therefore less “haze effect”. In short, the background brightness of the sky tends to be lower in winter, resulting in darker skies and stars that “stand out” more.
The winter sky is darker also because it is in the cold season the Sun is lower below the horizon at night compared to what happens in summer. This is a direct consequence of the fact that in winter the Sun is less inclined to the horizon due to how the Earth’s axis is tilted (which is why it is cold in winter). This contributes to increasing the contrast between the darkness of the sky and the brightness of the stars, and therefore the perception of a dazzling night sky.
Winter stars are brighter: astronomical causes
If the stars seem brighter in winter, the reason is also there different direction of the sky where we are looking. In fact, in winter and summer the night sky shows opposite directions in the firmament.
In summer the northern hemisphere faces the center of the Milky Waya region extremely rich in very distant stars, whose diffuse brightness adds a background of brightness to the night sky. In winter, however, our hemisphere faces the outside of our galaxy at night, to be precise that of arm of Oriona minor arm of the galactic spiral in which the Sun is also located.
In spiral galaxies, such as the Milky Way, the arms are regions rich in massive and bright stars, which therefore in winter we in the Northern Hemisphere can observe from a relatively close distance. It is no coincidence that it is precisely in winter in which we can enjoy the spectacle of particularly bright stars such as Sirius (the brightest ever in our sky), Betelgeuse And Rigel (both in the constellation of Orion) e Aldebaran. Therefore, if in winter the stars seem brighter to us, it is also because we are actually observing particularly bright stars.
