As seen during Northern Hemisphere winter (or Southern Hemisphere summer), the stars seem brighter. Why? It’s partly because – on December, January and February evenings – the part of Earth you’re standing on is facing into the spiral arm of the galaxy to which our sun belongs.
Consider the sky at the opposite time of year. In June, July and August, the evening sky seen from the entire Earth is facing toward the center of the Milky Way galaxy. The galaxy is about 100,000 light-years across, and its center is some 25,000 to 28,000 light-years away. We don’t see into the exact center of the Milky Way, because it’s obscured by galactic dust. But during those Northern Hemisphere summer months (Southern Hemisphere winter months), as we peer edgewise into the galaxy’s disk, we’re gazing across some 75,000 light-years of star-packed space (the distance between us and the center, plus the distance beyond the center to the other side of the galaxy).
Thus – on June, July and August evenings – we’re looking toward the combined light of billions upon billions of stars.
On December, January and February evenings, we’re looking the opposite way – into the suburbs of the galaxy and into the spiral arm of the galaxy in which our sun resides.
There really are some gigantic stars located in this direction, and they are relatively close to us – in our own neighborhood, so to speak, our own spiral arm.
So we’re seeing fewer stars on Northern Hemisphere winter evenings (or Southern Hemisphere summer evenings), as we look across only about 25,000 light-years of Milky Way, toward the deep space beyond our galaxy’s boundaries.
And that’s why, while the combined light of so many distant stars visible on June, July and August evenings gives the sky a hazy quality, the evening sky in December, January and February looks clearer and sharper.
Our spiral arm of the galaxy is called the Orion Arm, or sometimes the Orion Spur. It’s not one of the primary spiral arms of the Milky Way but only a “minor” spiral arm. Our local Orion Arm is some 3,500 light years across. It’s approximately 10,000 light years in length. Our sun, the Earth, and all the other planets in our solar system reside within this Orion Arm. We’re located close to the inner rim of of this spiral arm, about halfway along its length.
The Orion Arm is also sometimes called the Local Arm, the Orion-Cygnus Arm, or the Local Spur.
Perhaps you know the bright stars of the prominent constellation Orion the Hunter? This constellation is visible in the evening during Northern Hemisphere winter (Southern Hemisphere summer). The stars of mighty Orion also reside within the Orion Arm of the Milky Way. In fact, our arm of the galaxy is named for this constellation.
Bottom line: During Northern Hemisphere winter (Southern Hemisphere summer), Earth’s evening sky is facing away from the star-rich center of the galaxy, toward the outskirts of our Milky Way galaxy. We’re all looking into the spiral arm of the galaxy to which our sun belongs. That’s why we see so many bright stars; they belong to our local spiral arm.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.