Astronomy Essentials

Why are stars so bright as the year turns?

A flat white spiral with several arms, arrows pointing outward from a dot labeled sun.
View larger. | On June, July and August evenings, we look toward the galaxy’s center, as indicated by the red arrows. On December, January and February evenings, we look away from the center, as indicated by the blue arrows. We’re seeing fewer stars now, and we’re looking into our local spiral arm. The overall effect is that the stars look brighter at this time of year. Artist’s concept via NASA/ JPL/ Caltech/ R. Hurt/ Wikimedia Commons.

Why are the stars so bright?

Look outside on a northern winter (southern summer) evening. You’ll see many bright stars. The evening sky also looks clearer and sharper than it did six months ago, assuming no clouds are in the way. Why?

On December, January and February evenings our evening sky faces away from the center of our Milky Way galaxy. Instead, we look toward our galaxy’s outskirts at this time of year. There are fewer stars between us and extragalactic space now. We’re also looking toward the spiral arm of the galaxy in which our sun resides – the Orion Arm – and toward some gigantic stars in this direction. These huge stars are relatively close to us, within our own galactic neighborhood and local spiral arm, so they look bright.

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Stars so bright: Bare trees glittering under a starry sky, the moon visible through the twigs.
Why are the stars so bright tonight? Even the bright moonlight can’t diminish these bright stars. Icy moonlit trees and a lovely view of Orion, Taurus, and the Pleiades, taken in early January 2017 by Cynthia Haithcock in North Carolina.

Comparing the winter and summer sky

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. Its center is some 25,000 to 28,000 light-years away from us here on Earth. 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. (75,000 light-years is 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. The combined light of so many distant stars gives the sky a hazy quality.

The Orion Arm

Our spiral arm of the galaxy is the Orion Arm. It also goes by the name of the Orion Spur, Local Arm, Orion-Cygnus Arm or the Local Spur. It’s not one of the primary spiral arms of the Milky Way, just 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 entire solar system resides within this this Orion Arm. We’re located close to the inner rim of of this spiral arm, about halfway along its length.

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, scientists named our arm of the galaxy for this constellation.

Several fuzzy white horizontal bands with labeled dots along them.
Our local arm of the Milky Way galaxy is the Orion Arm. Notice Orion’s Belt – the three medium-bright stars (see photo below) – and Orion’s brightest stars Betelgeuse and Rigel. If you visit this page on Wikipedia, you’ll find this image in interactive form.
Black sky with a number of extra-bright stars including Orion.
Sirius and Orion as they appear on a December, January or February evening. Image via EarthSky Facebook friend Susan Jensen in Odessa, Washington. Thank you, Susan!

Bottom line: In December, January and February, we are looking away from the thick stars and clouds of the Milky Way’s core toward the Orion arm, where bright stars reside.

Posted 
December 26, 2021
 in 
Astronomy Essentials

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