Summer is a wonderful time for stargazing. Not only is the nighttime weather usually pleasant, but the sky offers some remarkable sights — most notably, the gossamer arch of the Milky Way. It’s a time you’ll often find me out with a telescope, helping groups enjoy the cosmos.
Over many years as a celestial tour guide, I’ve discovered how many folks are surprised to learn that stars do not all appear the same but actually display different colors. Those that appear white are hotter than those that are orange or red. And bluish stars are the hottest of all.
Trying to see these hues, however, can be quite challenging since the human eye’s color receptors do not respond well to faint light. You know this is true if you’ve ever looked around a dark movie theater; shapes and shades of gray are pretty easy to spot, but colors are virtually nonexistent.
Yes, star colors are subtle, and anyone with even the slightest colorblindness might miss them completely, especially since colorful stars are often widely separated in the sky.
But there is a place in the heavens where two very differently tinted stars lie side by side and, when viewed through a small backyard telescope, always elicit “oohs” and “aahs.” Astronomers know this binary star as Beta Cygni, but its proper name is Albireo.
We can find Albireo within the boundaries of the Summer Triangle — formed by the three bright stars Deneb, Vega and Altair — nearly straight overhead after dark this week. It marks the head of Cygnus, the swan, which stargazers also know as the Northern Cross.
With our unaided eyes, we see Albireo as a single star, no matter how good our vision. But aim a small, low-powered telescope in its direction and you’ll be able to resolve its light into that of two separate stars.
In 1905, the astronomy writer Agnes Clerke wrote that the “golden and azure” tints gave perhaps “the most lovely effect of color in the heavens.” And anyone peering at Albireo for the first time will agree that she was absolutely correct.
While viewing this beautiful stellar pair — or any celestial object, for that matter — it’s fun to contemplate exactly what we’re seeing. Albireo’s stars not only represent stunning colors but a fundamental property of stars as well. One of its components is a yellow star, about 7,700 degrees Fahrenheit, and the other a bluish star, at 19,500 degrees.
The stars of Albireo are known to lie about 400 light-years, or about 2,400 trillion miles, from us. Sounds far, but they are only 0.4 percent of the way across our Milky Way galaxy — very close neighbors indeed.
Despite this pair’s brightness and relative proximity, astronomers still aren’t sure if these two stars orbit one another. If they do, they would require at least 75 centuries to complete one orbit.
Albireo will be visible for another few months, drifting lower into the west with each passing week. Before they disappear for the winter, be sure to aim a small telescope toward the head of the celestial swan. You may be surprised to learn just how colorful our cosmos can be!
— Dennis Mammana is an astronomy writer, author, lecturer and photographer working from under the clear dark skies of the Anza-Borrego Desert in the San Diego County backcountry. Contact him at mammana@skyscapes.com and follow him on Twitter: @dennismammana. The opinions expressed are his own.