Brightest Stars

Exquisite Albireo, a much-loved double star

A star map showing Cygnus against a black sky with nebulous hints of the Milky Way Galaxy in the background. Stars are in white, with constellation lines in blue.
Larger view. The constellation Cygnus lies within a larger star pattern known as the Summer Triangle. See the three bright stars here: Vega, Deneb and Altair? See how the pattern of the cross (Cygnus the Swan) lies inside the triangle made by those three stars? Once you’ve located it, you’ll find Albireo at the end of the cross. Image via Stellarium app.

Albireo is 2 stars

Albireo, also known as Beta Cygni, is the 2nd-brightest star in the constellation Cygnus the Swan. At first glance, it doesn’t particularly stand out. But viewing this star through a small telescope can take your breath away. It resolves into a striking double, with one component a lovely gold star and the other a dimmer blue close by. Although the two stars appear close in the sky, from our perspective, astronomers still don’t know for sure if they’re gravitationally bound to each other. It doesn’t matter. The color contrast between the two is so striking and so beautiful that Albireo is one of the sky’s most beloved stars.

How can you spot Albireo in the night sky? It’s easy to find, if you can locate Cygnus the Swan. Cygnus has an easy-to-recognize shape, that of a cross, and the constellation is also known as the Northern Cross. The brightest star in Cygnus, called Deneb, marks the head of the Cross or the Tail of the Swan. Albireo marks the base of the Cross or the Head of Cygnus.

And how can you see Albireo as two stars? They’re best viewed at 30X (“30 power” or a magnification of 30). Unless you have exceedingly powerful binoculars, mounted on a tripod, binoculars won’t show you Albireo as two stars. But any small telescope will. When you do see Albireo as two stars, be sure to notice the striking color contrast between the two.

Double star Albireo - 2 bright stars close together - in a star-studded field of mostly faint stars. One star is orange-yellow and the other one a dimmer blue.
An image of Albireo taken with a camera attached to a 12-inch telescope. Image via Tom Wildoner / Flickr.
A grey, black, and white star map with black dots indicating the positions of stars, and green lines showing constellation lines.
A star map of Cygnus. Image via IAU an Sky & Telescope / Wikimedia Commons.

Science of Albireo

The brighter gold star, known as Albireo A, is about 430 light-years away. Albireo B, the dimmer blue star, is around 400 light-years distant. It’s not yet clear if these two stars are distantly-spaced binary stars. An analysis of their motion in space, from ground- and space-based telescopes suggests they aren’t gravitationally bound to each other. But, astronomers say they need more measurements to come to a definitive conclusion.

So is it just by chance that “A” and “B” appear close to each other in our night sky? According to a paper published in January 2021, there’s evidence from spectroscopic data that they formed around the same time. Astronomers also found a few other stars in close proximity to them with similar motions in space. They suspect these stars and the Albireo system are what’s left of an open star cluster, a family of stars formed together about 100 million years ago.

Albireo A’s binary star system has an orbital period of 121.6 years. The brighter star is responsible for the gold color you see through a telescope; it’s a red supergiant star, about 5 times the mass of the sun. It outshines its fainter companion, a hot main sequence star that’s 2.7 times the sun’s mass. However, in a recent analysis of the Albireo A binary system, astronomer were surprised to find that there may be another yet undetected star in the mix, possibly making Albireo A a triple star system.

Albireo B, the fainter blue star of the pair when viewed through a small telescope, appears just 34 arc seconds away from gold-colored Albireo A. It’s a hot blue star, about 3.7 times the sun’s mass.

An antique star chart showing stars with illustrations of the constellations — Lacerta the lizard, Cygnus the swan, Lyra the harp, with Vulpecula the fox holding a small swan by the neck in its mouth. The larger swan Cygnus appears to be diving down towards the fox to protect the smaller swan.
The constellations Lacerta, Cygnus, Lyra, with Vulpecula and Anser (depicted in modern star charts as just “Vulpecula)“ from Urania’s Mirror, a set of celestial cards by Sidney Hall, published in 1825. Image via Wikimedia Commons.

Bottom line: Through a telescope, Albireo, a seemingly nondescript star in the constellation Cygnus, pops as a stunning gold and blue double star. Astronomers still don’t know if they’re binary stars but think they were formed around the same time.

July 31, 2021
Brightest Stars

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