A Beautiful Double Star


Some of my favorite objects to observe with a small telescope are double stars. These multiple star systems are generally close by – astronomically speaking – and most are fairly bright so they’re easy to find. They come in two varieties: optical doubles, in which the stars are aligned by chance from our viewpoint here on Earth, and physical doubles, true binary star systems in which the stars are gravitationally bound and actually in orbit around each other.

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The object of our post this week is one of the most famous and beloved double stars in the sky. It’s located in the constellation of Cygnus, where it makes up the tip of the swan’s beak. It’s easily identified, but to the unaided eye it appears merely as a single star so you’ll need a telescope to reveal its individual components. Its formal name is Beta Cygni, but it’s commonly known as Albireo.

The most visually appealing feature of Albireo is the striking colors of the component stars. In the eyepiece, one star – the brighter one – appears as a warm amber color and the other is a sharp, brilliant blue. The colors are apparent even to an inexperienced observer and it’s often been called the “Cub Scout” star. The brighter star, called Albireo “A”, is an older, cooler Class “K” star about 40 times larger than our sun while the “B” star is a much younger Class “B”. Albireo “B” is only about 2-1/2 times the size of our sun, but much more luminous and with a much higher surface temperature of around 11,000 degrees Kelvin. Both stars lie around 430 light years from Earth.

Ironically, the true nature of Albireo is unknown. Astronomers have so far been unable to determine whether the system is an optical or physical double. Observations, even with modern techniques, have not revealed any motion of the stars relative to each other, so if Albireo is in fact a binary star, then the orbital period of its components is probably in excess of 100,000 years. Regardless of whether the stars actually interact with each other, the system is a delight to observe and a very popular object at star parties.

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Modern astronomy has revealed that most known stars actually reside in multiple systems, so most of the stars that we see in our night sky have unseen companions. In fact, evidence is strong that the two visible components of Albireo each have several unseen companions of their own! Studies in this field continue and are important to our understanding of stellar evolution. Systems like Albireo are close enough for direct observation and so provide valuable scientific data, as well as being pleasant to observe.

We’ll finish up this tour of Cygnus here, but the Swan will grace our evening skies for a while yet and will be visible into early winter. Plenty of sights await in the night sky!

Until next time,

Clear Skies!

Leonard Ferguson

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