Seeing double – Observing double stars, that is

By Chris Fellows, Serenity Mobile Observatory

[Click any image to enlarge.]
In this series of articles I like to keep my targets to objects that anyone can observe. (This isn’t always possible but it is a goal.) This week I am going to focus on observations that can be made with just the naked eye or a small set of binoculars.

Double stars or binary star systems are much more common than most people might think. It is estimated that about 4/5ths of the stars in the sky are actually binary or multiple star systems.

Binary stars are two or more stars orbiting a common center of gravity. They come in a wide variety of star types and masses and some of them are absolutely beautiful to see with your own eyes. There are wide binaries, where the pair are so far apart that they don’t really affect each other’s evolution, and close binaries, where the two are close enough to swap mass between them and in some cases can completely consume their partner.   

Double stars are extremely satisfying to observe and there are always a few good ones in the sky. Let’s take a look at some of my favorites.

Nu Draconis is one of the finest doubles in the heavens. Nu Draconis consists of two white stars of equal brightness (magnitude 4.9), with a relatively wide separation that is easy to spot in modest binoculars. The system is about 100 light years from Sol.

This system makes a pretty large field of view and has relatively few surrounding stars, making it very easy to pick it out in the sky. Nu Draconis is easy to find, and since it is near the celestial pole it is in the sky for most of us, most of the time. Simply find Polaris, the North Star, which is the last star in the handle of the little dipper, then look carefully for the little dipper and you will see a constellation that basically surrounds it. This is Draco, the dragon. Draco’s head is made up of a trapezoid of smallish stars. Nu Draconis is one of these. Use your binoculars or a small telescope to scan these stars and you will find this lovely pair.

Another favorite pair is Albireo, located in the constellation Cygnus, the swan. Albireo is a tight pair of vastly different stars. Although Albireo itself is only fairly bright, a magnitude 5ish star and its companion Beta Cygni B is much dimmer, together they look like a single magnitude 3 star to the naked eye. Use a pair of binoculars or a small telescope to split them into two colorful stars.

This object is also very easy to find in the summer sky. The constellation Cygnus lies in the band of the Milky Way about half way between Sagittarius and Cassiopeia and looks like a large cross in the sky. Albireo is the last star in the long arm of the cross.

Lastly, let’s look at Epsilon Lyrae, also known as the Double Double! This is an easy to find (I’m not going to give you a map!) and very beautiful pair with a hidden secret. It is really 4 stars, or a pair of pairs. Splitting the twins isn’t hard but splitting the tight pairs is a real challenge.

As you can see in my accompanying photo, even through my telescope with a 10-second exposure you can’t see the tighter doubles that make up this system. This is a real astronomical challenge and I would like to hear from anyone who has successfully spotted the tighter double stars in this system. Leave me a comment if you were able to do it!

There are many, many more double and multiple star systems in our galaxy and they are always a joy to spot. I have spent many hours showing them to eager star party attendees and they always get an “ooooh” at the eyepiece. Have a look for yourself and let me know which ones you have spotted.

Till next time!

Clear Skies,
Chris Fellows, Serenity Mobile Observatory

Find Chris on Facebook (or, if you’re lucky, at your campground). (Editor: Check out his amazing photos on his Facebook page!)