These pictures and text are supplements to the article I wrote for Sky & Telescope magazine in their September 2014 issue. You can skip this introductory material and go directly to the list of 20 Fun Naked-Eye Double Stars and by clicking on the links in the table there you can go to the images. You can also go through them one by one by clicking on the > link at the top right of each page.

Introduction

Because I take astrophotos all the time, some people think that I never do any observing. But once I get an exposure run going, I start observing visually.

If I’m out with friends, I will observe through their scopes. If I’m by myself, and I am out by myself a lot, then I either observe with binoculars or with my naked eyes.

Over the years I have done a lot of naked-eye observing. Sitting alone under a clear dark sky as the rest of the universe passes overhead in majestic splendor is a profound and astonishing experience.

A favorite pastime that I have come to enjoy over the years is naked-eye double star observing.

My favorite showcase naked-eye doubles are Theta Tauri, Alpha Capricorni, Epsilon Lyrae and Mu Scorpii.

Mizar and Alcor have always been known as a famous double star “test” of unaided eyesight but with a separation of almost 12 arcminutes they are not really that hard to split for anyone with normal vision.

Here we will look at some other naked-eye double stars. Some are very easy and some are quite a challenge.

Resolving close doubles depends on the quality of the optical instrument being used – the eye in this case, the experience of the observer, the elevation of the star above the horizon, atmospheric conditions such as haze, the brightness and separation of the pair of stars involved, and the difference in brightness between the two stars.

For close naked-eye doubles, it is our eyesight, and the brightness and separation of the stars that will play the most important roles.

“Double Stars”

The generic term “double star” can refer to a true binary star or an optical binary star.

True binary stars, such as Epsilon Lyrae, orbit each other and are gravitationally bound.

Optical binary stars, such as Algedi, do not interact with each other and just happen to lie along the same line of sight and are usually at greatly different distances.

Most stars form out of clouds of dust and gas in pairs, groups and clusters. Astronomers estimate that as many as 85 percent of all stars are either true binaries or members of multiple star systems. Our Sun probably formed in a star cluster, but has moved on to a solitary life in its middle age.

The Eye

The eye is an incredible and fascinating optical instrument that has evolved over millions of years to give us our main source of information about the world. Indeed, our consciousness interacts directly with photons, one of the universe’s elementary particles.

The eye is similar to a camera in many ways. It has a lens that focuses light on a sensor (the retina) that detects photons.

It has an aperture that can vary from about 1.5 mm to 9 mm, a focal length of about 22 mm, and a focal ratio that varies from about f/2.4 to f/14.6 depending on the aperture. There are, of course, variations on the size of both the aperture and focal length because of variations in the physiology of the human population. Most people fall in the 2 mm to 8 mm range for the aperture when they are young, which corresponds to a focal ratio of about f/2.75 to f/11. The eye will not dilate as large as observers get older. It has a field of view of about 180 degrees, but we only see sharp detail in a small portion of the center of this field.

Light first passes through the transparent cornea. It then goes through the liquid aqueous humor and through the iris – a variable aperture that controls the amount of light that is let into the eye. The aperture that the iris forms is called the pupil. Light then passes through the eye’s lens. The ciliary body adjusts the shape of the lens in a process called accommodation to focus the light on the retina for objects at different distances.

The light then passes through the vitreous humor, a transparent gel that fills the inside of the eyeball, a network of bl