The Famous Double Stars

Stefan Binnewies / Josef Pöpsel / Capella Observatory -

Stefan Binnewies / Josef Pöpsel / Capella Observatory –

Let’s get to know them better.

A double star is exactly what you would think, two small stars positioned close and visible to the naked eye. In some cases, these are simple random alignments of objects, known as double optics, which are actually very far apart. But there is usually a real physical connection. In most cases, the stars orbit each other, usually taking from a few decades to many centuries to complete a tour. These are binary stars. (Some of them are so close that their orbital periods are measured in a few minutes! But these require professional telescopes and special tools to be observed.)

In other systems, the stars are so widely separated that they probably do not orbit each other. Even in cases where the system is made up of three or more suns, it will tend to be referred to as the “double star”.

Good examples of double and multiple stars can be found anywhere in the sky at any time of the night throughout the year. This is an advantage if the view of the sky is limited by trees, houses or other obstructions, as usually happens. Moreover, as they are often very bright and easy to spot, these stars can be our subjects during evenings where it is not possible to observe nebulae and galaxies due to haze, moonlight or light pollution. Despite their profusion, no couple looks exactly like the other! They present a seemingly infinite combination of brightness, separations and configurations.

But it is the perceived colors of these objects that are particularly fascinating. For many amateur astronomers, double stars are the colored jewels of the sky. I used the adjective “perceived” because in most cases the color is illusory, a contrast effect between stars of different brightness. However sometimes the shades are real. The true color differences between the stars derive mainly from temperature differences, with the red ones being relatively cold and the blue ones quite hot.

A trick to improve the perception of color is to slightly blur the image seen through a low magnification eyepiece. Even couples that do not have contrasting tones (this happens when they have similar brightness) or no color apparently can still be exciting considerations. To the naked eye, they typically appear as radiant diamonds in the darkness of the sky.

The double stars analyzed below are among the most fascinating to observe towards the end of the summer. In many cases the dominant (primary) star is so bright that it is seen by a reasonably dark sky with the naked eye. And while any pairing can be solved with a small telescope, the view typically becomes more exciting (brighter images and sharper separations) in telescopes with larger apertures.

Generally, the best visual effect is obtained by using the lowest magnification that separates the two stars well – typically 25 × to 50 × for large attachments. To double the narrower ones, you’ll need to push yourself up to 100 × and good seeing.

The Double Double (Epsilon Lyrae): This famous multiple system is located in the same field as Vega, making it easy to spot. Here you will find two pairs of double stars distant from each other and the system is easily visible with a 3 × 4 inch 100 × telescope that will show both the systems and the 2 stars that compose them. Both binary stars have periods of revolution measured over hundreds of centuries, and together they are orbiting around a common center of gravity with a period of up to a million years! Quadruple systems like this are relatively rare and Epsilon Lyrae is definitely one of the best to look at.





Magnitude: 5.0, 6.1 and 5.2, 5.5

Separation: Very close

Optimum magnification: 100 ×

Colors: All white

Distance: 160 light years

Alya (Theta Serpentis): This pretty double system is located just west of Altair. Composed of two almost identical stars, it is easily solved even with a 2-inch (or 60mm) telescope at 25 ×, while the 4-inch view at 50 × it is truly incredible. This is one of those cases in which the stars show no color contrast, both are white and of similar magnitude. Although this double track system is noticeably neglected today, in the past it enjoyed great fame among the first amateur astronomers thanks to the ease of observation.





Magnitude: 4.5 and 5.4

Separation: Wide

Optimum magnification: 30 ×

Colors: Both white

Distance: 140 light years

Albireo (Beta Cygni): Considered one of the most beautiful double stars in the sky, Albireo is located at the tail of Cygnus (also known as the Swan Constellation), located in the middle of the summer triangle formed by Vega, Deneb and Altair. If you think that the stars have no color, then just take a peek into a telescope and observe Albireo. The primary is a rich orange that tends to golden and its partner is a sapphire blue. These celestial shades are real – they are not just a contrasting effect. Albireo is a large double system, even with an aperture range of 2 inches to 25 ×, this system shows a vivid contrast. Both of its suns are roughly the same distance from us, floating on the rich background of the Milky Way.





Magnitude: 3.1 and 5.1

Separation: Wide

Optimum magnification: 30 ×

Colors: light blue and yellow

Distance: 380 light years

Omicron-1 Cygni: This impressive system is located west of Deneb and consists of three widely separated stars with different brightness and colors. A 3-inch to 30-inch aperture will offer a superb view. The shades here are very varied, like orange, white and blue! This is one of the simplest, brightest and most colourful triple stars in the sky.







Magnitude: 3.8, 7.7, and 4.8

Separation: Very wide

Optimum magnification: 30 ×

Colors: Reddish, white, and blue

Distance: 200 light years

Gamma Delphini: You will find this beautiful colored gem that marks the “nose” of the small constellation of the dolphin. A 50 × 3 or 4-inch telescope shows a clear binary system of stars that differ slightly in brightness. Observers note various shades, with most of them seeing a primary and pale emerald mate. Seen in a small telescope at a magnification of at least 150 ×, these points of light become spurious diffraction discs that make the colors more evident. Astronomers have found little indication about their orbital movement.




Magnitude: 4.5 and 5.5

Separation: Neighboring

Optimum magnification: 75 ×

Colors: Golden and greenishDistance: 100 light years

Enif (Epsilon Pegasi): Here is another “nose”, this time in Pegasus, the flying heavenly horse. Although not an impressive double star by itself, Enif is one of the few objects in the sky that “does something” while you look at it. In fact, by gently touching or shaking the telescope tube, the less bright star seems to move back and forth like a pendulum. This phenomenon was first noticed by the great observer John Herschel, who rightly deduced that the light of the coldest star requires more time to stimulate the retina, so its movement seems slower than that of the brightest. You can observe this effect with a 4-inch to 50 × telescope, but it becomes increasingly impressive as the aperture increases.





Magnitude: 2.4 and 8.5

Separation: Very wide

Optimum magnification: 50 ×

Colors: Yellow and purple

Distance: 700 light years

Delta Cephei: In the upper right corner of the Cepheus constellation, we find an object with a double personality. It is an attractive, widely separated double star reminiscent of Albireo. While its pale orange hues are a nice sight in a 3-inch to 30-inch field, you’ll find that they do not match the vivid shades of Albireo. But Delta Cephei is not only a double star, it is also a variable star of considerable importance, as the Cepheid variable is usually used by astronomers to measure distances with nearby galaxies. Every 5.4 days the primary star reaches its peak with a factor of 2½ times, also evident to the naked eye.





Magnitude: 3.5-4.4 and 6.3

Separation: Wide

Optimum magnification: 30 ×

Colors: Pale orange and blue

Distance: 1,100 light years

Almach (Gamma Andromedae): This radiant duo is at the end of the curved line of stars that mark Andromeda, in the north-eastern sky during the autumn evenings. Its magnificent colors and shades of aquamarine are a joy to see in any telescope. Many observers believe that the colors of Almach are more intense than those of rival Albireo! Since both doubles can be observed at the same time in the autumn sky, why not compare them alone? To conclude, no orbital movement has been identified.





Magnitude: 2.3 and 5.5

Separation: Wide

Optimum magnification: 75 ×

Colors: Yellow and light blue

Distance: 350 light years

As the examples above testify, it is possible to observe double stars even by using small telescopes. You just have to give it a try!


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