Of all multiple star systems consisting of at least three stars, perhaps the finest and most celebrated of all is Epsilon (ε Lyr) Lyrae, better known as "The Double Double." What is great about this star system is that it offers something for everyone, from the naked-eye observer to those with the largest of amateur telescopes.
Epsilon Lyrae is located in the constellation of Lyra close to the bright star Vega (mag. 0.0). For northern hemisphere observers Vega dazzles brilliantly during the summer months, appearing high in the sky during the warm nights. Positioned just over 1.5 degrees to the northeast of Vega and shining at magnitude 3.9 is Epsilon Lyrae. The star at first glance probably won't seem at all remarkable, but on closer inspection and especially if you have sharp eyesight, you may notice that Epsilon Lyrae is in fact a double star consisting of two almost identical white stars. The stars are named Epsilon1 and Epsilon2 Lyrae, have a separation of 208 arc seconds and a position angle of 172 degrees. It has often been said that if you can visually split the famous Mizar/Alcor grouping in Ursa Major then you have good eyesight. With a separation of 708 arc seconds, the Mizar/Alcor pair is a piece of cake to resolve compared to the much tighter Epsilon1/Epsilon2 Lyrae pairing. Split the latter with the naked eye and you have no need to worry, your eyesight is of the highest order!
Even if you can't split Epsilon Lyrae with the naked eye, it is easily done with binoculars. Epsilon1 and Epsilon2 are split in 7x35, 10x50 and 20x80 bins without problem. Of the two stars, the northern one is Epsilon1 and the southern one, Epsilon2. Remarkably, this is not where the story ends as both Epsilon1 and Epsilon2 themselves are double stars; hence the nickname "The Double Double". However, you won't be able to further split these two stars with binoculars; it requires much higher magnification. A small 70mm to 80mm telescope at about 120x magnification will do the job, if the seeing conditions are good. In fact, this example is a good test of the seeing conditions. If you can split Epsilon Lyrae into its four components with a small telescope, then you're night is off to a good start as the observing conditions are superb. In larger instruments such as a 200m (8-inch) scope, the view is similar to that of smaller telescopes but the pairs appear brighter and easier to split, especially if you ramp up the magnification. All four stars appear off white in colour.
The component stars of Epsilon1 are of magnitudes 5.15 and 6.10 and are currently separated by 2.3 arc seconds at a position angle of 347 degrees. The two stars of Epsilon2 are fractionally further apart at 2.4 arc seconds, with magnitudes of 5.25 and 5.38. The current position angle of these stars is 79 degrees. All four stars are gravitationally bound and it is estimated that the two components of Epsilon1 take 1200 years to complete one orbit compared to 585 years for the stars of Epsilon2. The two pairs themselves are separated by 0.16 light-years, but at such a distance they would take thousands of years to orbit.
The Epsilon Lyrae system is located 162 light-years from Earth. An observer at one pair would see the other pair shining with the light of a quarter Moon, less than a degree away from each other. In 1985, a fifth component of the system orbiting one of the Epsilon2 pair was detected by speckle interferometry. In addition, they're maybe a number of other nearby stars that are also members.
The Epsilon Lyrae system is a remarkable multiple star system. It is a must see object in the night sky and arguably the finest example of its type. When the opportunity arises, with or without optical aid take the time to marvel at this celestial wonder.
Epsilon Lyrae Data Table
|Epsilon1 Lyrae||Epsilon2 Lyrae|
|Flamsteed||4 Lyr||5 Lyr|
|RA (J2000)||18h 44m 20s||18h 44m 23s|
|DEC (J2000)||39d 40m 12s||39d 36m 46s|
|Other Designations||WDS 18443+3940|
|Component Stars||A and B||C and D|
|Component Stars Mag.||A = 5.15, B = 6.10||C = 5.25, D = 5.38|
|Component Stars Separation (arc seconds)||AB = 2.3||CD = 2.4|
|Component Stars Position Angle (degrees)||AB = 347||CD = 79|
|Epsilon1 Lyrae / Epsilon2 Lyrae Separation (arc seconds)||AB / CD = 208|
|Epsilon1 Lyrae / Epsilon2 Lyrae Position Angle (degrees)||AB / CD = 172|