Epsilon Lyrae is a binary star system in the constellation Lyra, appearing just to the northwest of Vega. Commonly known as the "double-double", Epsilon Lyrae is one of the most well known binary stars.
Observation
Separated by 208", ε Lyr can easily be resolved into two components when viewed through binoculars, or even by the naked eye under excellent conditions. The northern component is called ε1 Lyr, and the southern one is ε2 Lyr; they both lie around 180 light years from Earth and orbit each other. When viewed at higher magnifications, both ε1 Lyr and ε2 Lyr can be further split into binaries; that is, the system contains a pair of binary stars orbiting each other. Being able to view the components of each is a common benchmark for the resolving power of telescopes, since the individual doubles are so close together.
Components
The component stars of ε1 Lyr have magnitudes of 4.7 and 6.2, separated by 2.8". With an orbital period that can only be crudely estimated at 1200 years, they are roughly 160 AU apart. The component stars of ε2 Lyr have magnitudes of 5.1 and 5.5, a separation of 2.2" or 140 AU, and an orbit with perhaps 600 years. ε1 and ε2 Lyr themselves are no closer than 0.16 light years apart, and would take hundreds of thousands of years to complete an orbit. An observer at either pair would see the other shining with the light of a quarter Moon, less than a degree away from each other.
From west to east, the four are labelled A through D as ε1 Lyr A, ε1 Lyr B, ε2 Lyr C, ε2 Lyr D. While they all look similar, there really are differences among them. From A through D, we find spectral classes A3, A7, A5, A5; temperatures of 8000, 7700, 8200, 8200 K; luminosities of 18, 8, 17, 14 suns; and masses 1.9, 1.5, 1.9, 1.8 times solar. The whole system is probably 800 or so million years old. We still do not know how such quadruples are formed, with pairs so reasonably close and the pairs of pairs so far apart. The only way such a quadruple star can survive is to be in a double-double hierarchy; if the four are mixed up close together, their combined gravitation will kick out one component, then another, leaving just a double behind. Nevertheless, the gravitational influence of passing stars will most likely separate ε1 and ε2 Lyr into doubles that eventually go their separate ways.
A fifth component of this system, orbiting one of the ε2 Lyr pair, was detected by speckle interferometry in 1985 and confirmed in two subsequent observations. No orbit can be computed from such limited data, but its rapid motion suggests a period of a few decades. Its maximum observed separation of 0.2" precludes direct visual observation.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]