EPS LYR (Epsilon Lyrae). Among the sky's myriads of stars, few carry more "weight" than the famed "Double-Double in Lyra," Epsilon Lyrae. While there are numerous other examples of multiple stars, even on the scale of Epsilon, this one is the favorite. Look just to the northeast of brilliant Vega, and if your eyes are very good -- or upon using binoculars -- you see that fourth magnitude (3.88) Epsilon is really two nearly identical stars close together, separated by 208 seconds of arc (3.5 minutes). The northern of the naked-eye double is called Eps-1, the other Eps-2. A decent telescope then shows each of these to be
A closeup view of Vega (the bright star at top) and its surroundings reveals the duplicity of the famed "double-double star" Epsilon Lyrae at left (west is up in the picture). The left- hand star of the pair is Epsilon-1, the right hand star Epsilon-2. Each of the two are also double. Zeta Lyrae is at the center of the right-hand edge, while the unrelated pair Delta-1 and Delta-2 Lyrae are at bottom right, Delta-2 the brighter.
divided into two more nearly identical class A stars, those of Eps- 1 split by 2.8 seconds, those of Eps-2 by 2.2 seconds, all located 160 light years away. From west to east, the four are labelled A through D as Eps-1 A, Eps-1 B, Eps-2 C, Eps-2 D. Much brighter Mizar and Castor are similar, but the individual pairs cannot be split by eye, and require the spectrograph or other sophisticated techniques. Epsilon Lyrae is one you can see complete. The stars of each of the pairs orbit each other with periods estimated at around 1000 years, placing them some 140 Astronomical Units apart. Eps-1 and Eps-2 are vastly too far apart for any orbital motion to be noticed, as they are separated by at least 10,000 AU and must take at least half a million years to make a complete circuit. While they look similar, there really are differences among them all. From A through D, we find apparent magnitudes 5.1, 6.0, 5.1, 5.4; spectral classes A3, A7, A5, A5; temperatures 8000, 7700, 8200, 8200 K; luminosities 18, 8, 17, 14 times that of the Sun; masses: 1.9, 1.5, 1.9, 1.8 solar. The whole system is probably 800 or so million years old. We still do not know just how such quadruples are formed, with pairs so reasonably close and the pairs of pairs so far apart. From each of the pairs, an observer would see the other shining with the light of a quarter Moon under a degree away from each other. The only way such a quadruple star can survive for any period of time is to be in a double-double hierarchy; if the four are mixed up close together, the combined gravitational influences will kick out first one, then another, leaving just a double behind. Nevertheless, gravitational influences of passing stars and tides raised by the Galaxy will most likely separate Eps-1 and Eps-2 into doubles that go their separate ways. Read more about Epsilon Lyrae in Jim Kaler's "Hundred Greatest Stars."
Written by Jim Kaler. Return to STARS.