EPS LYR (Epsilon Lyrae). Among the naked-eye sky's myriads of stars, few are more distinctive 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 may be the favorite. Writing from the nineteenth century, Smythe and Chambers tell us that "This system forms a very elegant object, and merits the closest attention." Look just to the northeast of brilliant Vega, and if your eyes are very good -- or upon using binoculars -- you see that fourth magnitude (3.85) Epsilon is really two nearly identical stars close together, separated by 208 seconds of arc (3.5 minutes). The western of the naked-eye double is Epsilon- 1, the eastern Epsilon-2. A decent telescope then shows each of these to be divided into two more closely similar class A stars, those of Eps-1 split by 2.8 seconds, those of Eps-2 by 2.2 seconds, all located 161 light years away (second Hipparcos reduction), give or take half a dozen.

At far left, a closeup view of Vega (the bright star at top) and its surroundings reveals the duplicity of the famed "double-double star" Epsilon Lyrae, which lies down and to the left of Vega (west is up in the picture). The left-hand star of the pair is Epsilon-1, the right hand star Epsilon-2. As seen at near left, each of the two is also double. The stars between the pairs are irrelevant. At far left, Zeta Lyrae is down and a bit to the right of Vega, while the unrelated pair Delta-1 and Delta-2 Lyrae are at bottom center, Delta-2 the brighter. (Far left: J. B. Kaler; near left from W. H. Smythe as revised by G. F. Chambers, A Cycle of Celestial Objects, Oxford University Press, 1881.)

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. Orbital motion is seen for each pair, but the errors are significant. In what seems to be the most likely solution of the limited data, the Eps-1 pair (Epsilon A and B) orbit with a period of around 1800 years at an average separation of 235 Astronomical Units, a high eccentricity bringing them as close as 73 AU and as far apart as 400. The Eps-2 pair (Eps C and D) take 724 years at a mean separation of 145 AU (95 to 195). 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 from the masses below must take at least 400,000 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; and from the theory of stellar structure and evolution, masses of 1.9, 1.5, 1.9, 1.8 solar. The luminosities might be increased by at most 20 percent or so if some dimming by interstellar dust is taken into account. The whole system is probably 800 or so million years old. All are fast spinners, with minimum equatorial rotation speeds of 200, 150, 177, and 212 kilometers per second, which precludes odd abundance anomalies caused by separation of elements. 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 might see each of the other shining with the light of a quarter Moon perhaps 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. The easternmost of the Eps-2 pair may also be double with a faint companion 0.2 seconds or so away, yielding at least five stars in the system. Read about Epsilon Lyrae in Jim Kaler's "Hundred Greatest Stars."

Written by Jim Kaler 7/04/03; revised 11/15/13. Return to STARS.