Epsilon Hydrae

EPS HYA (Epsilon Hydrae). Hydra (the Water Serpent), by far the longest constellation in the sky, is well known for its bright, mid-second magnitude Alpha star, Alphard (the solitary one), which lies to the south and a bit west of Regulus in Leo. South of Cancer, you can find Hydra's glowering head, made of a small distorted circlet whose stars, as noted in Allen, are collectively called "Min al Az'al," from Arabic meaning "belonging to the uninhabited spot." None of these stars, including Bayer's Epsilon, has a proper name to itself, though third magnitude (3.4) Epsilon is singled out as the most northerly of them. Appearing undistinguished to the eye, Epsilon Hya, 129 light years away (give or take 5), is a remarkable multiple star with at least four members. The principal star is a fourth magnitude (3.8) yellow-white class G (G5 or G8) giant that has a very close (0.2 seconds of arc away) fifth (5.0) magnitude class A (A8) subgiant (but see below) companion. The two orbit each other every 15.09 years at an average separation of 10.0 Astronomical Units, about the distance of Saturn from the Sun, on eccentric paths that takes them from 16.6 AU apart to as close as 3.5 AU (that point passed in 1991). Kepler's Laws give the two a total mass of 4.4 times that of the Sun.

The image scales are both in seconds of arc. At left, Epsilon Hydrae B orbits the brighter class G giant, Epsilon A (at the cross), with a period of 15.09 years. While on the average the two are separated by 16.6 Astronomical Units, a high eccentricity carries them between 3.5 and 16.6 AU apart. At right is a much more (13 times) expanded view in which Epsilon Hydrae C orbits the close AB pair (again at the cross) every 590 years with a mean separation of 130 AU. In reality, each of the pairs goes around a common center of mass between them. The orbits are distorted into apparent ellipses in which the stars are not at the foci because of the orbital tilts against the plane of the sky (50 degrees at left, 33 degrees at right) and the orbital orientations. (W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, US Naval Observatory Double Star Catalog, 2006.)

The G star (Epsilon Hydrae A), at 4900 Kelvin (from the G8 class), shines with a light of 52 Suns. The class A star (Epsilon B: don't confuse the spectral classes with the star names), at 7700 Kelvin, radiates at 12 solar luminosities. Theory shows Eps A to be a stable helium-fusing giant and Eps B to be an ordinary dwarf. More important perhaps, theory gives respective masses for "A" and "B" of 2.5 and 1.7 Suns, which totals 4.2 times solar, remarkably close to that derived from the orbit (the difference even explained: again see below). Separated from this close double by 3 to 4 seconds of arc is a class F (F7 or so) seventh magnitude (6.7) dwarf that from a series of observations takes 590 years to orbit the inner pair at a mean distance of about 130 AU. At 6000 Kelvin, it shines 2.8 times as brightly as does the Sun, and carries just 1.2 solar masses. Subtracting the mass of the AB pair gives "C" a much larger mass of 2.4 Suns that, given the uncertainties inherent in the larger orbit and in the evolutionary masses, is not surprising. And that's not all. The spectrum of the class G giant, Epsilon Hydrae A, reveals yet another member with a period of 9.91 days. A mass of just 0.2 Suns makes up the difference between that derived for "A" and "B" from Kepler's laws and that derived from evolutionary theory. Probably a red dwarf, it must then orbit at a mean separation from "A" of just 0.13 or so AU. Eps A, The G star, is somewhat magnetically active, and has been detected as an X-ray source, possibly as a result of spin-up of the G star by the gravitational effect of the close companion, though the activity might come from the red dwarf itself. And once again, that's not all. Three more "companions" are listed in the double-star catalogs, 13th magnitude D 18 seconds of arc away, 11th magnitude "E" at 340 seconds, and 11th magnitude "F" at 407 seconds. All are moving too quickly relative to "A" to belong to the system, and are just line of sight coincidences. We don't need them: the wonderful inner quartet is plenty enough.

Written byJim Kaler 4/18/03; rewritten 6/17/15). Return to STARS.