PSI DRA (Psi = Psi-1 = 31 Draconis), with an excursion to Psi-2 = 34 Draconis). In the nineteenth century, Smyth and Chambers wrote of fourth magnitude (4.23) Psi Dra in Draco: "A neat double star on the Dragon's back...easily identified, being on the same parallel as Gamma Ursae Minoris...A 5 1/2, and B 6, both pearly white." That is just the beginning of a remarkable tale. First, we need deal with the names. Psi Dra is commonly called Psi-1 in conjunction with otherwise unrelated fifth magnitude (5.43) Psi-2 Dra a degree to the east. Unfortunately, the two components of Psi-1 (Psi-1 A and B) have also been respectively labeled Psi-1 and Psi-2. To avoid confusion, "Psi-1" is thus often dropped and the binary called just "Psi Dra" or by its Flamsteed number 31 Dra, distant Psi-2 being 34 Draconis. We'll get to 34 Dra later. Hipparcos gives a distance of 74.9 light years (plus or minus 0.4) to Psi-1 = Psi, while more recent analysis yields 72.4 l-y. It does not make much difference in the final results. Psi A and B are clearly drifting through space together at a speed of about 30 kilometers per second relative to the Sun, and have maintained nearly constant separation, now 30.2 seconds of arc, over the past two centuries, so there is no doubt as to their binary nature. Farther out are 11th magnitude Psi-1 C (78 seconds), 13th magnitude D (101), and 15th magnitude E (69). "D" has been a constant companion for a century, so it might belong to the system.

For now focus on Psi A and B. Psi A is a class F (F5) subgiant- dwarf with a temperature of 6340, a luminosity of about 6 Suns, and a mass of 1.43 times solar. It hosts a dim spectroscopic red dwarf companion of perhaps half a solar mass in an 18-year eccentric orbit at an average distance of 8.7 Astronomical Units (called Psi C, but not the same "Psi C" as above, so we need watch the context). Of much greater interest is Psi B, a rather sunlike class G (G0) star with a temperature of 6200 K, a luminosity double the Sun, and a mass of 1.19 Suns. Its velocity variations show the presence of not one, but TWO orbiting planets. The primary planet, Psi Bb, has a minimum mass of 1.53 times that of Jupiter and orbits with an average separation from Psi-B of 4.43 AU with a period of 8.53 years. More subtle variations in velocity point to another Jupiter-like planet averaging 6.8 AU out. In summary, Psi-1 Dra is a hierarchical triple, with a low mass star orbiting Psi-1 A and a pair of massive planets in a solar-system-like orbit going about the lesser star Psi-1 B, making it look rather like 61 Cygni. Separated by at least 700 Astronomical Units, Psi B with its planets and Psi A with its red dwarf take at least 10,000 years to orbit each other. Is there an earth going about Psi-B as well? If so, Psi-A would glow in its sky with a brightness at least six times that of our full Moon.

What about old Psi-2 = 34 Dra? The fifth magnitude (5.43) class F2-3 bright giant stands out quite nicely by itself. At a distance of 1055 light years (give or take 68), it is quite unrelated to Psi-1. With a temperature of 6530 Kelvin, it radiates at a rate of 560 Suns and has a radius of 18.5 solar radii. Its projected equatorial rotation speed of 48 kilometers per second gives a rotation period under 20 days. Application of theory yields a substantial mass of 4.2 Suns, an age of about 20 million years, and shows the star to have a dead helium core in transition to becoming a true red giant, whereupon the core will burn to a mixture of carbon and oxygen. After sloughing off its outer layers, the C/O core will be revealed as a white dwarf with a mass about 80 percent that of the Sun. From its vantage point, Psi-1 would be an 11th magnitude dim bulb that would merit little consideration. (Most of the discussion of Psi-1 Dra is from M. Endl, E. J. Brugamyer, W. D. Cochran et The Astrophysical Journal, 818:34, 2016 February 10, with thanks; thanks also to Jerry Diekmann for suggesting the star system.)
Written byJim Kaler 09/23/16. Return to STARS.