CHI-1 HYA (Chi-1 Hydrae) Odd coincidences abound among the stars, probably because there are so many of them. Chi-1 Hydrae (in Hydra, the very long Water Serpent) lies just 8.4 minutes of arc almost exactly west of Chi-2, seven-tenths the separation between Mizar and Alcor in Ursa Major. The latter two are pretty clearly a real pair (Mizar itself quadruple, Alcor double). Chi-1 and Chi-2 Hya, on the other hand, have nothing to do with each other except angular proximity. What is a bit weird is that both are doubles with confused memberships. At a distance of 141 light years (give or take just 2), a fifth the distance to Chi-2, Chi 1 is a fifth magnitude (4.94) class F (F3) subgiant binary with nearly equal components (which is not all that unusual), the individuals both of magnitude 5.7 and separated by a mere two or so tenths of a second of arc. From "Burnham's Celestial Handbook," as of 1978: "This is a binary system with one of the shortest periods known for any visual pair, but too close for small telescopes." The two go around each other with a period of just 7.60 years.

Chi-1 Hydrae The orbit of Chi-1 B around Chi-1 A (at the cross) is not only fairly eccentric (e=0.35) but is tilted at a high angle to the sky, 95 degrees, which together give the orbit its skinny look. The tilt is almost enough to make the double into an eclipsing binary. In reality, each star has its own orbit about a common center of mass, though for this pair they are almost identical. Note the scale: the stars at best are only a few tenths of a second of arc apart, the two taking only 7.603 years to go around each other at an average distance of 5.9 Astronomical Units (3.9 AU at periastron, 8.0 at apastron). They were physically (though not on the sky) last closest together late in 2013. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.

The orbit shows them to average 5.92 Astronomical Units apart, an eccentricity of 0.35 taking them from as close together as 3.9 AU to as far as 8.0 AU. Rather obviously a planet could not exist unless it orbited both of them very far away. Using Kepler's third law, the sum of stellar masses is 3.8 Suns, which makes them 1.8 solar masses each. We can check the calculation with evolutionary masses derived from theory. With temperatures of 6750 Kelvin, the duo radiates by far most of its light in the optical spectrum, each shining with the light of 7.7 Suns, from which we derive radii of 2.0 Suns and masses of 1.6 times solar, which sum to 3.2 Suns, 84 percent that of the sum of the orbital masses. Given that the orbital masses are dependent on the mean orbital radius cubed, the agreement is quite good; squeezing the orbit down by just four percent makes a perfect match. Published projected equatorial rotation velocities differ by a factor of 2! Adoption of the later measure of 93 km/s applied to either or both stars gives a rotation period of under 2.0 days. Theory, which this binary well supports, shows that the stars, while not actually subgiants that have run out of core hydrogen, are getting close, with ages that approach two billion years. All this looks pretty good. The problem is the discovery of sixth magnitude Chi-1 Hydrae C at a separation of 18 seconds of arc from the Chi-1 AB pair. But that would bring the visual magnitude of the trio to 4.62, which it very much is not. Since "C" has been observed just once, we can only conclude that the "discovery," or listing, is bogus and that "C" does not exist. To add to the confusion, Chi-2 has therefore been called Chi-1 D, implying a connection that also does not exist Such are the perils of double-star astronomy. (Thanks to Bill Hartkopf for discussion.)
Written byJim Kaler 5/20/16. Return to STARS.