CHI-2 HYA (Chi-2 Hydrae). Given that sixth magnitude (5.71) Chi-2 Hydrae (in Hydra, the water Serpent) is only 8.4 minutes of arc east of Chi-1 (such numbers generally running from west to east), we might expect them to be a naked-eye binary. But even though Chi-2 is catalogued as Chi-1 Hydrae D, as if they actually composed a true double, the two Chi's are not at all connected except by accidental alignment. Chi-1 is 141 light years away, while Chi-2, at 692 light years (give or take 40), is nearly five times farther (which tells us that Chi-2 is by far the more luminous: more about that below). What the two stars do oddly share is confusion regarding their memberships. Chi-1 has a putative third star called Chi-1 C that does not exist (though it lives on through Chi-2 being called Chi-1 D). But Chi-2's problem goes deeper. It's an eclipsing binary made of two class B stars (a B8 giant- subgiant coupled with a B8.5 dwarf) in which one star periodically gets in front of the other, which drops combined brightness, in Chi-2's case by about 0.3 magnitudes in the visual band (making the eclipse detectable by eye through the telescope, especially since it's so close to Chi-1, which can be used as a reference).

Chi-2 Hydrae In mutual orbit, the two stars of Chi-2 Hydrae partially eclipse each other every 1.134 days. At left, the dimmer secondary component covers part of the primary, while at right the primary covers part of the secondary. The actual orbital period is thus twice the interval between the eclipses, or 2.267 days. The light curve (magnitude vs. time as compared with a nearby non-variable star) is presented in three colors or wavelength bands: standard blue (B), shorter-wave blue (b), and ultraviolet (v). If the eclipses were total, the minima would have flat bottoms as one star disappears behind the other. Instead, the recovery is instantaneous. The variation outside of eclipse is the result of mutual tidal distortion, the orbiting stars thus continuously presenting different angular areas to the observer. (Light curves from H. Mauder and U. Koehler in Astronomy and Astrophysics, 1, 147, 1969.)

The problem is with the orbital period, the time it takes the stars to go around their common center of mass. Three periods are given in various sources: 1.134 days, 2.267 days, and 16.54 days. The last makes it look as there is a third star going around a double with a shorter period. But like Chi-1 Hydrae C, the longer period is a chimera; the responsible star does not exist. The shortest period listed, 1.134 days, is exactly half that of 2.267 days, and is clearly the interval between primary and secondary eclipses (star A getting in front of star B to star B getting in front of A), understandable since the two eclipses are difficult to distinguish from each other. So once again we are left with a simple binary with a period of 2.267 days. Like Algol, the eclipses are partial, from our point of view neither star fully covering the other. Moreover, the binary varies even outside of eclipse as a result of considerable tidally-induced oblateness. Solution from all the data, including the light curve (variation with time), gives respective temperatures for the two of 11,750 and 11,100 Kelvin, luminosities of 344 and 66 Suns, radii of 4.48 and 2.21 solar radii, and masses of 3.61 and 2.63 Suns. The separation between the two is only 13.4 solar radii, less than four times the size of the larger star, making tidal distortion inevitable. With an age of about 700 million years, the secondary is still fusing hydrogen in its core. The more massive primary however is ready to give it up if it has not done so already. As the primary evolves, it will expand toward the secondary and eventually pass mass toward it. As the secondary expands with its new-found mass, it will eventually send matter back toward the current primary, and the two will come into contact looking something like a three-dimensional figure-8. Eventually they may merge into one star with serious loss of mass, though the theory is not yet good enough to tell us exactly what will happen. (Stellar parameters from Y.Meng and Q.S.Zhang in the Astrophysical Journal, 787:127, 2014.) 11111111111
Written byJim Kaler 6/03/16. Return to STARS.