CHI-1 ORI (Chi-1 Orionis). Proximity in the sky, sharing a bond of mythology, and similar apparent brightnesses can fool us into thinking that pairs of stars must be similar. Chi-1 (by usual tradition, the westerly one) and Chi-2 of Orion (the easterly), which make the tip of the Hunter's upraised club, instruct us otherwise, as it is hard to see how two stars could be more different. Chi-2 is a barely-fifth magnitude blue-white class B (B2) aging supergiant 4900 light years away that shines with a spectacular luminosity of nearly half a million suns, while class G (G0) faint-fourth magnitude (4.41) Chi- 1 is not only one of the closer stars to the Earth (just 28 light years away), but is also a near-solar replica, though one with a couple of serious differences. Its temperature of 5940 Kelvin is just 160 Kelvin warmer than the solar surface, while the luminosity is a mere eight percent greater, the radius just two percent smaller, the chemical composition about the same, and the mass just a hair over solar. A rotation period less than a third that of the Sun produces some magnetic activity and X- ray radiation. We could be quite happy under the star's light except for its companion, which would seriously disturb any "solar system" that Chi-1 might ever have attempted to have. Just barely detectable, the companion orbits in 14.1 years at an average
Chi-1 Ori Chi-1 Orionis, the solar-type naked-eye star, is hidden at the center (its position indicated by the cross-marks) behind a dark "occulting disk" in the telescope so that it will not overwhelm the tiny, dim companion, which is indicated by the cross-marks to the left. The spots surrounding the bright component are optical artifacts. Chi-1 Ori B averages roughly Jupiter's distance from the Sun from much brighter Chi-1 Ori A. From "Direct Detection of the Companion of Chi-1 Orionis" by B. Koenig, K. Fuhrmann, R. Neuhaeuser, D. Charbonneau, and R. Jayawardhana in Astronomy and Astrophysics, vol. 394, L43, 2002.
separation of 6.1 Astronomical Units, a fairly high eccentricity causing it to sweep between 3.3 and 8.9 AU from its more massive neighbor. From it, we derive the above mass (which is consistent with luminosity and temperature), and see that the companion weighs in at a mere 0.15 solar, which is just double the limit for "real stars" that are able to fuse hydrogen to helium in their deep cores (below which they are seen as "brown dwarfs"). More interesting is that the system appears to be very young, only about 100 million years old, such that the low-mass companion may still internally be adjusting itself in the birth process and is not yet at fully stable "age zero," from which time we begin to measure its age, when it will be (roughly) a class M6 dwarf. Orion's two "Chi- stars" in a way really are paired, as each -- different as they are -- are not only fascinating in their own rights, but us give a fine sense of stellar variety.
Written by Jim Kaler. Return to STARS.