At magnitude 3.95, Chi Lupi is not one of the brightest stars in Lupus, but is one of the better known.
Properties
Chi Lupi is a spectroscopic binary, known to be double from two overlapping spectra, whose absorption lines dance back and forth as a result of the Doppler effect. It has an orbital period of 15.2565 days. The brighter of the pair is a class B9.5 IV subgiant with a temperature of 10650 K. The fainter is a cooler class A2 V main-sequence star at 9200 K. Their luminosities are difficult to separate, because each produces a different amount of ultraviolet light. The brighter class B star shines at 90 suns, the fainter at 25 suns. Luminosity and temperature yield respective masses of 3 and 2.2 solar masses, and a youthful age of less than 250 million years. These masses, the orbital period, and Kepler's third law indicate an orbital separation of 0.21 AU.
What stands out about the components of χ Lup is that they are both chemically peculiar, but in quite different ways. The fainter is a "metallic-line star." Such stars, typically of classes A and F, seem highly enriched in elements like copper, zirconium, barium, and europium, while low in others such as calcium. χ Lup's brighter member is a wonderful example of a "mercury-manganese" star. Its surface gases are enhanced in platinum and gold by factors of tens of thousands, and it has a million or so times the solar abundance of mercury (but calcium is way down).
These strange stars are not really rich in such chemical elements, nor really depleted in others. Instead, they have undergone a process of diffusion, in which some elements fall inward through the action of gravity, while others are lofted upward by radiation. Such separation can take place only in a quiet stellar atmosphere that is not undergoing convection and is not rotating quickly. χ Lup is so important that it has been designated a "Hubble Pathfinder Star" that has been specially targeted for observation by the Space Telescope.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]