For a long time, astronomers were not quite sure on how to explain some of the properties of η Car. That it was a system of two stars was something astronomers had been considering for some decades, but it was not until as recently as 2005 that Eta was confirmed to be a binary system.
There is strong evidence that supports this insight. The models that predict an orbiting period of 2022±1.3 days accurately predict features of multiwavelength observations (radio, millimeter, optical, near-infrared, and X-ray data observed over the past few decades, Farmer, 2010). Moreover, the “line intensity and the radial-velocity curve display a phase-locked behaviour implying that the energy and dynamics of the event repeat from cycle to cycle” (Damineli, 1999, p.2).
The consensus is that the first component of the system, ηA, is a Luminous Blue Variable (LBV), and the second is probably a late-type nitrogen-rich O star. The semimajor axis of the orbit is 16.64 AU, with a very high eccentricity e~0.9. The primary star’s radius is estimated to be in the range 0.7-1 AU (Farnier, 2010). From spectroscopic observation it has been inferred that ηA has a current mass ≥ 90 M⊙, and a stellar wind with a mass-loss rate ~10-3 M⊙ yr-1 and terminal speed of ~500 km s-1 (Madurai et al., 2011). It has a luminosity 4.5×106 L⊙, and a surface temperature of 20,000K (Bednarek, 2011). We do not have access to such certainty in the parameters of its companion ηB, since ηA dwarfs its emission, but we know that its luminosity is of the order of 9×105 L⊙, its surfece temperature, ~40,000K, its radius ~1010m, and its mass-loss rate a more modest 10-5 M⊙ yr-1 (Bednarek, 2011).
Because both members of the system are massive stars producing stellar wind, a shock front is produced where the winds meet, causing the X-ray emission seen in Chandra’s photographs. A binary star system of these characteristics is called a colliding wind binary.