The massive evolved star \eta Carinae is the most luminous star in the Milky Way and has the highest steady wind mass-loss rate of any known star .
Radiative transfer models of the spectrum by Hillier et al .
predict that H \alpha is mostly emitted in regions of the wind at radii of 6–60 AU from the star ( 2.5–25 mas at 2.35 kpc ) .
We present diffraction-limited images ( FWHM \sim 25 mas ) with Magellan adaptive optics ( MagAO ) in two epochs , showing that \eta Carinae consistently appears \sim 2.5–3 mas wider in H \alpha emission compared to the adjacent 643 nm continuum .
This implies that the H \alpha line-forming region may have a characteristic emitting radius of 12 mas or \sim 30 AU , in very good agreement with the Hillier stellar-wind model .
This provides direct confirmation that the physical wind parameters of that model are roughly correct , including the mass-loss rate of \dot { M } = 10 ^ { -3 } M _ { \odot } yr ^ { -1 } , plus the clumping factor , and the terminal velocity .
Comparison of the H \alpha images ( ellipticity and PA ) to the continuum images reveals no significant asymmetries at H \alpha .
Hence , any asymmetry induced by a companion or by the primary ’ s rotation do not strongly influence the global H \alpha emission in the outer wind .