Introduction

Eta Carinae (η Car) has intrigued astronomers for well over a century, beginning with its brightening to —1 magnitude in the late 1830s, rivaling Sirius as the brightest star in the sky for nearly two decades, then fading below naked-eye sensitivity. Observers in the Southern Hemisphere have pointed their telescopes in its direction since the 1820s; some navigational records exist even back to the late sixteenth century with visual magnitudes noted between 2nd and 4th magnitude (Frew 2004). Characterization of this peculiar star has been a challenge; D. Frew (private communication, 2003) noted that η Car was monitored by observers at Sydney Observatory in the nineteenth century in the suspicion that it was a binary system. Yet still today not all are convinced as direct evidence of the secondary star is not in hand.

η Car is of great interest, as at least one of the companions is at the end of its hydrogenburning phase.