I compare the structures of the bipolar nebulae around the massive binary system η Carinae and around the low-mass binary system HD 44179. While η Car is on its way to becoming a supernova, the Red Rectangle is on its way to forming a planetary nebula. Despite the 2 orders of magnitude difference in mass, these two systems show several similarities, both in the properties of the stellar binary systems and in the nebulae. From this comparison and further analysis of the accretion process during the 20 yr Great Eruption of η Car, I strengthen the binary model for the formation of its bipolar nebula—the Homunculus. In the binary model a large fraction of the mass lost by the primary star during the Great Eruption was transferred to the secondary star (the companion). An accretion disk was formed around the companion, and the companion launched two opposite jets. I show that the gravitational energy of the mass accreted onto the secondary star during the Great Eruption can account for the extra energy of the Great Eruption, both the radiated energy and the kinetic energy in the Homunculus. I also conclude that neither the proximity of the primary star in η Car to the Eddington luminosity nor the rotation of the primary star are related directly to the shaping of the Homunculus. I speculate that the Great Eruption of η Car was triggered by disturbance in the outer boundary of the convective region, most likely by magnetic activity, that expelled the outer radiative zone.
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