wavelengths across the electromagnetic spectrum. The dominant feature is blackbody radiation from dust heated by the stars within. Overlaid on this is some light from the stars themselves reflected mostly from dense features within the nebulosity, showing strong visual and UV spectral lines in emission. There are also emission lines from ionised gas where it collides with slower moving material or is excited by high energy electromagnetic radiation from the stars. The ionisation emission is similar to a planetary nebula but at lower levels of ionisation due to the lower temperatures of the central stars. The strongest lines are [Fe ] and [N ], similar to those from the stellar winds of the stars themselves, but with narrower profiles. Shock waves at the outer edge of the ejecta are heated to millions of kelvin and emit x-ray radiation. The lobes of the Homunculus emit copious radio waves, including emission in the 21 cm line of hydrogen. The reflected spectrum of the Homunculus lobes varies with position, due to the central star emitting different radiation at different latitudes on its surface. This is the only star for which such an effect can be observed. Formation The Homunculus was ejected in an enormous outburst from Eta Carinae. Light from this event reached Earth in 1841, when Eta Carinae briefly became the second-brightest star in the sky, after Sirius; the ejected gas and dust have since obscured much of its light. The near-supernova explosion produced two polar lobes, and a large but thin equatorial disk, all moving outward at up to . The Homunculus Nebula is a virtually unique structure, believed to result from the extremely young age. This means that the shape and structure is almost entirely due to the original eruption rather than the interaction with surrounding interstellar material. Suggestions that the waist between the two lobes was formed by "pinching" from a dense surrounding material have been disproved, and the bipolar shells are now believed to be caused by concentrated polar outflows of material, with the equatorial skirt formed by breakout of faster ejected material through the thinnest parts of the shells. The ejection of material preferentially along the axis of rotation of the stars, or of the binary orbit, may be due to the rotation of Eta Carinae A itself resulting in stronger mass loss towards the poles. The thinness of the bipolar shells argues for their ejection within