From the Latin homunculus meaning Little Man, the nebula consists of gas which was ejected from Eta Carinae during the Great Eruption, which occurred ~7,500 years before it was observed on Earth, from 1838 to 1845.

[3] It also contains dust which absorbs much of the light from the extremely luminous central stellar system and re-radiates it as infra-red (IR).

At that time the shape of the nebula showed a central bulge with a single large lump to the northwest and two smaller extensions to the southeast, which was described as a Homunculus.

[7] The Homunculus consists of two lobes, referred to as northwest (NW) and southeast (SE) based on their orientation as seen from Earth.

It contains a much smaller mass of material than the lobes, shining mainly by reflected light which escapes most easily at equatorial latitudes.

The lobes contain the majority of the material in the Homunculus Nebula, in relatively thin shells concentrated towards the poles.

Older calculations had produced consensus estimates of 10-15 M☉[11] Early speckle interferometry showed that the central region of the Homunculus contains four point-like sources, originally designated A1, A2, A3, and A4.

[12] The four speckle objects were later referred to as A, B, C, and D. Higher resolution studies showed that only the brightest source A was truly stellar, and the other three were small nebular condensations.

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.

[9] Shock waves at the outer edge of the ejecta are heated to millions of kelvin and emit x-ray radiation.

The near-supernova explosion produced two polar lobes, and a large but thin equatorial disk, all moving outward at up to 670 km/s (1,500,000 mph).

[9] Irregularities in the otherwise very smooth structure of the shells are conjectured to result from interactions between the winds of the two central stars, and from their orbital motion.

Assuming an origin in Eta Carinae's Great Eruption and a constant expansion velocity, this gives the linear distance of the shell from the central star projected along the line of sight.