The Nebula consists of a huge cloud of ionised hydrogen surrounding the extraordinary star Eta Carinae. Eta Carinae itself is a binary system, consisting of two massive stars in a mutual embrace, each casting off spherical shrouds of matter to form a peculiar dumb-bell shape. It is one of the most massive and luminous stars known, and also one of the most variable. In the mid-nineteenth century, it was one of the brightest stars in the sky, but has now faded very considerably.

Surrounding the hourglass of Eta Carinae is an extensive cloud of ionised gas, within which several star clusters are in the process of forming. Clouds of dark obscuring dust swarm through the nebula, too, and the most prominent of these gives the nebula its name. This is a vast and rapidly expanding dark cloud in the central parts of the main nebula, that gives its common name of the 'Keyhole'.