Eta Carinae appears to be a relatively innocuous star as seen from Earth, though in fact it is a brilliantly luminous system with a remarkable history. It lies among a complex of nebulous clouds and star clusters in the depths of the Milky Way as it runs through northeastern Carina. It is a binary system consisting of two immense, hot stars, with Eta Carinae A being at least a hundred times the Sun's mass, and Eta Carinae B being at least thirty times that mass. Eta Carinae A in particular is one of the most massive stars known, and belongs to the rare classification of hypergiant.
Despite its inttrinic luminosity, Eta Carinae is so faint in the skies of Earth that is barely visible to the naked eye. One of the reasons for this is its immense distance, so great that it difficult to establish with certainty. Most estimates place the system at around 7,800 light years from the Solar System, and the recent Gaia data seems to confirm this, measuring a distance of 7,766 light years (within a margin of error) to stars in the same cluster as Eta Carinae.
That cluster, Trumpler 16, consists of several gigantic, highly luminous stars comparable to Eta Carinae, and forms part of Carina OB1, an association of comparable star clusters running along the Carina-Sagittarius Arm of the Milky Way Galaxy. This association lies within a vast nebulous structure known as the Eta Carinae Nebula (or simply the Carina Nebula) that stretches for some 350 light years. Trumpler 16 and Eta Carinae lies close to the Keyhole Nebula, a darker area within the main nebulous region.
The Great Eruption
Apart from its great distance from Earth, there is another reason why Eta Carinae appears so faint in the sky. It is shrouded within a double-lobed gaseous envelope, the Homunculus Nebula, which appears as two joined 'bubbles' of gas illuminated by the light of the binary star within. The existence of this Homunculus Nebula is tied to the history of Eta Carinae, and in particular to the event known as the Great Eruption.
Before 1837, Eta Carinae was little-regarded star, probably of third or fourth magnitude, but over the following few years its brightness suddenly began to increase until it became one of the brightest stars in the sky. It reached its maximum brightness in 1845 when, with an apparent magnitude of -1.0, it was outshone only by Sirius. From that point its brightness began to decrease, fading so rapidly that within a few decades it could no longer be seen by the naked eye. It brightened again in the last years of the nineteenth century, in a brief phenomenon dubbed the 'Lesser Eruption' before fading away once again.
These shifts in brightness seen from Earth were due to dramatic changes within the system. Eta Carinae A was at that time even more massive and luminous than it is today, with some estimates making it as much as 250 times as massive as the Sun. Through mechanisms that are still not fully understood, this star ejected much of its outer shell in a process similar to a supernova, but leaving the original star intact, albeit much reduced in size and power. That ejected matter is still visible today as the Homunculus Nebula, and indeed there is a smaller structure within the Homunculus showing that a second ejection took place at the time of the Lesser Eruption.
Over the decades since the Great and Lesser Eruptions, Eta Carinae has been gradually brightening again. At the turn of the twentieth century it varied around eighth magnitude (far too faint to be seen with the naked eye) but a hundred years later it had brightened to about fifth magnitude (that is, a roughly sixteen-fold increase in brightness over the course of a century). This trend is gradually accelerating, and it may be that Eta Carinae A is building toward another 'supernova impostor' event comparable to the Great Eruption.