Eta Carinae is a stellar system about 7800 lightyears away that is more than five million times as bright as the Sun. It was known to astronomers as an unremarkable star until 1837, when it began to grow brighter in an event known as the Great Eruption. For a brief time in 1843 it was the second-brightest star in the sky before fading again.
Since about 1940 it has been growing brighter again, with small fluctuations in brightness occurring on a roughly five and a half year cycle. The Great Eruption and the other changes in brightness are still a mystery to astrophysicists.
In the early twentieth century, astronomers noticed that Eta Carinae was growing fuzzy and no longer had the clean, point-like appearance of a typical star.
As the fuzziness grew and telescopes improved, it became clear that a billowing cloud of dust and gas – now known as the Homunculus Nebula, and shown here in a Hubble Space Telescope image – had been ejected during the Great Eruption of the 1840s and was expanding outwards.
Now about a light-year long, the expanding cloud contains enough material to make at least 10 copies of our Sun. Astronomers cannot yet explain what caused this eruption.