Eta Carinaeastronomy also called Keyhole Nebula,

peculiar red star and nebula (catalog number NGC 3372) in the southern constellation Carina and now known to be a binary star system. It is one of a small class of stars called luminous blue variables. The English astronomer Sir Edmond Halley noted it in 1677 as a star of about fourth magnitude. In 1838 Sir John Herschel observed it as a first-magnitude star. By 1843 it had reached its greatest recorded brightness, approximately −1 magnitude, or as bright as the brightest stars. Unlike the common types of exploding stars called novae and supernovae, it remained bright for several years. From about 1857 it faded steadily, disappearing to the unaided eye only about 1870. Since then it has varied irregularly about the seventh magnitude. The nebula around the star is an expanding shell of gas and dust, shaped like an hourglass with a disk at its centre. In 2005 astronomers studying far-ultraviolet spectral observations of Eta Carinae made by spacecraft found that it is a binary star system with an orbital period of 5.52 years. Its A component has a temperature of about 15,000 K; its B component, about 35,000 K.