Eta Carinae (pronounced "ATE-ah ca-RYE-nee") is the largest and most splendid diffuse nebula in the sky, surpassing even the famous Orion Nebula (M42) in size and glory. Only its position far south of the celestial equator prevents this nebula from being a household name. You can't see it from much of the Northern Hemisphere.

Eta Carinae is a colossal star-forming region of hot gas, heated by ultraviolet rays from its hot young stars. It covers a full two degrees of sky (four times the width of the full moon).

Eta Carinae is obvious to the naked eye as a bright, elongated glow, but that only hints at its telescopic grandeur. With binoculars it appears large and bright. Its nebulosity seems split in two by a V-shaped dark lane of dust, called the Keyhole Nebula.

The field of view in most binoculars is just stunning, with Eta Carinae embedded in the star-studded Milky Way. A telescope will reveal many bright wisps, dark lanes, and subtle details that will keep the most ardent observer busy for years. And don't neglect the two open clusters—NGC 3532 and NGC 3114—that flank the Eta Carinae Nebula.

The star at the center of the nebula has a unique story to tell. Eta Carinae is one of the most massive stars known, 100 times the mass of our sun. It emits four million times as much light as our friendly local star, making it the brightest object in the sky when viewed through an infrared detector. Eta Carinae is a highly unstable star which fluctuates greatly in brightness. It will undoubtedly explode as a supernova soon.

On the cosmic time scale, "soon" means anytime in the next few hundred thousand years. But it could happen in your lifetime. It could happen tonight! Today the star is at the edge of naked eye visibility, but an outburst in the mid-18th century temporarily made Eta Carinae the second brightest star in the night sky, trailing only Sirius—and not by very much! This outburst ejected gas that now surrounds the central star and is known as the Homunculus Nebula, itself a great "fuzzy" for southern sky watchers with telescopes.

Click here to see if this object is visible in your sky tonight.