The Eta Carinae Nebula

The Eta Carinae Nebula (NGC 3372), the brightest star forming region visible from Earth (credit: John Mansur and Terry Hancock).

The Eta Carinae Nebula (NGC 3372), the brightest star forming region visible from Earth (credit: John Mansur and Terry Hancock).

The Eta Carinae Nebula, the jewel of the southern-hemisphere constellation Carina, the Keel, is the most spectacular example of an active star factory in all the heavens. The nebula is about 260 light years across, some seven times larger than the Orion Nebula. And while it’s 7,500 light years away, five times farther away than Orion, it’s still easily visible to the even the most casual stargazer as a large frosty patch three times as wide as the full Moon in the Milky Way west of the constellation Crux, the Southern Cross.

Like all emission nebulae, the Eta Carinae Nebula is powered by many giant and supergiant stars shrouded within. The largest of these stars is the binary star η (eta) Carinae itself. Though it’s not directly visible, astronomers estimate eta Carinae has a combined mass of 100 solar masses and luminosity of a million Suns. Only a few dozen stars in the galaxy are as large and bright. Such stars burn their fuel quickly. Eta can only be a few million years old and is not expected to burn much longer before exploding once and for all in a tremendous supernova explosion which will light our skies in the day and night for many weeks. This event will occur soon, in the next few years, perhaps, or the next million years. It is impossible to know.

Because of its great distance and dusty shroud, the star eta Carinae is currently just on edge of visibility to an observer without optical aid. But the star has flared up dramatically in the past. In the early 1800’s, the star flared up until in 1843 it was the second brightest star in the sky except for Sirius for some 20 years. This remarkable brightening of the star in 1838 is sometimes called the “Great Eruption”. Astronomers of the day recorded the variation in brightness and color of eta Carinae, and made a few drawings. But the key tools of photography and spectroscopy were still decades in the future, so the nature of the outburst remained unexplained.

Eta Carinae slowly faded again and became impossible to see without a pair of binoculars in the late 1960s. It has since brightened slightly.

Astronomers are still unable to explain what caused eta Carinae to brighten by more than a factor of 1000 for decades. The event in 1843 produced a bi-lobed emission nebula around the star. This feature, known as the Homonculus Nebula, is embedded within the greater Eta Carinae Nebula and is visible with a small telescope as a bright orange star near the center of the nebula.

The light from the Great Eruption still travels through interstellar space. Light reached us directly from the star more than 150 years ago and has long passed us by. But some of the light took other paths through the galaxy, then bounced off clouds of gas and dust in the region of eta Carinae, then reflected again back into our line of sight. This phenomenon is known as a “light echo”. In 2012, a group of scientists announced they had detected a light echo from the Great Eruption of Eta Carinae in images taken with the 4-meter Victor Blanco Telescope in Chile. By analyzing the light echo and comparing it with models of massive stars, the astronomers have already learned the outburst was cooler than expected if the star simply blew off some stellar wind as do many massive stars. This means another mechanism was at work. One possibility: the companion star of Eta Carinae, which revolves in an elliptical orbit, made a close approach to the star and triggered gravitationally the Great Eruption.

The Carina Nebula is also home to two open clusters of stars and a fascinating range of structure. Dark lanes of interstellar dust adjacent to the star Eta Carinae give the center of the nebula a keyhole shape, so the nebula is often called the Keyhole Nebula. Detailed images with the Hubble Space Telescope show evaporating gaseous globules (EGGs) where new stars may be forming, as well as “bow shocks” around stars facing Eta Carinae that are probably a result of the “Great Eruption.

NGC 3372 (the Eta Carinae Nebula) lies in the Milky Way west of the constellation Crux, the Southern Cross (created with SkySafari 4 Plus)

NGC 3372 (the Eta Carinae Nebula) lies in the Milky Way west of the constellation Crux, the Southern Cross (created with SkySafari 4 Plus)

The nebula spans an amazing 2º of sky, so only wide-field telescopes can fit the entire nebula into the field of view. The nebula reveals a well-structured group of bright patches and dark lanes, along with embedded star clusters. The size and complexity of the nebula invite frequent inspection, and a single night of observation does not do justice to this awesome star-forming region.

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