INTERLUDE 19-2  Eta Carinae
At the heart of the Carina emission nebula (shown at left in the figure) lies a remarkable object called Eta Carinae (right-hand images). With an estimated mass of around 100 times the mass of the Sun and a luminosity of 5 million times the solar value, Eta Carinae is the most massive star known. Formed probably only a few hundred thousand years ago, the short life of this star has been explosive indeed. In the mid-nineteenth century, Eta Carinae experienced an outburst that made it one of the brightest stars in the southern sky (even though it lies some 3000 pc away from Earth, a very long way compared to most of the bright stars visible in our night sky). The star released as much visible energy as a supernova explosion (see Chapter 21), yet it somehow survived the event. The object shown at bottom right below is the result of that outburst.

The bottom-right image, obtained by HST and carefully processed to reveal fine detail, is the highest-resolution view of the explosion obtained to date. Dust lanes, tiny condensations in the

outflowing material, and dark radial streaks of unknown origin all appear with exquisite clarity. The star itself is the white dot at the center of the image. The two ends of the "peanut" (at top right and bottom left) are blobs of ejected material racing away from the star at hundreds of kilometers per second—perhaps enough to expel the surrounding nebular gas and convert the Carina Nebula into the Carina Cluster. Perpendicular to the line joining these two blobs is a thin disk of gas, also moving outward at high speed.

The details of the events leading to this outburst are unclear; quite possibly such episodes of violent activity are the norm for supermassive stars. Smaller outbursts in the radio and X-ray parts of the spectrum have also been observed over the past few years. In addition to helping unravel the mysteries of Eta Carinae itself, astronomers hope that continued studies of this object will also provide insights into the less violent jets and other outflows associated with star formation.