The Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer (NICMOS)has peered into the dust-obscured heart of the Egg Nebula, revealing a detailed portrait of a star's last gasps.
The Egg Nebula, also known as CRL 2688 and located 3,000 light-years away in the constellation Cygnus, is an expanding cloud of gas and dust ejected by a dying Sun-like star that has burned most of its fuel. Studying the death of Sun-like stars is important for understanding how two of the elements crucial for human life carbon and nitrogen, formed from hydrogen and helium inside stars are expelled into the interstellar medium. Eventually, these elements become the building blocks of new stars and planets. Most of the carbon and nitrogen in your body were formed inside a star like CRL 2688 and were expelled back into space in processes that are now being better understood as a result of Hubble's new infrared eyes NICMOS.
A long-held model for dying Sun-like stars is that they eject matter in a slowly-expanding spherical wind. But objects like the Egg Nebula are forcing a shift in this model, showing that dying stars also eject matter at high speeds preferentially along their poles, and may even have multiple jet-like outflows from their surfaces. The signature of the collision between the fast and slow outflows is the glow of hydrogen molecules captured in the NICMOS image. The detailed structure of the hydrogen-emitting region tells us about the earlier slow ejections of mass and the current jet-like wind.
The NICMOS image shows two spindle-like bubbles of molecular hydrogen and dust along the long axis of the nebula. The red tips of the bubbles directly trace the shock front where the high-speed outflow (expanding at more than 62 miles per second or 100 kilometers per second) collides with the denser and slower-moving (at 12 miles per second or 20 kilometers per second) material of the "arcs" seen in the Wide Field and Planetary Camera 2 (WFPC2) image. The bubbles are seen to be closed at their ends by bright caps of dense material, directly showing that high density gas in the nebula is blocking the flow of high velocity material escaping from the top and bottom of the obscuring dust cocoon. These features thus directly confirm that the dark region between the searchlight beams seen in the WFPC2 image does not result from a lack of matter, as had been previously believed, but from a lack of illumination. The bright walls of the bubbles lie just inside the outer edges of the searchlight beams. This observation is consistent with the hypothesis that the high-velocity outflow is streaming out through the same holes as the starlight.
The NICMOS image also shows emission from hot hydrogen molecules in the regions that are dark in the WFPC2 image. With the far superior sensitivity and detail of Hubble, we had expected to see this ring-like region of glowing molecular hydrogen to extend inwards, like the spindle-shaped lobes, into the center of the nebula. Surprisingly, this region remains very dark in the infrared. It is possible that the dust in this region is extremely thick and blocks the infrared light from molecular hydrogen, which is produced on the far side of the dust from us.
The Egg Nebula is an important prototype of similar nebulae which surround both dying and newborn stars. It also represents a nearby small-scale model of the structure at the center of quasars, where a luminous compact object is embedded in a dust torus with radiation and mass flowing out of a hole in two oppositely directed beams.