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Hubble instrument lets scientists see dying star

An MIT scientist and colleagues have peered into the dust-obscured heart of the Egg Nebula to reveal a detailed portrait of a star's last gasps.

Dr. Joel H. Kastner, a research scientist at the Center for Space Research, made the observations with Drs. Raghvendra Sahai and John Trauger of the Jet Propulsion Laboratory and David Weintraub of Vanderbilt University, using the new Near Infrared Camera and Multi-Object Spectrometer (NICMOS) aboard the recently refurbished Hubble Space Telescope.

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, where they eventually become the building blocks of new stars and planets.

Most of the carbon and nitrogen in the human body formed inside a star like CRL 2688 and was expelled back into space. The processes involved in this expulsion are now being better understood as a result of the capabilities of the Hubble with its new near-infrared camera.

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 theory, 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 tips of the bubbles directly trace the shock front where the high-speed outflow (expanding at more than 62 miles per second) collides with the denser and slower-moving (12 miles per second) material of the "arcs" seen in a second image taken two years ago by the Wide Field and Planetary Camera 2 (WFPC2).

The NICMOS image also shows emission from hot hydrogen molecules in the regions that are dark in the WFPC2 image. This emission had been observed previously using ground-based telescopes, and it has presented a puzzle, since it's not clear to astronomers how shock fronts could develop along the star's equator and not just toward its poles.

With the far superior sensitivity and detail provided by the Hubble images, it should be possible to unravel this mystery. "One possibility is that an explosive event occurred on the surface of the star several hundred years ago, scattering material in many directions at high speed, and the glowing hydrogen molecules trace the collision of this ejected material with previously ejected gas," Dr. Kastner said.

The current data, obtained through the Early Release Observation program in cooperation with the NICMOS instrument team, are part of a more detailed NICMOS and WFPC2 study of the Egg Nebula by Drs. Sahai, Trauger, Kastner and Weintraub.

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 a hole in two oppositely directed beams.

The work was funded by NASA.

A version of this article appeared in MIT Tech Talk on May 21, 1997.

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