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NASA's Hubble Space Telescope has shown us that the shrouds of gas surrounding dying, sunlike stars (called planetary nebulae) come in a variety of strange shapes, from an "hourglass" to a "butterfly" to a "stingray." With this image of NGC 6210, the Hubble telescope has added another bizarre form to the rogues' gallery of planetary nebulae: a turtle swallowing a seashell. Giving this dying star such a weird name is less of a challenge than trying to figure out how dying stars create these unusual shapes.
The remarkable features of this nebula are the numerous holes in the inner shells with jets of material streaming from them. These jets produce column-shaped features that are mirrored in the opposite direction. The multiple shells of material ejected by the dying star give this planetary nebula its odd form. In this image, the brighter central region looks like a "nautilus shell"; the fainter outer structure (colored red) a "tortoise." The dying star is the white dot in the center. This picture is a composite image based on observations taken Aug. 6, 1997 with the telescope's Wide Field and Planetary Camera 2.
Material flung off by the central star is streaming out of holes it punched in the nautilus shell. At least four jets of material can be seen: a pair near 6 and 12 o'clock and another near 2 and 8 o'clock. In each pair, the jets are directly opposite each other, exemplifying their "bipolar" nature. The jets are thought to be driven by a "fast wind" - material propelled by radiation from the hot central star. In the inner "nautilus" shell, bright rims outline the escape holes created by this "wind," such as the one at 2 o'clock. This same "wind" appears to give rise to the prominent outer jet in the same direction. The hole in the inner shell acts like a hose nozzle, directing the flow of material.
The column at 6 o'clock, which appears to be a series of vertebrae-shaped structures, suggests that the jets occur episodically. The broadest, most prominent of these are near the bottom and are curved upward, facing the central star. This column seems well aligned with the opening in the bottom of the nautilus shell.
This picture is a composite of images taken with three filters which are used to make a representative picture of the true colors of the object. Red represents hydrogen, which constitutes most of the nebula; blue, oxygen that is singly ionized; and green, oxygen at even higher ionization (doubly ionized). The ionization, in this case, is caused by ultraviolet light from the dying star stripping electrons from atoms.
NGC 6210 is about 6,600 light-years away in the constellation Hercules. The nebula measures 1.6 light-years from the very top of the turtle-shaped form to the tip of the bottom. The inner nautilus shell is about 0.5 light-years in diameter.
October 22, 1998
Credit: Robert Rubin and Christopher Ortiz (NASA Ames Research Center), Patrick Harrington and Nancy Jo Lame (University of Maryland), Reginald Dufour (Rice University), and NASA