A Turtle-Shaped Nebula, NGC 6210, and the Structure Around Its Star (inset)
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 larger image shows the entire nebula; the inset picture captures the complicated structure surrounding the dying star. 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 the "full nebula" 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. Both pictures are composite images based on observations taken Aug. 6, 1997 with the telescope's Wide Field and Planetary Camera 2.
Material flung off by this central star is streaming out of holes it punched in the nautilus shell. At least four jets of material can be seen in the "full nebula" image: 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.
Although the central star is visible in both pictures, it is more prominent on the inset image. Another clear feature on the inset image is a very interesting red, arrowhead-shaped protrusion emanating from a hole (seen nearly edge-on) at 4 o'clock. On the main image, the "arrowhead" is colored a subtle magenta. The "arrowhead" appears to be driving an outward swelling of material at the 4 o'clock border. This too appears to have a counterpart in the opposite direction. Some evidence is visible at the 10 o'clock position (inset). These features suggest a more recent shaping of the nebula by the fast stellar wind, because the material does not appear to be as far away from the central star as the outlying jets.
The column at 6 o'clock in the main image, 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 seen in both the main and inset images.
The main 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.
The inset picture is a composite of the inner nautilus shell generated by combining the Hubble telescope images in a different way. This picture enhances some of the inner structure that is not as clear in the main photo due to color blending. The inset is a two-color composite with red and green now depicting the radiation from singly ionized and doubly ionized oxygen, respectively. (This combination is useful for separating the less highly ionized gas from more highly ionized gas.)
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.