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FOR RELEASE: October 22, 1998

PHOTO NO.: STScI-PRC98-36

TURTLE IN SPACE DESCRIBES NEW HUBBLE IMAGE

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.

Credits: Robert Rubin and Christopher Ortiz (NASA Ames Research Center),
Patrick Harrington and Nancy Jo Lame (University of Maryland), Reginald
Dufour (Rice University), and NASA