Gemini Images a "Shocking" Skull of Gas (Forwarded)

Gemini Observatory
Hilo, Hawaii

Friday, 07 April 2006

Gemini Images a "Shocking" Skull of Gas

A new Gemini Observatory image of NGC 246, nicknamed the "Skull
Nebula," shows what can happen as the outer atmosphere of a fast-
moving, dying star like the Sun (in several billion years) pushes
through the complex soufflé of gas and dust that lies between the
stars of our galaxy.

NGC 246 is a planetary nebula that lies about 1,600 light-years
away in the constellation Cetus. Its central star shines at 12th
magnitude and is whisking through space at about 80 kilometers
(50 miles) per second.

Astronomers have known since 1969 that a planetary nebula interacts
with the interstellar medium as it expands. For NGC 246, this means
that the rapidly moving gases on the nebula's leading edge (which
shares the central star's direction of motion) slow down and
compress as they collide with the interstellar medium. Meanwhile,
gas clouds on the shell's trailing edge continue to expand freely.

The complex braided structure of NGC 246's blue outer ring is most
likely due to the action of shock waves. As the compressed outer
shell piles up against the interstellar medium like plowed snow
on a roadway, it heats up and becomes unstable. The ring starts
to fray and fragment. High-velocity gases pushing outward from the
hot (~200,000 degrees K) central star may also contribute to the
visible chaos. Collisions between these fast winds and the shocked
shell could explain the distinctive green lobes and darker voids
visible in the leading half of NGC 246's interior shell.

The unequal expansion along the nebula's major axis should result
in a visible asymmetry. As this highly detailed Gemini image shows,
NGC 246's outer structure is indeed shaped like an oval ring. Its
leading (top) edge appears brighter and sharper than its trailing
(bottom) edge. The "central" binary star is slightly displaced
toward the shell's leading edge, as astronomers predicted. (For
more details on the dynamics of planetary nebulae, and this object
in particular, see the papers by Muthu et al. and Soker et al.)

The star at the heart of NGC 246 is part of a binary pair. The
primary star is highly evolved, having exhausted all of its
nuclear fuel. This star has become a white dwarf with a radius
only a fraction of its original size (typically about the size
of the earth). Aside from NGC 246's central binary star, several
other conspicuous stars and some distant galaxies appear
superimposed on the gas shell; still others lie just beyond the
nebula's confines.

Technical Data:

Filter* Color FWHM Exposure Time

H-alpha Yellow 0.38" 4x300 seconds
[OIII] Red 0.52" 4x300 seconds
[SII] Blue 0.38" 4x300 seconds

Field of View: 5.57 x 5.57 arcminutes

Orientation: Image rotated 57 degrees counter-clockwise from North
up, East left

Data for this image was obtained on November 21-22, 2005

NGC 246 -- An Amateur's View

Discovered by German-born English astronomer William Herschel in
1785, NGC 246 is located a little more than 6 deg north-northeast
of 2nd-magnitude Beta Ceti and about 1.5 deg south-southeast of
4.8-magnitude Phi1 Ceti.

The nebula is relatively small (4.6' x 4.1') and dim (11th-
magnitude). In a small telescope the nebula's feeble light is
almost overpowered by the "cluster" of stars superimposed on it.
Larger telescopes will show the nebula more clearly, especially with
an oxygen III filter. NGC 246's central star is worth monitoring.
In 1930, its photographic magnitude was as bright as 9th magnitude.
In 1969 it dipped to 11.2 and today it shines at about 12th
magnitude.

Gemini is an international partnership managed by the Association of
Universities for Research in Astronomy under a cooperative agreement
with the National Science Foundation.

The Gemini Observatory provides the astronomical communities in each
partner country with state-of-the-art astronomical facilities that
allocate observing time in proportion to each country's contribution.
In addition to financial support, each country also contributes
significant scientific and technical resources. The national research
agencies that form the Gemini partnership include: the US National
Science Foundation (NSF), the UK Particle Physics and Astronomy
Research Council (PPARC), the Canadian National Research Council
(NRC), the Chilean Comisión Nacional de Investigación Cientifica y
Tecnológica (CONICYT), the Australian Research Council (ARC), the
Argentinean Consejo Nacional de Investigaciones Científicas y
Técnicas (CONICET) and the Brazilian Conselho Nacional de
Desenvolvimento Científico e Tecnológico (CNPq). The Observatory is
managed by the Association of Universities for Research in Astronomy,
Inc. (AURA) under a cooperative agreement with the NSF. The NSF also
serves as the executive agency for the international partnership.

IMAGE CAPTION:
[Full Resolution TIF (7.04MB),
http://www.gemini.edu/images/stories.../fig1_full.tif

Full Resolution JPG (68KB)
http://www.gemini.edu/images/stories.../fig1_full.jpg

Med Resolution JPG (28KB)
http://www.gemini.edu/images/stories.../fig1_med.jpg]

Gemini South Image of the planetary nebula NGC 246

Credit: Gemini South GMOS image/Travis Rector University of Alaska
Anchorage