In one of the largest and most detailed celestial images ever made,
the coil-shaped Helix Nebula is being unveiled tomorrow in celebration
of Astronomy Day (Saturday, May 10).

The composite picture is a seamless blend of ultra-sharp NASA Hubble
Space Telescope (HST) images combined with the wide view of the Mosaic
Camera on the National Science Foundation’s 0.9-meter telescope at Kitt
Peak National Observatory, part of the National Optical Astronomy
Observatory, near Tucson, Ariz. Astronomers at the Space Telescope
Science Institute assembled these images into a mosaic. The mosaic was
then blended with a wider photograph taken by the Mosaic Camera. The
image shows a fine web of filamentary “bicycle-spoke” features embedded
in the colorful red and blue gas ring, which is one of the nearest
planetary nebulae to Earth.

Because the nebula is nearby, it appears as nearly one-half the diameter
of the full Moon. This required HST astronomers to take several
exposures with the Advanced Camera for Surveys to capture most of the
Helix. HST views were then blended with a wider photo taken by the
Mosaic Camera. The portrait offers a dizzying look down what is actually
a trillion-mile-long tunnel of glowing gases. The fluorescing tube is
pointed nearly directly at Earth, so it looks more like a bubble than a
cylinder. A forest of thousands of comet-like filaments, embedded along
the inner rim of the nebula, points back toward the central star, which
is a small, super-hot white dwarf.

The tentacles formed when a hot “stellar wind” of gas plowed into colder
shells of dust and gas ejected previously by the doomed star.
Ground-based telescopes have seen these comet-like filaments for
decades, but never before in such detail. The filaments may actually lie
in a disk encircling the hot star, like a collar. The radiant tie-die
colors correspond to glowing oxygen (blue) and hydrogen and nitrogen (red).

Valuable Hubble observing time became available during the November 2002
Leonid meteor storm. To protect the spacecraft, including HST’s precise
mirror, controllers turned the aft end into the direction of the meteor
stream for about half a day. Fortunately, the Helix Nebula was almost
exactly in the opposite direction of the meteor stream, so HST used nine
orbits to photograph the nebula while it waited out the storm. To
capture the sprawling nebula, HST had to take nine separate snapshots.

Planetary nebulae like the Helix are sculpted late in a Sun-like star’s
life by a torrential gush of gases escaping from the dying star. They
have nothing to do with planet formation, but got their name because
they look like planetary disks when viewed through a small telescope.
With higher magnification, the classic “donut-hole” in the middle of a
planetary nebula can be resolved. Based on the nebula’s distance of 650
light-years, its angular size corresponds to a huge ring with a diameter
of nearly 3 light-years. That’s approximately three-quarters of the
distance between our Sun and the nearest star.

The Helix Nebula is a popular target of amateur astronomers and can be
seen with binoculars as a ghostly, greenish cloud in the constellation
Aquarius. Larger amateur telescopes can resolve the ring-shaped nebula,
but only the largest ground-based telescopes can resolve the radial
streaks. After careful analysis, astronomers concluded the nebula really
isn’t a bubble, but is a cylinder that happens to be pointed toward Earth.

Credit: NASA, NOAO, ESA, the Hubble Helix Nebula Team,
M. Meixner (STScI), and T.A. Rector (NRAO).

Electronic image files and additional information are available at
http://hubblesite.org/newscenter/2003/11.

The Space Telescope Science Institute (STScI) is operated by the
Association of Universities for Research in Astronomy, Inc. (AURA), for
NASA, under contract with the Goddard Space Flight Center, Greenbelt,
MD. The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency (ESA). Kitt Peak National
Observatory is part of the National Optical Astronomy Observatory,
which is operated by AURA under a cooperative agreement with the
National Science Foundation.