The Caldwell Objects

Society. Accepting that distance, we find the
nebula measures 1.8 x 1.5 light-years. The Helix's
proximity also makes it unique among the
planetaries, because it allows astronomers to
study its small-scale structures with groundbased
telescopes and extremely fine details with
the orbiting Hubble Space Telescope.
In ground-based images the Helix looks like a
typical annular planetary nebula. The vacuous
region inside the ring shows evidence of oxygen
atoms being excited by ultraviolet radiation
flowing out from the extremely hot (120,000°
Kelvin) central star. That star shines at
magnitude 13.4 (though it maybe variable), so it's
a bit brighter than Pluto is when closest to the
Sun. The ring itself glows with the light of
excited nitrogen and hydrogen atoms.
Everything looks normal until you look closely at
the ring and see that it has a helical structure
with two twists. But this is an optical illusion. In
January 1999 a team of astronomers released a
three-dimensional, globelike map of the Helix
based on data from a submillimeter-wave
telescope atop Mauna Kea. Close inspection of
the 3-D image reveals that the Helix Nebula
behaves much like a double-headed garden
sprinkler. For each blob of gas that was ejected
on one side of the nebula, a similar structure was
ejected on the opposite side. The researchers say
this behavior likely has been caused by a
companion star that affects how the Helix's
progenitor ejects its outer layers.
The Helix is also the only planetary in which
gaseous knots with cometlike tails have been
observed. This does not necessarily make the
Helix unique; more distant planetaries probably
have similar but irresolvable features. A few
hundred of the Helix's knots were first detected
from the ground, but HST increased the count to
3,500. None of the knots is found close to the
central star. Rather,
The Caldwell Objects
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they form a thick ring, with the most visible knots
lying along the inner edge of the luminous
annulus, trillions of km from the central star. At
first, astronomers believed these features might
be evidence of a gigantic cloud of comets like the
Oort Cloud postulated to surround our Sun. But
it turns out that these features are much too large
to be comets. Each gaseous knot is at least twice
the size of our solar system, and each tail
stretches for more than 150 billion km. The
gaseous knots display crescent-shaped heads that
point to the central star. Meaburn says that each
crescent is an ionized skin on the surface of a
dusty molecular globule. And since the "comet
tails" point away from the central star (which is
ionizing the crescents), it's clear that each globule
is being shaped by a wind from that star. In fact,
we're seeing a sort of cosmic rear-end collision in
which hot gas spewing from the surface of the
central star rushes through a shell of colder gas
that the star ejected when it was still a red giant,
some 10,000 years earlier. When the hot, rarefied
gas mixes with the cooler, denser gas, instability
results, and the cooler cloud fragments into these
gaseous knots. A close look at the HST image
shows the cool ring writhing with these ghoulish
"comets." The ring looks as if it's effervescing like
an Alka-Seltzer tablet in water. Astronomers
expect the gaseous knots, each several billion km
across, to eventually dissipate into the cold
blackness of interstellar space. "If this
phenomenon is common among stars," says Rice
University astronomer C. Robert Ο'Dell, who
acquired the images, "then our galaxy could be
littered with trillions of these objects." The Helix's
knots are moving outward at about 15 km per
second on average. This radiating array of
pseudocometary pellets led Houston to coin the
object's other popular nickname, the Sunflower
Nebula.
So how does this celestial marvel hold up
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