05.06.2013 Views

The Caldwell Objects

The Caldwell Objects

The Caldwell Objects

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

Society. Accepting that distance, we find the<br />

nebula measures 1.8 x 1.5 light-years. <strong>The</strong> Helix's<br />

proximity also makes it unique among the<br />

planetaries, because it allows astronomers to<br />

study its small-scale structures with groundbased<br />

telescopes and extremely fine details with<br />

the orbiting Hubble Space Telescope.<br />

In ground-based images the Helix looks like a<br />

typical annular planetary nebula. <strong>The</strong> vacuous<br />

region inside the ring shows evidence of oxygen<br />

atoms being excited by ultraviolet radiation<br />

flowing out from the extremely hot (120,000°<br />

Kelvin) central star. That star shines at<br />

magnitude 13.4 (though it maybe variable), so it's<br />

a bit brighter than Pluto is when closest to the<br />

Sun. <strong>The</strong> ring itself glows with the light of<br />

excited nitrogen and hydrogen atoms.<br />

Everything looks normal until you look closely at<br />

the ring and see that it has a helical structure<br />

with two twists. But this is an optical illusion. In<br />

January 1999 a team of astronomers released a<br />

three-dimensional, globelike map of the Helix<br />

based on data from a submillimeter-wave<br />

telescope atop Mauna Kea. Close inspection of<br />

the 3-D image reveals that the Helix Nebula<br />

behaves much like a double-headed garden<br />

sprinkler. For each blob of gas that was ejected<br />

on one side of the nebula, a similar structure was<br />

ejected on the opposite side. <strong>The</strong> researchers say<br />

this behavior likely has been caused by a<br />

companion star that affects how the Helix's<br />

progenitor ejects its outer layers.<br />

<strong>The</strong> Helix is also the only planetary in which<br />

gaseous knots with cometlike tails have been<br />

observed. This does not necessarily make the<br />

Helix unique; more distant planetaries probably<br />

have similar but irresolvable features. A few<br />

hundred of the Helix's knots were first detected<br />

from the ground, but HST increased the count to<br />

3,500. None of the knots is found close to the<br />

central star. Rather,<br />

<strong>The</strong> <strong>Caldwell</strong> <strong>Objects</strong><br />

63<br />

they form a thick ring, with the most visible knots<br />

lying along the inner edge of the luminous<br />

annulus, trillions of km from the central star. At<br />

first, astronomers believed these features might<br />

be evidence of a gigantic cloud of comets like the<br />

Oort Cloud postulated to surround our Sun. But<br />

it turns out that these features are much too large<br />

to be comets. Each gaseous knot is at least twice<br />

the size of our solar system, and each tail<br />

stretches for more than 150 billion km. <strong>The</strong><br />

gaseous knots display crescent-shaped heads that<br />

point to the central star. Meaburn says that each<br />

crescent is an ionized skin on the surface of a<br />

dusty molecular globule. And since the "comet<br />

tails" point away from the central star (which is<br />

ionizing the crescents), it's clear that each globule<br />

is being shaped by a wind from that star. In fact,<br />

we're seeing a sort of cosmic rear-end collision in<br />

which hot gas spewing from the surface of the<br />

central star rushes through a shell of colder gas<br />

that the star ejected when it was still a red giant,<br />

some 10,000 years earlier. When the hot, rarefied<br />

gas mixes with the cooler, denser gas, instability<br />

results, and the cooler cloud fragments into these<br />

gaseous knots. A close look at the HST image<br />

shows the cool ring writhing with these ghoulish<br />

"comets." <strong>The</strong> ring looks as if it's effervescing like<br />

an Alka-Seltzer tablet in water. Astronomers<br />

expect the gaseous knots, each several billion km<br />

across, to eventually dissipate into the cold<br />

blackness of interstellar space. "If this<br />

phenomenon is common among stars," says Rice<br />

University astronomer C. Robert Ο'Dell, who<br />

acquired the images, "then our galaxy could be<br />

littered with trillions of these objects." <strong>The</strong> Helix's<br />

knots are moving outward at about 15 km per<br />

second on average. This radiating array of<br />

pseudocometary pellets led Houston to coin the<br />

object's other popular nickname, the Sunflower<br />

Nebula.<br />

So how does this celestial marvel hold up<br />

251

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!