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Star Power

Comet-like knots provide fireworks for curious astronomers.

Star Nebula. Photo courtesy NASA

Nebula Knots: A Hubble space telescope image depicting comet-shaped ‘knots’ extending toward the center of the Helix Nebula. MU astronomer Angela Speck is now using Earth-based images to help scientists unravel the mystery of the knots’ origin and composition.

The Helix Nebula, a dying star located some 700 light years from the Earth, has long been an object of intense astronomical interest. Chief among its attractions? A luminous multitude of comet-shaped “knots” projecting inward toward the nebula’s vast, tubular center.

The knots had been visible from ground-based telescopes for years. But it took images taken in 1996 from the Hubble space telescope and its Wide Field Planetary Camera to really pique the interest of professional stargazers.

Astronomers were astounded, and not a little perplexed, by the gigantic, comet-shaped objects revealed in the Hubble photos. Each exhibited a huge, comet-like head—by some estimates as big as our entire solar system—that glowed impressively. These were trailed by wispy tails of gas billions-of-miles long. All radiated spoke-like from the nebula’s outer shell toward the nebula’s vast, cloudy core.

At the time, Hubble astronomer C. Robert O’Dell, in a news release from his lab at Houston’s Rice University, speculated that the strange objects were perhaps created when the star responsible for the nebula, fading fast, blasted great streams of hot hydrogen gasses into space. These then encountered a “cloud” of denser, cooler gas and space dust already surrounding the doomed star. The resulting collision, he said, destabilized the dusty cloud and created the curious knot phenomenon recorded by Hubble.

So far, so good, says Angela Speck, an MU astronomer and professor at MU. But now new high-resolution infrared images, available via an infrared camera on the Japanese Subaru Telescope in Hawaii, are revealing more of the knots’ secrets.

The images have, for example, shown the knots to be more plentiful than astronomers had previously anticipated. The new technology, Speck says, “showed us tens of thousands of previously unseen comet-shaped knots that look like a massive fireworks display in space.” Current thinking is that the Helix Nebula may have as many as 40,000 knots with a total mass that might be equal to 30,000 Earths.

The images have also changed scientists’ understanding of the knots’ life cycle. “Originally, we thought hydrogen molecules ejected from a dying star did not survive very long because of strong ultraviolet light,” says Speck. “We have found that the dust clouds prevent light from reaching and destroying the molecules. When the light can’t come into the dusty clumps in the nebula, the molecules can’t die. The hydrogen molecules can survive as long as they remain in the knots.”

And while the origin of the knots remains a mystery, the Subaru Telescope is helping Speck and her colleagues move closer to resolving this question, too.

“This new image provides us a better understanding of the process that creates the comet-shaped knots and helps us determine what really is going on. Based on our observations, we can’t attribute the cause of these knots to any one mechanism. In actuality, multiple mechanisms work together to create the knots,” she says.

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University of Missouri

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