Thousands of Gaseous Knots Surround Dying Star

first_imgShareCONTACT: Lia Unrau PHONE: (713) 831-4793 THOUSANDS OF GASEOUS KNOTS SURROUND DYING STARThousands of huge tadpole-shaped objects surround a dying star astronomically close to Earth, and the Hubble Space Telescope (HST) recently captured the first strikingly clearlook at these abundant and mysterious objects. Rice University astronomers C. Robert O’Dell, Andrew Hays Buchanan Professor of Astrophysics, and graduate student Kerry P. Handron observed the objects, which probably resulted from the final outbursts in the star’s death throes. Prior ground-basedobservations showed only blurry streaks with a hint of structure. Called “cometary knots” due to their knotted glowing heads and streaming tails, like those of a comet, they are positioned in a ring around the dying the star, which is now entering its white dwarf phase. The glowing knots and tails radiate out like spokes on a wheel, lying at a distance of trillions of miles from the star,their probable source. The gaseous knots became visible when O’Dell used the extraordinary resolving power of the Wide Field Planetary Camera 2 to explore the Helix nebula in the constellation Aquarius. The Helix nebula is a planetary nebula, a series of rings or shells of gases thrown off a sunlike star at the violent end of its life. At 450 light-years away, it is the closest planetary nebula to Earth. The most visible cometary knots reside around the inner edge of the ringof gases. O’Dell and Handron have detected 313 of the cometary knots and project a total number for the entire nebula of 3,500, more than have ever been seen in this nebula. All other planetary nebula are too far away for astronomers to see clearly without HST. While previous ground-based observations showed a faint spoke-like pattern and a hint of structure in the Helix, the Hubble images captured layers of many more objects in greater detail. The scientists’findings appear in the April issue of Astronomical Journal. While two of our solar systems could fit inside one of the knots, its mass is only twice that of Earth, O’Dell determined. O’Dell expects that the gaseous knots, each about 10 billion miles in diameter, (at least twice the size of our solar system) will follow a standard model, and expand and dissipate into thereaches of space within a few hundred thousand years. Alternatively, if the knots have achieved a high enough density, they will continue to condense and become solid planet-sized bodiesof icy dust and particles, O’Dell speculates.“If that scenario is right, it would mean that stars are spewing out thousands of gigantic cometary-type bodies into interstellar space as they die, and they are roaming around out there,” he says.In examining the Helix, O’Dell originally sought to test whether the objects are true comets, brought out of “cold storage” when the dying star’s outburst boiled gases off the icy, dormant comet nuclei. While the knots have the right shape and lie at the right distance from the dying star to be a “comet cloud,” similar to the theorized Oort Cloud that orbits the Sun at the outskirts of our solar system, each gaseous knot is much too large to be the gaseousshell, known as a coma, that surrounds a comet. O’Dell says the knots and tails most likely formed when the dying star jettisoned shells of gas into space. The dying star expelled these gaseous shells in stages, and toward the end of the process, a faster moving shell collided with slower moving gasexpelled 10,000 years earlier. As hot, lower density gas collided with cooler, higher density gas, an unstable condition formed, in which the two gases swirled together, breaking up the previously smooth cloud. Through this process, known as a Rayleigh-Taylor instability, the cloud fragmented and clumped intosmaller and denser droplets-the gaseous knots. The image or idea that the objects are moving at great speed, enhanced by the comet-like tails that stream out for 100 billion miles, is likely an illusion, O’Dell says, created by wind abrasionas the star continues to produce a wind of charged particles, similar to that of the Sun’s, but vastly more intense.“About one dying star each year passes through this planetary nebula phase, each producing these gaseous knots,” says O’Dell. This process serves as the usual ending for the billions of sunlike stars in the Milky Way galaxy. “Our galaxy could be strewn with trillionsof these bodies,” he says. Hubble will be used to search for similar objects in more distant planetary nebulae. O’Dell hopes Hubble will again turn its observational powers on the Helix nebula in a few years to revealthe extent and direction of the motion of the knots. ### AddThislast_img read more