Space / Telescopes

The dwarf is not hot enough to fuse carbon or oxygen, so it no longer burns like a star, but for a time it is still plenty hot enough-more than 50,000 degrees Fahrenheit-to expel the remnants of its atmosphere and to blast all that departed gas with ultraviolet light. The ultraviolet rays shred the molecules and atoms in the blown-off atmosphere, and that causes the gas to glow brightly. That glow is what we see as a planetary nebula.

By all rights, such a nebula should be spherical, like the original star itself. Some are: Abell 39, for instance. But look at some of the pictures made with the Hubble over the past decade and simple, glowing gas rings is not what you see. You see a Hamburger and a Garden Sprinkler. You see a Butterfly and an Ant. And inside some of the nebulas-the Helix or Eskimo, say-you see peculiar knots and clumps of gas that look like comets plunging in toward the star. A couple of longtime students of planetary nebulas, Bruce Balick of the University of Washington and Adam Frank of the University of Rochester, recently summed up the impact of the Hubble images. “Each new image was greeted with a combination of aesthetic delight and interpretive apprehension, if not terror,” they wrote. Researchers who thought they had these nebulas figured out discovered how little they knew.

As Balick and Frank continue to struggle with what they’re seeing, one thing has become clear about how stars die: Many perfectly ordinary ones do not go gently. As they near the end of their lives, they revert to the aggressive behavior they showed billions of years earlier in their vigorous youth. They emit focused jets of gas that travel at hundreds of miles per second. Those jets catch up with the more lazily drifting star smoke and knock it about, which may help explain the weird shapes.

The Stingray nebula (left) is even younger than the Ant; its central star got hot enough to light up the surrounding gas and make it visible just within the past 25 years. The central star in the Blinking Planetary Nebula (middle) is shedding mysteriously bright clumps of gas, which glow red. The Little Ghost (right) is a more classic planetary nebula: Its doughnut is the steadily expanding ring of star gas that has been ionized and set aglow by ultraviolet light from the central white dwarf. (middle photo courtesy of Bruce Balick (University of Washington), Jason Alexander (University of Washington), Arsen Hajian (U.S. Naval Observatory), Yervant Terzian (Cornell University), Mario Perinotto (University of Florence, Italy), Patrizio Patriarchi (Arcetri Observatory, Italy), and NASA)

The Spirograph nebula (left) contains enigmatic internal filaments. Similarly, the Retina nebula (middle), which is shaped like a doughnut seen from the side, exhibits light-absorbing dust and gas that have been swept into dense, dark lanes, seen silhouetted against the expanding doughnut of ionized gas. The Eskimo nebula (right) is older than the Retina, says astronomer C. Robert O’Dell. Here, the bright orange lanes have been whittled away by ultraviolet radiation from the star, turning them into knots with cometlike tails.

Those things that look like curved metal arms in the Garden Sprinkler are really pulsing jets of gas, emitted by some kind of nozzle close to the central star that is wobbling like the axis of a top, says Angels Riera of the Polytechnic University of Catalonia in Spain. The jets in another nebula, K3-35, are so powerful, reports Luis Miranda of Spain’s Astrophysical Institute of Andalucia, that they are squashing clouds of water vapor at their tips, 450 billion miles from the star, and causing the water to mase-emit laserlike beams of microwaves. Water masers also line the jets that Hiroshi Imai of the Joint Institute for Very Long Baseline Interferometry in Europe has traced to W43A, a red giant that appears to be on the verge of shedding its atmosphere. In the Ant nebula, Joel Kastner, a young astronomer at the Rochester Institute of Technology, and his collaborators have mapped what seem to be X-ray-emitting jets that extend from the center of the nebula to the tips of its bizarre, bright lobes. “It looks like we’re seeing the nozzles that are inflating the two bubbles,” Kastner says.