Hubble's image of the Cygnus loop. The smooth supernova shock wave was a surprise to astronomers.

Sinuous Shock Wave in Space

AUSTIN, TEXAS--A shock wave from a giant star that blew up 10,000 years ago, still plowing through space at 170 kilometers per second, has come under the discerning eye of the Hubble Space Telescope. The blast continues to light up filaments of gas in surprisingly delicate and gently rolling sheets, according to research presented here last week at a meeting of the American Astronomical Society.

Massive stars die in titanic detonations called supernovae, which pop off in our galaxy once or twice per century. These explosions seed space with the raw materials for new generations of stars and planets. In addition, supernovae unleash shock waves that persist for millennia and rip through the gas-filled space between stars called the interstellar medium. One such shock front encircles a glowing cloud of debris in the constellation Cygnus, known as the Cygnus Loop. The faint cloud is about 1500 light-years from Earth, but is so big--70 light-years across--that it covers an area of the sky 35 times larger than the full moon.

In November 1997, Hubble's optical camera zeroed in on the leading edge of the Cygnus Loop to reveal details 10 times more clearly than astronomers can see from the ground. The still-energetic shock wave of hot electrons ionizes hydrogen atoms to form red glowing strands, says astronomer Ravi Sankrit of Johns Hopkins University in Baltimore, Maryland. The image's smooth sinuous flow came as a "great surprise," notes Sankrit. Theoretical models had predicted that supernova shocks would light up sharp waves of hydrogen, but with a more ragged pattern.

Astronomers can use images like this one to gauge the distribution of hydrogen in the galaxy, says astronomer Denis Leahy of the University of Calgary in Alberta. Denser regions of gas--containing perhaps two atoms per cubic meter--glow more brightly as the shock passes. "Supernova remnants tell us the most about the density structure of the interstellar medium," Leahy says. "The Cygnus Loop is the prototype."