The University Record, June 6, 1994

Mysterious ‘knots’ in Crab Nebula may unlock secrets

By Sally Pobojewski
News and Information Services

University astronomers have discovered a line of clouds or “knots” of gas, strung out like pearls on a string, that extends more than halfway across the Crab Nebula.

Stephen S. Lawrence, graduate student in astronomy, presented an image and spectra of the knots during a news conference last week at the American Astronomical Society meeting in Minneapolis.

A huge expanding cloud of gas and dust 6,000 light-years from Earth, the Crab Nebula was created thousands of years ago when a star exploded in a massive supernova. The explosion was so large it was visible from Earth and was first documented by Chinese and Arabian observers in 1054 A.D.

Gordon M. MacAlpine, professor of astronomy, and Lawrence discovered the knots while obtaining data for Lawrence’s Ph.D. dissertation on a three-dimensional model of the Crab Nebula.

“The Crab is the most famous, most studied and possibly the least understood supernova remnant in our galaxy,” MacAlpine said. “The discovery of these extraordinary knots in the Crab Nebula is extremely important, because understanding the mechanisms that produced the knots will help us understand the unusual astrophysical processes and distribution of chemical elements in the nebula itself.”

Lawrence and MacAlpine first noticed the knots in an image taken in one narrow band of the spectrum. The small, dim knots in the Crab do not show up well in images taken with a full spectrum of light, because they are drowned out by brighter light from the surrounding gas.

Spectral analysis of light from the knots shows them to be unlike anything ever seen before, MacAlpine said. When compared to surrounding filaments of gas, the knots show extremely high levels of argon emissions, according to Lawrence.

“There appear to be 11 of these ‘argoknots’ aligned in arcs on either side of the pulsar [the remains of the star that exploded to create the Crab Nebula],” Lawrence said. “The knots seem to be positioned within a corridor running through the north-south axis of the nebula.”

Since the discovery, Lawrence and MacAlpine have been struggling to develop a theory that could explain the knots’ existence and their high concentration of argon emissions.

MacAlpine believes the knots may be related to instabilities involving the back part of the nebula. “The fact that argon emissions are higher on the side of the knots facing the pulsar suggests some kind of high-energy wind or streams of charged particles emanating from the pulsar’s vicinity,” he said.

Recent images of the knots have produced another mystery, according to Lawrence. They show dark regions or possible breaks in gas filaments behind the knots. “The knots may have been removed from the filaments by instabilities or by clumps of matter that existed prior to the supernova’s explosion,” he said. “Or the knots may contain high levels of dust, which obscures the background filaments.”

The digital image of the knots was taken with the Hiltner 2.4-meter (94-inch) telescope at the Michigan-Dartmouth-MIT Observatory on Kitt Peak, Ariz., using a Fabry-Perot imager on loan from NASA’s Goddard Space Flight Center. Spectra of the knots were obtained with the 4.5-meter (176-inch) Multiple Mirror Telescope of Whipple Observatory on Mt. Hopkins near Amado, Ariz. The MMT is operated jointly by the Smithsonian Institution and the University of Arizona.

Others assisting with the research include Beth A. Brown, U-M graduate student, Bruce E. Woodgate of NASA-Goddard, and Alan Uomoto of Johns Hopkins University. The research was funded in part by the U-M and NASA.