Cosmic Housecleaning? Building Blocks Of Stellar Systems Go Missing

The team leader was graduate student Tea Temim. Her advisers, astronomy professors Robert Gehrz and Charles Woodward, are among the co-authors of the paper. They are all infrared astronomers interested in how cosmic dust forms and recondenses into a new generation of stars with planets. Our sun is at least a second-generation star, built from the dusty, gaseous wreckage of previous stars.

Cosmic dust is composed of any fine solid particles in space except ices. It is much finer than beach sand; it's more like smoke particles. It forms from the aggregation of gas molecules and very fine particles into even bigger particles--like the nucleation of water to make raindrops. Since very fine dust has been observed to form in the ejected material of another supernova only a few years after it exploded (Supernova 1987A, detected in 1987), the scientists expected to find plenty of it in a relatively young supernova remant like the Crab Nebula. Instead, the Spitzer data showed only much coarser dust--particles that, although only a few millionths of a meter in size, were still 10 to 100 times larger than the fine stuff. The big question is, what could have destroyed or spirited away the smallest dust particles?

One possible culprit is the rapidly spinning neutron star at the core of the Crab Nebula. It is pumping out intense ultraviolet radiation, which might be evaporating small particles. The core is also throwing out protons and electrons at speeds approaching the speed of light, and these could also be destroying the fine dust. One reason this is an attractive explanation is that fine dust forms within a year of an explosion, but the intense radiation from the core doesn't develop until long after coarse dust has coalesced. Temim has mapped the energy distribution from these ultrafast particles (called synchrotron radiation) coming from the neutron star. This will show how these particles spread out and mix with the rest of the ejected material.

The Crab Nebula is 6,500 light-years away--that's 30,000 times the distance of Earth from the sun. The nebula is five light-years across, which is bigger than the distance between the sun and its nearest star. The Crab Nebula has been photographed in the X-ray, radio and visible parts of the spectrum.

"The images are exciting because we're filling in the puzzle with infrared," said Temim. "Infrared is where we can find out information about the formation of dust."

These images could not have been made from a ground-based telescope because Earth's atmosphere filters out much infrared light. Spitzer can pick up faint infrared signals only because it is cooled to minus 450 degrees F; if it were much warmer, its own heat would drown out the signals from the Crab.

A note on how dust forms in a supernova. A massive star, during the course of its life, derives its energy from synthesizing elements as heavy as iron (of the 92 naturally occurring elements, iron is the 26th heaviest). When the star uses up its fuel, its core collapses, forcing protons and electrons together, forming neutrons. But, as Isaac Newton said, every action has an equal and opposite reaction. After collapsing, the star explodes, shooting neutrons and small atomic nuclei into each other to form new nuclei. Some of these nuclei are bigger than iron. Ordinarily, they would undergo radioactive decay, but as they are shot away from their parent star by the explosion they cool before decay can occur. All these elements coalesce to form dust. The iron in our blood comes from the iron forged in these big stars, and the gold, silver, mercury, lead and other heavy elements we find so useful are forged in the supernova explosion.

One of many pictures of the Crab Nebula is at http://antwrp.gsfc.nasa.gov/apod/ap991122.html. Pictures taken for this study are available at ftp://ftp.astro.umn.edu/pub/users/gehrz/crab_press_release_images/. The Spitzer Space Telescope is managed by NASA's Jet Propulsion Laboratory in Pasadena, Calif. Launched in August 2003, it circles the sun in the same orbital path as Earth but millions of miles behind. This work was funded by NASA.