Astronomers sweep space for the sources of cosmic dust

Long before the love song "Smoke Gets in Your Eyes" made its debut in 1933, astronomers had to contend with a smoky pall that dulled their view of the universe. Dark, sooty particles and fine, sandlike grains drift among the stars, obscuring attractions such as the cores of galaxies and the nurseries where new stars emerge. "Dust was a thing that just got in the way," says astronomer Angela Speck of the University of Missouri, Columbia.

Today, that dirty reputation has faded. Astronomers know that interstellar dust illuminates the erratic deaths of stars, and it traces a direct link from stars to the birth of our solar system — and ultimately, to Earth. Researchers can deduce the histories of ancient stellar grains, embedded for billions of years in meteorites and cometary debris. Yet astronomers still have a poor grasp of where these flakes of the cosmos puff into existence.

New observing tools are making inroads. Most notably, NASA's Spitzer Space Telescope is sensing the infrared warmth of dust motes near and far, within our Milky Way and in galaxies from the early universe. Much of the dust has an organic component, showing that old stars and ultraviolet light can combine to create a pervasive prebiotic haze.

But Spitzer and other telescopes have not yet resolved a key puzzle: Does most dust condense in gentle breezes of gas emitted in the dying gasps of stars like our sun, or as a result of the much rarer concussive blasts of supernova explosions? Models predict that vast volumes of dust, roughly equal in mass to our sun, should form in the aftermath of a supernova. However, observers have spotted less than 1% of that amount in the debris from these detonations. "This is a real conundrum," says astronomer Robert Gehrz of the University of Minnesota, Twin Cities.

By examining individual grains within primitive meteorites, researchers can unlock what astronomer Donald Clayton of Clemson University in South Carolina calls the "cosmic chemical memory" of interstellar dust. "It's a beautiful thing," says one of Clayton's former students, Eli Dwek of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Each dust particle locks in the composition of the source where it formed."

Our sun will reach this brief phase of evolution in several billion years, as will all stars with about 0.8 to 8 times the sun's mass. When such stars run out of hydrogen at their cores, they start to fuse helium. That reaction releases more energy, bloating the stars into red giants. Later still, the helium begins to run dry. The stars then contract and expand in on-again, off-again pulses of helium burning, creating unstable orbs that would envelop the orbit of Mars in our solar system. For hundreds of thousands of years, stars in these rhythmic last gasps of fusion reside on what astronomers call the "asymptotic giant branch" (AGB) of a diagram that plots stellar evolution.

Gravity at the surfaces of distended AGB stars is so low that the outer layers escape with each expansive throb. When this liberated gas cools below 2000 kelvin, it starts to form tiny grains of dust. Their nature depends on the proportions of two elements forged by the stars' nuclear fires: carbon and oxygen, which quickly combine to make stable carbon monoxide gas. If there's carbon left over, a fraction of the gas will condense into sooty compounds such as graphite, silicon carbide, and complex organic molecules called polycyclic aromatic hydrocarbons. Oxygen-rich atmospheres spawn aluminum and titanium oxides as well as silicates with calcium, magnesium, and iron — the stuff of sand and rocks.

As more dust forms, radiation from the luminous stars — thousands of times brighter than our sun--pushes on the grains. The dust accelerates away and drags more gas with it, making the stars shed mass copiously. Late-stage AGB stars may vanish in optical light as the new dust screens our view, but they shine with a dazzling infrared glow. A new Spitzer image of the nearby Andromeda galaxy features thousands of false-color red dots that astronomers believe are shrouded AGB stars.

Each low-mass AGB star is a modest dust factory, but there are so many of them that they may be the predominant sources of cosmic dust. Indeed, most of the presolar isotopes in dust grains embedded in meteorites appear to have arisen by capturing neutrons inside AGB stars. The stars then ejected the isotopes in gentle stellar winds, says cosmochemist Ernst Zinner of Washington University in St. Louis, Missouri. "Supernovae get a lot of the glory," Speck observes. "But the isotopes we see indicate that most of these grains formed at a much slower rate, not explosively."

•	Astronomers Sweep Space for the Sources of Cosmic Dust Tiny interstellar grains dim the brilliance of many stars and galaxies, but the origins of the universe's ubiquitous dust remain hazy Science Magazine online, Friday, Oct. 28, 2005 Byline: Robert Irion