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Source of cosmic dust found
15 JUNE 2006

Balancing the dust budget -- almost
One of the larger problems of science is where we all came from. Before there were bacteria or monkeys, water or rocks, there was gas and dust -- various clumps of junk built from heavy atoms that had formed inside stars. The universe began with simple atoms: hydrogen, mainly, and some helium, which fused into all heavier atoms inside stars.

Galaxy is shot through with red bands, indicating dustThe Andromeda galaxy, seen from Spitzer, is an ocean of dust. Visual and infrared observations are combined to show how enormous quantities of dust, seen here for the first time, help comprise this nearby galaxy. NASA/JPL-Caltech /P. Barmby (Harvard-Smithsonian CfA).

But how did the heavy atoms combine into dust? In particular, what was the source of dust seen around a giant, blazing galaxy called a quasar that glowed in the deepest past, just half a billion years after the Big Bang?

The question seems arcane, but it matters if you want to understand our origins. Dust is built of elements heavier than hydrogen, helium or lithium -- including the familiar carbon, iron, and oxygen that are so essential to life. Dust carrying these elements coalesced with gas to form our solar system and planet more than 4.5 billion years ago.

Astrophysicists have imagined three possible sources for the dust seen around the early quasar, but none seemed to work fast enough to account for the amount of dust they saw in that ancient quasar. But until they could explain the source of the dust, astrophysicists could not explain the origin of the molecules inside Jen's baby, or even the molecules in the tabloid covers showing the all-smiles Brad or that sobbing Jennifer.

Spiral arrangement of stars against black space A supernova (star explosion) in this nearby spiral galaxy produced a gob of dust -- helping explain the origin of the "stuff" in Earth and your big toe. Box shows location of detailed photos, below. Courtesy Ben Sugerman, Space Telescope Science Institute.

Now a group of astronomers has detected a big gob of dust around a supernova -- a massive-star explosion -- first seen in 2003. In the years since the supernova blew up, it apparently made about 1,000 times the amount of dust that was previously measured around two other supernovas. The star blew up about 30 million light years from Earth.

If similar dust-making processes occurred in the ancient past, they could account for up to 50 percent of the ancient quasar's dust. That's a big step toward a complete explanation, says first author Ben Sugerman, a post-doctoral fellow at the Space Telescope Science Institute in Baltimore.

Left: Arrow points to bright yellow star. Right: Arrow points to dark location where star once was
LEFT: The exploding star (arrow) briefly glowed. As the heavy atoms it made expanded and cooled, many gathered into dust -- primitive clumps of atoms that eventually gathered into planets.
RIGHT: 678 days later, the supernova has disappeared. But the cloud of dust is still expanding. Eventually, it will be incorporated in stars and planets.
Courtesy Ben Sugerman, Space Telescope Science Institute.

The idea that a supernova could make dust was proposed back in 1965, Sugerman says, but until now, dust had been detected at only two supernovae, and then in tiny quantities. Supernovas are rare to begin with, and dust is seldom seen in them because astronomers usually turn their attejntion from the explosion after a few hundred days, even though atoms must cool for a few years before they can coalesce into dust.

In dust we trust
And the dust is best seen from space: dust from the 2003 explosion, for example, was not visible from a ground-based infrared telescope.

The researchers ogled dust from the 2003 explosion using the Spitzer Space Telescope, an orbiting spyglass that was, coincidentally, launched in 2003. Spitzer "sees" infra-red light -- the kind that warm dust emits.

Cylinder of metal floats through galaxy snapping photos of cosmos
The Spitzer Space Telescope "sees" in the infra-red - a band of electromagnetic radiation where dust becomes visible. Courtesy: Cal Tech

Using a more realistic estimate of how the expanding cloud of star-stuff was shaped, the researchers say they saw enough dust to equal up to 2 percent of our sun's mass -- a great leap forward in terms of accounting for all the dust there is (even the cooties behind the couch!) in space.

Beyond helping account for the heavy atoms in an interstate highway in Montana or the cobblestones of ancient Rome, this trusty dusty knowledge should give a better picture of how galaxies originate and evolve. Conventional physics can explain how dust formed in the early universe.Galaxies, like your body and the clothes you are wearing, all contain dust made during previous star explosions. "Dust is probably the most poorly understand factor that controls galactic evolution," says Sugerman, "so getting a sense of where it comes from is a good step in understanding how galaxies evolve."

The dust seen in the early universe has been a thorny problem for science, Sugerman says, because scientists "believed that supernovae could not have formed this dust," so explanations require "exotic physics, the kind of processes we don't see today." Exotic physics has been pressed into service to explain data showing that the universe is expanding faster, not slower, as time passes. Physicists have speculated that a repulsive force, never actually detected, may be counteracting gravity and causing the speed-up.

If the dusty supernova is representative of other star explosions, it could help explain most of the dust there is with hum-drum, conventional physics, Sugerman says. "We can say with certainty that today's physics can explain a lot of what happened in stellar deaths during the early universe."

-- David Tenenbaum

• Massive-Star Supernovae as Major Dust Factories, by Ben Sugerman et al, Science, June 9, 2006.

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