Published online 8 July 2011 | Nature | doi:10.1038/news.2011.404

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A supernova detected in 1987 may explain why galaxies in the early universe were so dusty.

In this 2006 Hubble Space Telescope image of Supernova 1987A, a shock wave from the supernova explosion almost 20 years earlier lights up material around the stellar remnant. NASA, ESA, K. France (University of Colorado, Boulder), and P. Challis and R. Kirshner (Harvard-Smithsonian Center for Astrophysics)

An article from Scientific American.

Dust on earthly objects is often an indicator of antiquity. But that is not always the case for cosmic objects, some of which have quite a bit of dust despite their relative youth.

Galaxies out toward the edge of the visible universe, so distant that astronomers see them as they existed less than a billion years after the big bang, seem to already harbor large quantities of interstellar dust. But just how that dust appeared in such a short time remains unsettled.

Now a group of astronomers reports that exploding stars known as supernovae could be a major dust producer in those early galaxies. The researchers based their conclusion on new observations of a recent, relatively nearby supernova whose light first reached Earth on February 23, 1987, from the Large Magellanic Cloud, a satellite galaxy of the Milky Way about 160,000 light-years away. Known as Supernova 1987A, the stellar cataclysm has provided a unique opportunity for astronomers to watch a supernova's aftermath.

With the help of a new spaceborne infrared observatory, the European Space Agency's Herschel telescope, Mikako Matsuura of University College London and her colleagues were able to make an accounting of the material ejected during Supernova 1987A. In a study published online 7 July in Science1, Matsuura and her co-authors reported that the supernova produced copious amounts of dust grains made of carbon, silicates and possibly iron. The researchers estimated that the supernova ejecta contains, in dust alone, 40 to 70 percent the mass of the sun. (The progenitor star of Supernova 1987A was likely a giant 20 times the sun's mass whose core collapsed, triggering the explosion.)

"What we found is this supernova is making significant amounts of dust using the elements that have formed in the star and during the supernova event," Matsuura says. If supernovae at high redshifts — that is, at great cosmic distances — behave similarly, that could explain why galaxies in the early universe were so dusty, even though those galaxies had not existed long enough to contain so-called asymptotic giant branch, or AGB, stars, another ready source of dust. (AGB stars are aging celestial objects that often shed mass by blowing dust into the interstellar medium.)

"Since the discovery of dusty, high-redshift galaxies, people have been asking, 'Where did this dust come from?'" says study co-author Michael J. Barlow, a UCL astrophysicist. "For high-redshift galaxies — these are galaxies less than a billion years after the big bang — they seem too young to have old stars."

Supernovae had been proposed as another source of interstellar dust, but they are rare at relatively close range, where they can be carefully monitored, and observational evidence was lacking. "Lots of people had these models where you could create dust in these supernovae, but no one had really seen it," says Haley Gomez, an astrophysicist at Cardiff University in Wales who did not contribute to the new study.

Gomez was part of a group that in 2003 reported significant dust production2 from an older but closer supernova known as Cassiopeia A, or Cas A, which went off in the Milky Way in the 17th century. The remnant of that supernova is still visible, but its proximity actually poses something of a challenge. "The problem with it being in the Milky Way is there's also a lot of other stuff in the Milky Way," Gomez says. "What we interpreted as coming from Cas A could also have come from intervening material."

Further research3 strengthened the case that Cas A, in fact, produced a large mass of dust, but the case for Supernova 1987A is even more clear-cut and should solidify the supernova-dust connection. Even though Supernova 1987A resides in another galaxy, there is very little dust along the line of sight between Earth and the supernova remnant, allowing researchers a clear view. "This is why this will make a big splash, because it's a confirmed case where there really is no other possibility for it," Gomez says.

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Having shown that Supernova 1987A created lots of dust, Matsuura and her colleagues would now like to know the dust's fate over the coming decades. Can it persist long enough in the supernova's violent wake to fill interstellar space? "One of the things is because the supernova happened so recently, we want to see what happens to the dust," she says. "The thing that we want to see is whether the dust can stay around; can the stuff survive the shock waves?"

The question of dust is not just one of interstellar detritus — the particulate debris between stars can eventually become the stuff of life. "One of the reasons why we're interested in dust is it gets incorporated in stars and planetesimals," or planetary building blocks, Barlow says. "The Earth itself is formed from interstellar dust; the elements in our bodies were once in interstellar dust."

Gomez adds, "It's really the building blocks of what goes into planets, asteroids and even us. We all come from the same thing."

This article was first published by Scientific American on 7 July, 2011.