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Explosive Origin of Cosmic Dust Discovered

08 January 2008


The first definitive evidence of cosmic dust, important in building planets like our Earth and ultimately ourselves, has been found in the remains of a massive star explosion 11,000 light years away in our own Galaxy.

Cosmic dust is made up of tiny particles of solid material and is present throughout the cosmos. It helps developing stars to ignite and also forms the matter making up planets and all living creatures - but there has been controversy over where it is formed.

Scientists have long suspected that exploding stars, or supernovae, are an important source of cosmic dust, but now an international team including a Cardiff University astronomer has provided unambiguous proof.

Dr Haley Gomez, of the University’s School of Physics and Astronomy, played a part in analysing images from NASA’s Spitzer Space Telescope of the remains of the supernova Cassiopeia A (Cas A). Cas A would have been 30 times the mass of our own Sun but would have taken just 10 million years to reach the explosion stage, providing a rapid source of dust. Dr Gomez was part of a team of UK astronomers who had previously hunted for dust grains in this enigmatic object. In this previous work, although dust was detected in Cas A, it was difficult to pinpoint where exactly it was coming from – whether it was made in the supernova or elsewhere.

Using Spitzer’s sensitive infrared detectors, the international team led by Dr Jeonghee Rho from NASA's Spitzer Science Center, have now found enough cosmic dust in Cas A to make up 10,000 Earths. Its composition was analyzed and found to be made up of proto-silicates, silicon dioxide, iron oxide, pyroxene, carbon, aluminium oxide and other compounds, all located in the same place as the supernova gas. Dr Rho said: "Now we can say unambiguously that dust - and lots of it - was formed in the ejecta of the Cassiopeia A explosion."

However, the hunt for cosmic dust does not end there. The findings from the Spitzer telescope do not explain where all of the dust seen in distant galaxies comes from. Cardiff astronomers are now working on cameras for the European Space Agency’s new Herschel Space Telescope, due to be launched this year, which could provide a final answer.

Dr Gomez said: "At the moment we’re missing something. The dust Spitzer is looking at is quite warm – about 100 degrees Kelvin. We think there’s colder dust in there, which Spitzer doesn’t see – at around 20 degrees Kelvin. We’re hoping that Herschel will allow us to see the colder dust. Herschel could completely change the way we see the Universe."

The report on the Spitzer Telescope findings is due to be published in the January 20 issue of the Astrophysical Journal. Dr Jeonghee Rho is the lead author along with Lawrence Rudnick of the University of Minnesota. Other co-authors include W.T. Reach of the Spitzer Science Center; J. D. Smith of the Steward Observatory, Tucson, Arizona; T. Delaney of the Massachusetts Institute of Technology, Cambridge; J.A. Ennis of the University of Minnesota; A. Tappe of the Spitzer Science Center and the Harvard Smithsonian Center for Astrophysics, Cambridge, Mass; and Takashi Kozasa, Hokkaido University in Japan.

The previous study of Cassiopeia A was published in Nature in 2003. The team was led by Dr Loretta Dunne of Nottingham University and Professor Rob Ivison of the ATC, Edinburgh as well as Cardiff astronomers, Professors Stephen Eales and Mike Edmunds.

Picture caption: The remains of the Cassiopeia A supernova shown in an infrared composite from NASA's Spitzer Space Telescope. Silicon gas is blue and argon gas is green, while red represents about 10,000 Earth masses worth of dust. Yellow shows areas where red and green overlap - indicating that this supernova is synthesizing dust and gas together. This is the smoking gun indicating that supernovae were significant suppliers of fresh dust in the very early universe.