How cosmic dust is created has been a mystery for some time. Although the textbooks tell us that the dusty stuff that builds the planets - and, ultimately, the complex chemistry that forms life (we are, after all, made of ‘star stuff') - comes from supernova explosions, astronomers have been puzzled as to how delicate grains of dust condense from stellar material and how they can possibly survive the violent shock waves of the cataclysmic booms.
But now, with the help of a powerful ground-based telescope, astronomers have not only watched one of these supernova ‘dust factories' in action, they've also discovered how the grains can withstand the violent supernova shock.
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"The problem has been that even though dust grains composed of heavy elements would form in supernovae, the supernova explosion is so violent that the grains of dust may not survive," said co-investigator Jens Hjorth, of the Niels Bohr Institute at the University of Copenhagen. "But cosmic grains of significant size do exist, so the mystery has been how they are formed and have survived the subsequent shockwaves."
Like many astronomical studies, the road to discovery started with a huge stroke of luck. In 2010, a very bright supernova erupted in the nearby galaxy UGC 5189A. The supernova, called SN2010jl, signified the death of a massive star around 40 times the mass of our sun.
Researchers used a special spectrograph called the "X-shooter" attached to the Very Large Telescope (VLT) at the Paranal Observatory in northern Chile to zoom-in on the supernova and, over the following two and a half years, make 10 observations of the supernova aftermath. This work builds on previous observations of the famous supernova 1987A (SN 1987A) remnant made by the by the Atacama Large Millimeter/submillimeter Array (ALMA). These ALMA observations revealed a supernova remnant brimming with fresh dust - but little information about how the dust was formed could be deciphered.
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By observing SN2010jl in visible and near-infrared wavelengths simultaneously with the VLT, the researchers were able to detect the absorption of light by newly-formed dust grains immediately after the explosion. These direct measurements of which wavelengths were absorbed by the dust revealed, for the first time, a timeline of how the dust is created by a supernova, the physical size of the dust particles and what material they are made of.
"Previously astronomers have seen plenty of dust in supernova remnants left over after the explosions," said lead author Christa Gall, of Aarhus University, Denmark. "But they also only found evidence for small amounts of dust actually being created in the supernova explosions. These remarkable new observations explain how this apparent contradiction can be resolved."
The VLT observations, which have been published in the journal Nature, show that dust formation starts soon after the supernova erupts and continues for a long period after the initial explosion. The researchers discovered that shortly before the main supernova explosion, the star blasts gases (rich in hydrogen, helium and carbon) into space, forming a dense shell around the star.
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"When the star explodes, the shockwave hits the dense gas cloud like a brick wall," said Gall. "It is all in gas form and incredibly hot, but when the eruption hits the ‘wall' the gas gets compressed and cools down to about 2,000 degrees.
"At this temperature and density elements can nucleate and form solid particles. We measured dust grains as large as around one micron (a thousandth of a millimeter), which is large for cosmic dust grains. They are so large that they can survive their onward journey out into the galaxy."
The surprising size of newly-formed dust grains, argue the researchers, is the reason they can withstand the violent shockwave and avoid being blown apart.
Until now we've known that dust is created by supernovae, but the processes behind dust formation were a mystery. Through this unprecedented campaign, observing a supernova from the initial blast and subsequent evolution of its dusty remnant, scientists are beginning to realize how these dust factories work, ultimately seeding the Universe with rocky planets like Earth and the organic chemistry that resides on them.