July 10, 2014
Supernova Explosions Release Building Blocks Of Planet Formation
John P. Millis, Ph.D. for redOrbit.com - Your Universe Online
The formation of planets of stars is still somewhat of a mystery to astronomers. While much progress has been made, particularly in the last few decades, there are still unanswered questions as to how the planetary building blocks form. But now, new research may be closing that knowledge gap.
Early in the life of the Universe, the cosmos was dominated by the basic elements of hydrogen and helium, with only trace amounts of lithium and little else. Then, as massive stars evolved, their massive nuclear engines fused heavier elements. Finally, supernova events led to even more massive nuclei forming through nucleosynthesis from the rapid outflow of compressed plasma during the event. Smaller stars, like our Sun, also fuse hydrogen into heavier elements, but typically stops with Carbon and Oxygen.
Ultimately, the result is that over time the Universe is turning hydrogen and helium into heavier elements through the creation of stars, particularly those several times the mass of our Sun. However, the elements produced escape in gaseous form. How then, do these elements eventually form the building blocks of planets?
While the hydrogen and helium gas that still outweighs all else in the Universe forms the basis for stars like our Sun, the planets are composed of cosmic grains – solid forms of heavier elements such as carbon. But the question remains, where do the solid forms of these elements originate? At what point do the elements enter a system where they are compressed enough to enter a solid state, yet not be completely obliterated in the process?
Physicists have suspected that supernovae may provide the answer -- after all they are responsible for creating many of the nuclei in the first place.
"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. But cosmic grains of significant size do exist, so the mystery has been how they are formed and have survived the subsequent shockwaves. Our research casts new light on this – both on how dust is formed and how it survives the shockwaves," explains Professor Jens Hjorth, head of the Dark Cosmology Centre at the Niels Bohr Institute at the University of Copenhagen.
Using an instrument known as the X-shooter on the European Southern Observatory’s Very Large Telescope in Chile, the team was able to simultaneously image Ultraviolet, Visible, and Infrared light with very high sensitivity. They simply had to wait for a very bright supernova in order to map the light, from which they could infer how the dust grains would form and travel outward.
Luckily, a very bright supernova – some 10 times brighter than the average event – was captured when a star 40 times the mass of our Sun exploded. After more than two and a half years of analysis, the team believes they have found an exciting result.
"Dust absorbs light and from our data we could calculate a curve that told us the about the amount of dust, the composition of the dust and the size of the dust grains. This showed something very exciting," explains Christa Gall, a postdoc at Aarhus University and affiliated with the Dark Cosmology Centre at the Niels Bohr Institute at the University of Copenhagen.
After a series of smaller outbursts, a dense cloud of hydrogen, helium, and carbon form an outer shell around the dying star. Then, finally, the star reaches its critical limit and explodes in a brilliant supernova.
"When the star explodes, the shockwave hits the dense gas cloud like a brick wall. 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," explains Gall.
A paper on the results of this study is published in the journal Nature.
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