martes, 14 de septiembre de 2010
Grains of 'Star Stuff' Found in Meteorite
One of the most mind-blowing revelations to come from astronomy is that, as Carl Sagan so succinctly put it, "We are star stuff."
Elements heavier than hydrogen and helium were made in the cores of stars or in supernovae, blasted into space to seed the next generation of stars and, eventually, planets. So although you can see the results of supernovae everywhere, some scientists are extracting the remnants of specific explosions in tiny particles in ancient asteroids.
A supernova can occur for two reasons. In a core-collapse supernova, a very massive star has reached the end of its lifetime, doing all the nuclear fusion that a star can do. The core of the star collapses upon itself, then explodes, ripping the outer layers of the star to shreds.
In a white dwarf supernova (also known as Type Ia) a small but dense white dwarf star pulls material onto itself from a companion star, until it gets too massive and explodes, ripping itself to shreds.
Both types of explosions are so powerful as to create new elements. In fact, a star can only create elements up to iron on the periodic table. Many radioactive elements are created in supernovae, and their decay can be measured in supernova remnants by astronomers for thousands of years.
A group of cosmochemists led by Nicolas Dauphas looked for the traces of supernovae that happened almost 5 billion years ago near the birthplace of our solar system. Such explosions would have left their chemical traces on the forming solar system, preserved in the most ancient asteroids.
Dauphas and his colleagues found the grains that carried chromium-54, found in greater abundance in some asteroids as a result of a nearby supernova, for the first time in an sample of the meteorite called Orgueil. The grains are just 100 nanometers in diameter or less. These very fine particles were "sprayed" into the proto-solar system, but today are not evenly distributed among planets and asteroids, so some other processes affected how these grains would be concentrated.
The researchers could not tell whether the chromium-54 came from a core-collapse or white dwarf supernova, though other evidence suggests that a core-collapse supernova did go off near the sun's birthplace, possibly triggering the star formation that led to the sun and solar system.
In a strangely poetic way, these supernova are our ancestors, and exploring these tiny grains is exploring our own origins.
Image: Transmission electron micrographs of the meteorite Orgueil, showing some of the larger micrograins, from Dauphas et al. 2010.
The research has been published in the Astrophysical Journal and the preprint is available on arxiv.org.