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Astronomy . Astrophysics

Convection thoroughly mixes the gas

Astronomers refer to this planetary cloud as the “Cat’s Eye” nebula. It bears witness to the slow death of a star with the mass of our own Sun. Zoom Image
Astronomers refer to this planetary cloud as the “Cat’s Eye” nebula. It bears witness to the slow death of a star with the mass of our own Sun. [less]

And an astonishing process takes place within the star: “The complicated interplay of forces creates the conditions for the nucleosynthesis of heavy elements,” explains Achim Weiss, “and violent convection flows are generated within the star.” These flows use particles to transport energy, and thoroughly mix the gas. The heat given off by a radiator is transported in the same way: hot air rises, while cool air falls. You need only hold your hand above a hot radiator to experience this phenomenon for yourself.

The resulting “eddies” in the star cause a certain amount of hydrogen from the outer layer to reach the helium that is burning in the shell beneath it. There, the protons are able to react with the carbon, resulting in neutrons being released. The neutrons are captured by the iron particles that were present in the star in small quantities from the beginning, resulting in the formation of neutron- rich iron isotopes.

If too many neutrons accumulate, radioactive beta decay occurs, which in turn creates stable cobalt nuclei. The neutrons are thus captured progressively by the atomic nuclei, which then become progressively heavier. This “s-process” (s for slow) produces all elements up to and including lead. According to Achim Weiss, “one day, the Sun will produce barium and other rare earths such as lanthanum.”

At any rate, the star’s death is now imminent. In the final phase, it loses several tenths of its mass within the space of a few tens of thousands of years, at the end of which 99 percent of its mass is accounted for by its carbon/oxygen core and only half a percent each by the thin hydrogen envelope and the helium shell. The carbon/oxygen core is effectively blasted clear in much the same way that the desert wind blasts a stone free of sand. The material that is carried off forms an expanding envelope surrounding the star; it is lit by the star, and it assumes the most diverse shapes, such as rings, spheres or asymmetrical structures. In the “hard core,” the fusion processes ultimately grind to a complete halt.

The star’s meager remains have a temperature of a few tens of thousands of degrees, and are now only as large as the Earth. The star now appears in the Hertzsprung-Russell diagram as a white dwarf: at first still hot and bright, but in the absence of nuclear fusion, cooling down and becoming dark, first quickly, then more and more slowly – just like the brown dwarfs. When the computer has churned out the state parameters for such a white dwarf – endless columns of figures for values such as the density, radius, mass and temperature – Weiss’ work is normally over; a white dwarf is the final stage of a star of low or medium mass.


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