the heavier elements. “We were very excited at finding this,” he adds. As the composition of stars is enriched
with heavier elements over time, comparison of the ratio of iron to hydrogen in a star with the present iron to hydrogen ratio of the Sun can be used to date stars. A star with a ratio 10 times lower is described by a metallicity of -1; 100 times lower gives a metallicity of -2, and so on. When observations indicated that
Fig. 2 - Supernova 1987A in the Large Magellanic Cloud (a close-by galaxy 160 000 lightyears away) after the explosion as supernova and before the explosion as star (Sanduleak −69° 202). ©Australian Astronomical Observatory/David Malin Images
burning in supernova explosions from millions to billions of years in stellar evolution. Astronomical data from the huge
observatories across the world are crucial for testing models. Astronomers determine the surface temperature of stars from the light spectrum they emit. “If
it’s very hot it’s more bluish if it’s
slightly cooler it’s more reddish,” explains Professor Thielemann. The spectra also provide information on the star’s composition as different elements absorb light at different frequencies/ wavelengths. Until about 12 years ago astrophysicists
were unable to identify situations where the conditions would be right for making the heavy elements up to uranium and thorium (and also gold and silver). “We knew what kind of conditions — temperature and what densities of neutrons — we needed to produce these heavy elements, but we had problems finding this in astrophysical sites and in our simulations of astrophysical events,” Professor Thielemann explains. He and his colleagues discovered the solution in a binary system of neutron stars, two originally massive stars that have shed a lot of their material in supernova explosions, forming neutron stars as end stages, which are bound to one another through gravitational attraction. The
58 Insight Publishers | Projects
neutron stars will lose energy through gravitational
radiation as they spin
around closer and closer to each other. Eventually they merge ejecting material in exactly the conditions needed to form
already the heavier nuclei were existing at metallicities of -3 confusion arose again. A metallicity of -3 is far too early for merging events of binary neutron stars to be possible, as not enough time has passed for the systems to evolve to the point of merging. “So we were looking for another option and we found a rare kind of these massive stars forming supernovae that make central neutron stars and explode the outer envelope with a composition enriched in heavy elements,” explains Professor Thielemann. “If they are fast rotators and if they have high magnetic fields — that’s probably only a few per cent of all these supernovae from massive stars — they form very neutron- rich jets of materials along the poles.” These very neutron-rich ejections also provide the right conditions for forming
Fig. 3 - Observed Eu/Fe ratios of old stars as a function of metallicity. In the notation [Eu/Fe], 0 indicates solar ratios, +1 10 times solar, -1 1/10 of solar, etc. The “metallicity” [Fe/H] measures the enrichment of heavy elements during galactic evolution and is a measure of time. The existence of Eu at the earliest moments requires that massive stars (with the shortest lifetimes) are responsible. The large scatter, before attaining the average (red line), means that rare events produced the r-process element Eu.
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