The chemical composition of a star provides important clues to its evolutionary history, including the age at which it formed. The interstellar medium of dust and gas from which stars form is primarily composed of hydrogen and helium with trace amounts of heavier elements. As nearby stars continually evolve and die, they seed the interstellar medium with an increasing portion of heavier elements. Thus younger stars will tend to have a higher portion of heavy elements in their atmospheres than do the older stars. These heavy elements are termed metals by astronomers and the portion of heavy elements is the metallicity.[29] The amount of metallicity in a star is given in terms of the ratio of iron (Fe), an easily observed heavy element, to hydrogen. A logarithm of the relative iron abundance is compared to the Sun. In the case of Tau Ceti, the atmospheric metallicity is roughly: \left [ \frac{\mathrm{Fe}}{\mathrm{H}} \right ] = -0.50 or about a third the solar abundance. Past measurements have varied from −0.13 to −0.60.[4][30] This lower abundance of iron indicates that Tau Ceti is almost certainly older than the Sun. Its age had previously been estimated to be about 10 Ga but is now thought to be around half that at 5.8 Ga.[8] This compares with 4.57 Ga for the Sun. However, computed age estimates for Tau Ceti can range from 4.4–12 Ga, depending on the model adopted.[25] Besides rotation, another factor that can widen the absorption features in the spectrum of a star is pressure-broadening. (See spectral line.) The presence of nearby particles will affect the radiation emitted by an individual particle. So the line width is dependent on the surface pressure of the star, which in turn is determined by the temperature and surface gravity. This technique was used to determine the surface gravity of Tau Ceti. The log g, or logarithm of the stars surface gravity, is about 4.4—very close to the log g = 4.44 for the Sun.[4]
Posted on: Mon, 04 Nov 2013 21:57:49 +0000
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