OUT THERE 



LETTERS 



(Continued from page 70) 

 the same place, all yon would have to 

 do is measure their respective elemen- 

 tal abundances. If they're roughly the 

 same, it's a good bet the stars are sib- 

 lings from the same cluster or galaxy 



Two big hurdles, however, have to 

 be overcome to carry out that simple 

 test. First, almost all the matter in the 

 universe is made up of just two ele- 

 ments, hydrogen (about 75 percent by 

 weight) and helium (about 25 per- 

 cent). All the other elements con- 

 tribute negligibly to the overall ele- 

 mental distribution. 



Worse, the relative abundances of 

 the elements change with time, be- 

 cause of what goes on inside the stars 

 themselves. Nucleosynthesis — includ- 

 ing the energy-producing fusion reac- 

 tions that make stars shine — turns 

 lighter elements into heavier ones. Fur- 

 thermore, most of the heavy elements 

 undergo radioactive decay, gradually 

 reducing the abundance ot some ele- 

 ments while increasing that of others. 



With all that in mind, Brewer and 

 Carney selected twenty-three long- 

 lived, stable stars from the thin and 

 thick disks for study. With the four- 

 meter Mayall telescope at Kitt Peak 

 National Observatory in Arizona and 

 the 2.7-meter Harlan J. Smith tele- 

 scope at McDonald Observatory in 

 Texas, they spread the visible portion 

 of the light from each of the target 

 stars into highly resolved spectra. The 

 spectra, in turn, yielded the signatures 

 of twenty-four elements for analysis, 

 including a number of trace elements. 



Brewer and Carney's painstaking 

 work shows that thin-disk and 

 thick-disk stars have many elemental 

 abundance patterns in common. At the 

 same time, though, the two kinds of 

 stars differ in several critical ways. The 

 differences imply that the oldest thick- 

 disk and thin-disk stars formed inde- 

 pendently, but that younger stars from 

 both disks may have formed in roughly 

 the same interstellar conditions. 



If Brewer and Carney are correct, a 

 fascinating, unifying picture of galac- 

 tic-disk evolution emerges — a scenario 

 that, as dwarf galaxies continue to 

 plunk into the Milky Way, may repeat 

 itself in the eons to come. Billions of 

 years ago, a small galaxy began failing 

 into the Milky Way. As the union pro- 

 ceeded, the gas ot the small galaxy 

 mixed with the gas already in the 

 Milky Way's thin disk, and new gener- 

 ations of stars formed out of that fresh 

 mixture. It may take a few billion 

 more years for the thick-disk, immi- 

 grant stars to settle completely into the 

 staid, swirling thin-disk environment. 

 In the meantime, though, their prog- 

 eny stars have already begun the proc- 

 ess of assimilation. They have taken on 

 some of the chemical characteristics of 

 their new surroundings, yet they still 

 carry the elemental roots and dynamic 

 motions of their extragalactic origins — 

 straddling the boundary between disks 

 thick and thin. 



Charlbs Liu is a professor of astrophysics at the 

 City I University of New York and an associate 

 with the American Museum if Natural History. 



74 NATURAl HISTORY March 2006 



(Continued from page 12) 

 views is probably anathema to those 

 who regard the Bible as offering a lit- 

 eral account of the origins of life. 

 Yet, turned against their Darwinian 

 opponents, "teach the controversy" 

 becomes verbal ju-jitsu. The diaboli- 

 cal cleverness of the creationist trap is 

 the almost irresistible appeal it makes 

 to the liberal mind. 



But it is quite possible to resist the 

 slogan and avoid the trap. The way 

 out is to note that scientific activity 

 has its own rules of engagement. Yes, 

 of course, science, like any other 

 human activity, can be afflicted with 

 authoritarianism, ego, political strife, 

 vested interests, and the like. But 

 doing science presupposes at least a 

 community whose participants can 

 agree, roughly, about the subject mat- 

 ter of their inquiry, what constitutes 

 relevant evidence, what counts as a 

 valid argument or a convincing refu- 

 tation. Those shared commitments 

 require time, maturity, and a certain 

 sophistication to develop — hence my 

 insistence on some form of a scien- 

 tific "union card." 



As for what goes on in the science 

 classroom, Peter Starr's proposals seem 

 sensible to me. But it also seems to me 

 both dangerous and discriminatory 

 not to give all students enough expo- 

 sure to science, say, to enable them to 

 respond as thoughtful citizens to the 

 ethical and social issues posed by sci- 

 entific and technical advances. If you 

 grant this, then there is little enough 

 time, at the pre-university level, to 

 teach a smattering of well-established 

 science, without wasting effort on 

 long-discredited views. Shall we teach 

 Aristotle in physics class, Ptolemy's 

 universe in astronomy, phlogiston the- 

 ory in chemistry? So it is with cre- 

 ationism in biology. 



Natural History welcomes correspon- 

 dence from readers. Letters should be sent 

 via e-mail to nhmag@naturalhistorymag. 

 com or by fax to 646-356-65 1 1. All 

 letters should include a daytime telephone 

 number, and all letters may be edited for 

 length and clarity. 



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