Evolution Of Highmass Stars

stars that have much higher masses than the Earth's sun evolve faster than the typical scenario for a one-solar-mass stellar evolution outlined in this entry. The evolutionary speed is particularly dramatic for the nuclear fusion stage, since these stars burn their fuel supply at a much higher rate than solar-size stars. For instance, whereas a sun-type star will typically spend about 10 billion years in the hydrogen-burning stage on the main sequence, a five-solar-mass star will spend only a few hundred million years in this stage.

After the hydrogen-burning stage is complete, high-mass stars also leave the main sequence and burn helium, albeit at a higher rate than their lower-mass cousins. However, since high-mass stars enter the helium-burning stage with lower internal densities than their lower-mass equivalents, electron degeneracy has not been attained in the core, and the star can burn helium smoothly and without the helium flashes that characterized the low- or normal-mass stars. Another difference is that since the core of a high-mass star can continue to contract after burning helium, it can attain temperatures high enough to start burning carbon (600 million K), and the high-mass star does not therefore enter a white dwarf stage. Instead, it continues to burn heavier and heavier elements, fusing hydrogen to helium, helium to carbon, carbon to oxygen, oxygen to neon, neon to magnesium, magnesium to silicon, and silicon to iron. As the star burns different fuels, its radius expands and contracts and its surface temperature goes up and down multiple times, instead of the simpler history of low-mass stars. The fusion reactions proceed at faster and faster rates as the star burns heavier elements, as if some catastrophic event is approaching. The fate of high-mass stars is an explosive ending, as the accelerating reactions and burning lead to one of the universe's most spectacular events, that of a supernova, or stellar explosion.

See also astronomy; astrophysics; binary star systems; constellation; cosmology; dwarfs (stars); galaxies; interstellar medium; planetary nebula; star formation; supernova.


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Steno, Nicolaus (1638-1686) Danish Anatomist, Bishop, Geologist, Paleontologist Nicolaus steno is probably the first scientist to clearly show that fossils are organic remains of formerly living organisms. He studied anatomy at Copenhagen and Leiden, and Florence. While dissecting a shark, he noted the similarity between the teeth of the modern shark and fossil shark teeth in local strata. After this revelation, steno traveled around Tuscany collecting as many fossils as he could, becoming obsessed with understanding the origin of fossils. He produced a major work on the origin of fossils, the Prodromus, in 1669, that led him to ponder differences between his observations and the history of the world as described in his version of the Bible. steno proposed the law of stratal superposition, clearly outlining that younger rocks are deposited over older rocks, and he recognized a sequence of changing fossil forms in the stratigraphic record. He also described metalliferous mineral deposits, recognizing crosscutting veins, and he described volcanic mountain building, erosion, and faulting. Nicolaus steno is considered the father of geology for formulating three major principles in a single geological work: the law of superposition, the principle of original horizontality, and the principle of lateral continuity.

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