Stars 2
Although they are tiny in size, white dwarfs have played a great role in astronomy. These compact stars are very different from familiar objects like our sun. They also pose a seemingly endless series of puzzles, whose solutions provide new insights into many areas of physics and astronomy. To unravel a wide variety of phenomena such as cataclysmic variables (novae, dwarf novae), planetary nebulae, and some types of supernova, we have to understand white dwarfs. These stars may even hold clues to one of the most fundamental questions of all- how old is the universe? White dwarfs come in a variety of types ranging from hot to luminous to cool and dim. Some are among the faintest of all stars, but paradoxically their study began with that of Sirius, the brightest star in our night sky. The Dog star is a slightly scaled-up version of the Sun, with about twice the mass and 25 times the luminosity. It is an unremarkable object comfortably fusing enough hydrogen into helium in its core to supply the energy radiated from the surface and to provide the thermal pressure needed to balance the inward force of gravity. Sirius appears bright to us because it is so close, less than 9 light-years away.
What remains of the expose core of the asymptotic giant branch star is called a planetary nebula nucleus, or PNN. Typically, one mass of 0.6 Sun evolves across the H-R diagram in only 10,000 years. As the shell-burning energy sources die out, the star's luminosity drops, and it rounds the "knee" of its evolutionary track. As the planetary nebula disperses, the hot, degenerate remnant emerges from its cocoon and settles down as a cooling white dwarf. Some very interesting physics controls the evolution of hot, new-formed white dwarfs. For most of their lives these degenerate objects cool by radiating away the kinetic energy of the bare nuclei they contain. However, another mechanism is active at an early stage, when copious numbers of neutrinos are produced deep in their interiors. These bizarre elementary particles barely interact with matter and escape from the star immediately, carrying away energy as they go. Theoretical estimates indicate that neutrino cooling dominates the evolution of a white dwarf for the first few million years. Eventually, however, the energies of the ions become so small that the electric forces dominate over increasing distances. More and more nuclei are bound together in a symmetric solid lattice-a crystal. This freezing out of the material is caused by the lowering temperature, but is aided by the high pressure that squeezes the nuclei together. Thus crystallization begins at the center of the white dwarf and marches inexorably outward.
Some common words found in the essay are:
Sirius Pup, STARS Theory, LIVELY AGE, Evry Schatzman, SandT June85, , Ralph Fowler, PNN Typically, Arthur Eddington, James Liebert, white dwarfs, white dwarf, h-r diagram, hydrogen helium, stellar evolution, planetary nebula, asymptotic giant branch, epoch star, oxygen carbon, star formation, dwarf cool, hot white dwarfs, white dwarf cool, epoch star formation,
Approximate Word count = 2614
Approximate Pages = 10 (250 words per page double spaced)
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