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Near the end of a star's asymptotic giant branch phase, the stellar wind increases substantially as nuclear energy is derived from the hydrogen shell and to a lesser extent from the helium shell. The degenerate core produces no nuclear energy and is composed almost entirely of carbon and oxygen. For stars less massive than 8 Mo, the wind blows away the envelope right down to the core in a very short period of time. The complete loss of the envelope ends the asymptotic giant branch phase of stars less massive than 8 Mo.
At the end, all that remains of a lower-mass star is a degenerate core of carbon and oxygen surrounded by two concentric thin shells of hydrogen-burning and helium-burning. The gas and dust expelled during the star's asymptotic giant branch phase are still moving outward at tens of kilometers per second. As the debris moves away from the hot, dense core, the star becomes visible. The star now begins to move quite rapidly towards the left in the H-R diagram. Once the shells burn out, the star moves downwards in the H-R diagram to the white dwarf region. The star (or what is left of it) is now a carbon-oxygen white dwarf.
As the faster moving portions of the envelope, expelled near the end, catch up to the slower moving portions, expelled earlier when the wind was not so rapid, a shock wave (a wave of compressed gas) is produced. The intense radiation emitted from the white dwarf causes the dense gas in the compression to glow, producing a planetary nebula which appears as a thick luminous ring surrounding the white dwarf remnant. Planetary nebula are named for their round appearance and have nothing to do with planets. The planetary nebula continues to expand, eventually cooling and growing darker as the material spreads into interstellar space.
Refer to your reading assignment for more on the formation of white dwarfs.
White dwarfs have been observed with surface temperatures from 4000 Kelvins (7200 oF) to 85,000 Kelvins (153,000 oF), but computer models predict that it is possible for them to have even higher temperatures. The masses of white dwarfs range from perhaps 0.6 Mo up to 1.4 Mo, the Chandrasekhar limit. Main sequence stars with masses between 2 and 8 Mo produce white dwarfs between 0.7 and 1.4 Mo. Main sequence stars with masses between 1 and 2 Mo produce white dwarfs between 0.6 and 0.7 Mo. White dwarfs with masses less than 0.6 Mo would evolve from main sequence stars with masses less than one solar mass. The main sequence lifetime of these stars are so long that the universe is not yet old enough for them to have become white dwarfs. Thus, there are no known white dwarfs with masses less than 0.6 Mo.
Theory suggests that white dwarfs with masses less than 0.6 Mo would have degenerate helium cores, not cores of carbon and oxygen that we observe in the more massive white dwarfs. The reason is that the cores of very low-mass main-sequence stars should never become hot enough to fuse helium when they leave the main sequence.
Despite the ordinary-sounding compositions of white dwarfs, their degenerate matter is unlike anything found on Earth. Gravity and electron degeneracy (repulsion) pressure have battled to a draw in white dwarfs. Hydrostatic equilibrium is established in the collapse of a one solar mass corpse when the core shinks to about the size of the Earth. The density (compaction) of such a white dwarf is so high that a pair of standard dice made from white dwarf material would weigh about five tons.
More massive white dwarfs are smaller and denser than less massive ones. For example, a 1.3 Mo white dwarf is half the diameter and about 10 times denser than a one solar mass white dwarf.
Refer to your reading assignment for more on the properties of white dwarfs and the Chandrasekhar limit.
|For this topic, study the true and false, fill in the blanks self-test, and review questions at the end of the Chapter(s) of your reading assignment. In addition, learn the key words and answer all questions that follow:|
Key Terms (refer to your text for some these terms)
Review Questions (refer
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Advanced Questions (refer to your text to answer some of these questions)
1. Sketch the H-R diagram for low-mass stars as they leave the asymptotic
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