Scholars Online Astronomy - Chapter 20: The Death of Stars
Reading: Astronomy, Chapter 20: Stellar Evolution: the Deaths of Stars
- Section 1: Supplies of hydrogen and helium in the core and upper layers of the star make possible or limit the rate of thermonuclear activity. As the fuel burns out, thermonuclear fusion rates drop, temperatures go down and the material contracts, which causes it to heat up again. If the temperature is high enough, thermonuclear reactions can restart, but even with lower temperatures, the outer layers of the star will expand. Stars may go through multiple cycles of expansion to red giant phase and contraction with increasing temperature but constant luminosity.
- Section 2: As stars contract and expand, convection circulation increases. Heavier elements formed by fusion in the nucleus rise to the surface and are blown off in the stellar wind. These elements can included carbon, oxygen, and nitrogen.
- Section 3: Stars with a mass of 0.4 M☉ to 4.0 M☉ will burn through their helium core, but when the core core collapses, the outer layers blow off, forming planetary nebula.
- Section 4: The burnt-out core of a moderate mass star which has ejected all of its outer layers collapses until the only thing preventing it from further collapse is degenerate electron pressure. The pressure generates enough heat to keep the star radiating at low luminosity even without thermonuclear activity: it becomes a white dwarf. Note that the mass of the white dwarf is inverse to its size: the more mass a degenerate star has, the smaller it becomes as gravitational collapse occurs.
- Section 5: Stars with masses over 4.0M☉exert enough pressure on their cores to trigger carbon-oxygen fusion. The cycle of core burnout and cessation of thermonuclear fusion, collapse with increasing pressure and temperature that reignites fusion of yet heaver metals continues through carbon, oxygen, silicon, and iron.
- Section 6: The iron core is so dense that even degenerate electron pressure can't support it; it collapses to the limit of nuclear degeneracy and bounces. The bounce creates a shock wave spreading outward and colliding with upper level matter falling toward the core, pushing it back out in an enormous explosion.
- Section 7: SN 1987A in the Magellanic Cloud provided astronomers with a rare opportunity to closely observe a supernova explosion. However, SN 1987A was not a typical red supergiant when it exploded. Instead it was a much smaller blue supergiant, with gas tightly held in its gravitational field. The explosion created an hourglass shape of expanding hydrogen and helium gas, rather than a spherical shell.
- Section 8: The model for supernova collapse predicted a brief, intense burst of neutrino emissions. The burst was actually observed and confirmed the model of Type II supernovae.
- Section 9: Type I supernovae occur when a white dwarf in proximity with a red giant absorbs its mass, heats up, and begins fusing carbon in the electron degenerate nucleus. The resulting runaway thermonuclear ignition blows carbon-oxygen core of the white dwarf apart.
- Section 10: The remnants of blown-off layers from a supernova continue to expand outward, creating supernova remnants similar to planetary nebula, but with significant differences in composition and structure.
- Section 11: The discovery of the neutron in 1932 led physicists to the realization that under certain circumstances, a proton and electron will merge. Walter Baade and Fritz Zwicky predicted that under the conditions of pressure and temperature in the core of massive stars, electron degeneracy would fail. Electrons and protons merge until only densely-packed neutrons are left. Such a core would not support thermonuclear fusion, but would still be very hot.
- Section 12: High gravity white dwarf and neutron stars attract surrounding gases, which collapse onto the stellar surface, heat up from the internal pressure of the star, and eventually ignite in a brief hydrogen thermonuclear reaction, creating nova and bursters that can repeat the cycle as long as the local gas clouds remain dense enough.
Key Formulae to Know
|No significant formulae in this chapter.|
Read the following weblecture before chat: The End of the Star
Planetarium Program: Use your planetarium program to investigate the following objects:
- Required: Complete the Mastery exercise with a passing score of 85% or better.
- Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!
Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.
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