Astro 201 Exercise 11

Stellar Life and Death It is of interest to compare the evolution and fate of a 1-solar-mass star (like our sun) with the evolution and fate of a 25-solar-mass star. For both, answer the following questions. (Ch. 21 and 22 material.) 1. How long do the stars stay on the main sequence? 1-solar-mass: 10 billion years. 25-solar-mass: 3 milliom years (a factor of 3000 shorter!) 2. What parts of the H-R diagram do the stars visit after they exhaust their hydrogen fuel? 1-solar-mass star: The aging sun will grow much more luminous and cooler, becoming a red giant. 25-solar-mass star: On the main sequence a 25-solar-mass star is very luminous, and it doesn't get THAT much brighter as it ages. It moves from the main sequence to the red supergiant corner of the H-R diagram. Depending on its exact mass, it may undergo "blue loops" that move it hotter in the H-R diagram, but it always stays a supergiant of one kind or another. 3. What nuclear fuels do the stars fuse at various stages in their lives? 1-solar-mass: H will fuse to He, then He will fuse to Carbon and Oxygen while the star is a red giant. 25-solar-mass: H, He, C, O, Ne, Si. Even more elements are produced during the supernova explosion. 4. Wimpy or violent, describe the events that put an end to the stars' lives. 1-solar-mass: as the He fusion stage comes to an end, the star will pulsate and shed its outer layers. The core of the star is a nascent white dwarf, and when this hot star is uncovered, it promptly ionizes all the gas in the neighborhood, becoming a planetary nebula. 25-solar-mass: During the last hours of this star's life, it burns all nuclear fuels possible as the core contracts and heats. Finally, with no further energy source available, the core collapses. A subsequent "bounce" throws a shock wave outwards through the star, creating a supernova explosion. 5. Describe the remnants left over after the stars are finished evolving. 1-solar-mass: a white dwarf has more than half of a solar mass of material in it, but is only the size of the earth. White dwarfs start out very hot but gradually cool and become dimmer. 25-solar-mass: This star will become a neutron star or black hole. A neutron star is only 10 or 20 kilometers across. All protons combined with electrons to become neutrons in the core of the star during the supernova explosion, and a neutron star really is made of neutrons. A tablespoon of neutron star weighs a million tons. A black hole is only slightly smaller than a neutron star. If one exceeds the limit of "neutron degeneracy pressure" no known force can stop a complete collapse of matter. General relativistic effects like time dilation, gravitational redshift, and warping of spacetime come into play near the even horizon.
Last modified: Mon Nov 23 22:18:30 CST 1998