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Final Study Guide

Final Study Guide - How do stellar diameters and...

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How do stellar diameters and interstellar distances compare with galactic diameters and intergalactic distances? What are the major lessons and tools of astronomy? • Neutron Stars, Pulsars, Black Holes, Ch. 22 neutron stars are one of the 3 possible end states of the life of a star neutron stars are being observed as pulsars by radio astronomy there are pulsars and neutron stars in nebulae which are the ejecta of relatively recent supernova explosions which together is beautiful evidence for the overall picture of stellar evolution A core-collapse supernova may leave behind a remnant, an ultracompressed ball of material called a neutron star . The processes that form neutron stars ensure that these stars are rapidly rotating and strongly magnetized at birth. Pulsars are objects that appear to emit regular bursts of electromagnetic energy. The accepted explanation for pulsars is the lighthouse model , in which a rotating neutron star sends a beam of energy into space. If the beam sweeps past Earth, we see a pulsar. The pulse period is the rotation period of the neutron star. A neutron star in a close binary system can draw matter from its companion, forming an accretion disk. The material in the disk heats up even before it reaches the neutron star, and the disk is usually a strong source of X-rays. As gas builds up on the star's surface it eventually becomes hot enough to fuse hydrogen. As with a nova explosion on a white dwarf, when hydrogen burning starts on a neutron star, it does so explosively. An X-ray burster results. Even more energetic are gamma-ray bursts , which may result from the violent merger of neutron stars in distant binary systems. The rapid rotation of the inner part of the accretion disk causes the neutron star to spin faster as new gas arrives on its surface. The eventual result is a very rapidly rotating neutron star—a millisecond pulsar . Many millisecond pulsars are found in the hearts of old globular clusters. They cannot have formed recently, so they must have been spun up by interactions with other stars. Careful analysis of the radiation received has shown that some pulsars are orbited by planet-sized objects. The origin of these "pulsar planets" is still uncertain. The upper limit on the mass of a neutron star is about 3 solar masses. Beyond that mass the star can no longer support itself against its own gravity, and it must collapse. No known force can prevent the material from collapsing all the way to a pointlike singularity , a region of extremely high density where the known laws of physics break down. Surrounding the singularity, at a distance of a few kilometers for a solar-mass
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