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Astronomy - Chapter 8 txtbook.docx

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Astronomy 1021
Stacey Hallman

Astronomy-Chapter 8 The Death of Stars Nova: from the Latin, meaning “new”, a sudden and temporary brightening of a star making it appear as a new star in the sky, evidently caused by an explosion of nuclear fuel on the surface of a white dwarf Supernova: a “new star” in the sky that is roughly 4000 times more luminous than a normal nova and longer lasting, evidently the result of an explosion of a star Giant Star: large, cool, highly luminous star in the upper right of the H-R diagram, typically 10 to 100 times the diameter of the Sun Supergiant Star: exceptionally luminous star whose diameter is 100 to 1000 times that of the Sun Horizontal Branch: The location in the H-R diagram of giant stars fusing helium Planetary Nebula: an expanding shell of gas ejected from a medium-mass star during the latter stages of its evolution Open Cluster: a cluster of 100 to 1000 stars with an open, transparent appearance, usually relatively young and located in the disk of the galaxy Globular Cluster: a star cluster containing 100 000 to 1 million stars in a sphere about 75 ly in diameter, generally old, metal-poor, and found in the spherical component of the galaxy Turnoff Point: the point in an H-R diagram at which a cluster’s stars turn off the main sequence and move toward the red-giant region, revealing the approximate age of the cluster Degenerate Matter: extremely high-density matter in which pressure no longer depends on temperature due to quantum mechanical effects Compact Object: one of the three final states of stellar evolution, which generates no nuclear energy and is much smaller and denser than a normal star Chandrasekhar-Landau: the maximum mass of a white dwarf, about 1.4 solar masses. Awhite dwarf of greater mass cannot support itself and will collapse Roche Lobe: the volume of space a star controls gravitationally within a binary system Angular Momentum: a measure of the tendency of a rotating body to continue rotating. Mathematically the product of mass, velocity, and radius Accretion Disk: the rotating disk that forms in some situations as matter is drawn gravitationally toward a central body Synchrotron Radiation: radiation emitted when high-speed electrons move through a magnetic field Supernova Remnant: the expanding shell of gas and dust marking the site of a supernova explosion Type I Supernova: a supernova whose spectrum contains no hydrogen lines Type II Supernova: a supernova explosion caused by the collapse of a massive star Neutron Star: a small, highly dense star, with radius about 10km, composed almost entirely of tightly packed neutrons Pulsar: a source of short, precisely timed radio bursts, understood to be spinning neutron stars Lighthouse Model: the explanation of a pulsar as a spinning neutron star sweeping beams of electromagnetic radiation around the sky General Theory of Relativity: Einstein’s theory that describes gravity as due to curvature of space-time Gravitational Radiation: expanding waves in a gravitational field that transport energy through space at the speed of light, as predicted by general relativity Millisecond Pulsar: a pulsar with a pulse period of only a few milliseconds Singularity: an object of zero radius and infinite density Black Hole: a mass that has collapsed to such a small volume that its gravity prevents the escape of all radiation. Also, the volume of space from which radiation may not escape. Event Horizon: the boundary of the region of a black hole from which no radiation may escape. No event that occurs within the event horizon is visible to a distant observer Schwarzschild Radius (Rs): the radius of the event horizon around a black hole Time Dilation: the slowing of moving clocks or clocks in strong gravitational fields Gravitational Redshift: the lengthening of the wavelength of a photon as it escapes from a gravitational field Gamma-ray Burst: a sudden, powerful burst of gamma rays Hypernova: produced when a very massive star collapses into a black hole. Thought to be a possible source of gamma-ray bursts • Massive stars use up their nuclear fuel at a furious rate • Low-mass stars use their fuel sparingly GIANT STARS • Amain sequence star generates its energy by nuclear fusion reactions that combine hydrogen to make helium • The giant star is the first step in the death of a star Expansion Into AGiant • The nuclear reactions in a main-sequence star’s core produce helium o Helium accumulates at the star’s center • When the temperature of the surrounding hydrogen becomes high enough, hydrogen fusion begins in a spherical layer, called a shell, surrounding the exhausted core of the star • Stars like the sun become giant stars 10 to 100 times the present diameter of the Sun and the most massive stars become supergiant stars as much as 1000 times larger than the sun • The expansion of a star to giant or supergiant size cools the star’s outer layers, and so the stars move toward the upper right in the H-R diagram • Because the core is not hot enough to fuse helium, gravity squeezes it to a relatively tiny size Helium Fusion • As a star becomes a giant, fusing hydrogen in a shell, the inert core of helium ash contracts ad grows hotter • The helium in the core changes the structure of the star. The star now makes energy in 2 locations by 2 different processes: o Helium fusion in the core o Hydrogen fusion in the surrounding shell • The point that represents the star on the H-R diagram moves downward, corresponding to lower luminosities, responding to higher surface temperatures to a region above main sequence called the horizontal branch • Helium fusion produces carbon and oxygen that accumulate in an inert core • The approximate rule is that if the core of a post-main-sequence star is “dead” the star is a red giant, and if the star is “alive”, the star is a yellow giant o Giants are rare Star Clusters • The differences among stars in one cluster must arise from differences in their masses 1. There are two kinds of star clusters 2. You can estimate the age of a star cluster by observing the distribution of the points on the H-R diagram 3. By comparing clusters of different ages, you can visualize how stars evolve THE DEATHS OF LOW-MASS STARS • Low-mass stars have little gravitational energy, they cant get very hot • Structural differences divide the lower-main-sequence stars into 2 groups: o Very-low-mass red dwarfs o Medium-mass stars (like the Sun) Red Dwarfs • They have very small masses, and very little weight to support • Their pressure-temperature thermostats are set low, and consume their hydrogen fuel very slowly • Have very long lives • Are completely convective o Stirred by circulating currents of hot gas rising from the interior a
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