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Download PHYS 175 Fall 2014 Final Recitation Ch. 16 The Sun
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PHYS 175 Fall 2014 Final Recitation Ch. 16 The Sun - describe the fusion process taking place in the present day Sun In a low-mass star, such as the Sun, the proton-proton chain fuses H into He. This process releases more energy than the kinetic energy required to bring the protons close enough for fusion to take place. This energy is carried off in the form of neutrinos and gamma rays (high energy photons). - describe a photon produced by fusion in the Sun’s core and how the energy from that photon eventually escapes the Sun’s outer atmosphere Photons released in the core (where fusion takes place) collide almost instantaneously with other core constituents. This energy gradually flows outward, until the density of the sun decreases sufficiently to allow for radiative diffusion of the energy. Again, the photons still undergo many collisions, but fusion has ceased. The collisions continue near the outer layers of the Sun, where convection (like in a boiling fluid) dominates the energy transfer and matter flow. Once out of the convection zone, photons in the Sun’s atmosphere are free to escape into space. The process of this energy transfer can take as much as 170,000 years, from core to atmosphere. Ch. 17 The Stars - review the inverse square law for stellar intensities Here I’m just looking for easy math that demonstrates the inverse square falloff with distance. Some examples, when you move a distance of 4 farther away, the intensity falls off by a factor of 16, etc. - plot the evolutionary track of the Sun on a blank H-R diagram Ch. 18 Star Birth - discuss the differences between dark nebulae, emission nebulae and reflection nebulae Emission nebulae: clouds of excited gas, caused by UV stimulation from nearby O & B stars in the nebula’s H atoms. Emissions are in the visible, red range. Reflection nebulae: dust and gas reflect the blue light from nearby O & B stars. Dark nebulae: star forming region, can contain a protostar. No visible emissions, but strong IR emissions from them. - label the following on an HR diagram: red giants, blue giants, supergiants, main sequence, white dwarfs, red dwarfs Ch. 19 Stellar Evolution - what determines when a star is on the main sequence? What happens to cause it to move off? A star is on the main sequence when it is converting H into He. This happens by either the p-p chain (low mass stars) or the CNO cycle (high mass stars). - what are Cephied variable stars and why are they so useful? These stars have a known relationship between period and luminosity. They are one of the standard candles used on the cosmic distance ladder. Since they are found at distances that overlap other techniques on the ladder, we know that the estimates of distance are valid (we have calibrated the technique). Ch. 20 Star Death - describe the characteristics of white dwarfs and neutron stars A white dwarf is the carbon core lefty over from a low mass star at the end of its H- and Heburning life. These objects are supported by electron degeneracy pressure, are very hot and not very luminous due to their small size. A neutron star is smaller than a white dwarf and is made up entirely of neutrons, supported by the neutron degeneracy pressure. Neutron stars often have radiation jets emanating along their magnetic axes – if this axis is mis-aligned with the spin axis, it can appear as a pulsar, when observed from a distance. - what is the “dividing line” between the two? The Chandrasekhar limit of 1.4M¤ governs whether or not a white dwarf core remnant will collapse into a neutron star (above this limit, the electron degeneracy pressure is overcome by the gravitational collapse). Above about 2-3M¤ the neutron degeneracy pressure is overcome and a black hole is formed. - how are the heavy elements formed? Once Fe beings to form via fusion, the reaction consumes more energy than it produces, so fusion ceases in the core very quickly. Massive stars can get to this stage. Elements heavier than Fe are formed when the shockwaves from a supernova event cause fusion in the stellar mass that has been ejected from the star’s outer layers. Ch. 21 Black Holes - describe gravitational redshift This type of redshift is caused by the distortion of spacetime near a massive object. In order to be observable, the density of the object must approach that required for a black hole. The redshift caused by gravity becomes infinite at an event horizon, so we can never observe matter falling in to one. - what are the main results of special relativity? The speed of light is the same for all observers, regardless of their relative motion. Space and time are both aspects of a single spacetime, and the perception of that spacetime is dependent upon the observer’s frame of reference. While special relativity only takes into account relative motion (not gravity), it correctly predicts many observed distortions of mass, length and time. Ch. 22 The Milky Way - why is radio astronomy so important for observations of our own galaxy? Gas and dust between the stars limits our ability to observe stars in the galactic plane from our vantage point within the galaxy. Using radio telescopes, we can peer into the very galactic core. - how does dark matter effect the observable matter in the galaxy? Without invoking a dark matter halo, astronomers cannot explain the rotational speeds of the stars with our galaxy (and others). As you move out from the galactic core, speeds should fall off as Kepler predicted; however, this falloff is not observed. The dark matter halo explains the observed speeds, but we still don’t exactly know what makes up this dark matter. Ch. 23 Galaxies - describe the basic characteristics of elliptical, spiral and irregular galaxies Elliptical: among the most massive galaxies, few younger stars (Pop. II and old Pop. I), about 20% of observed galaxies, stars formed quickly Spiral (barred and grand design): most common (~77%) type observed and span less mass scales, star formation active in spiral arms, supported by density waves, have dark matter halos, central bulges host elliptical-like star populations and supermassive black holes Irregular: least commonly observed (~3%), host star formation, possibly the result of galactic collisions - how does the Hubble Law work? when does its use become more uncertain? Discuss the image above, and how the certainly falls off a bit (but not badly) as the distance increases (this why ages are discussed in redshifts (known precisely), rather than the uncertain actual age, which depends on the value of Ho). Ch. 24 Quasars and Active Galactic Nuclei - what is an AGN? what are its characteristics? Quasars are one class of AGN. They are essentially a combination of supermassive black hole at the center, accretion disk and lobes of matter blown off in a dipolar fashion along the oppositely directed magnetic field lines. They may or may not have active radio emissions and may reside in spiral or elliptical host galaxies. Ch. 25 Cosmology - what is the cosmic microwave background? why is it important? It is the expected blackbody radiation remnant from the Big Bang event that occurred ~13.7 billion years ago. It is at a peak temperature of 2.725 K, and is present in all directions when looking with a microwave-tuned radio dish. There are fluctuations in the radiation, but these are very small compared to the background. This remnant radiation gives us string clues as to the nature of the very early universe and its subsequent evolution. - discuss the following plot and what it means for the expansion of the universe This plot was the winning evidence for the 2011 Nobel Prize in Physics. It shows that the expansion of the universe is actually speeding up when compared to earlier epochs. The data are best fit with a line that clearly (despite some uncertainty) puts the emphasis on an increasing rate of expansion.