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by: Sophie Stella

Study_Guide_3.pdf PHYS 104-01

Sophie Stella

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About this Document

Here is the study guide for the third midterm exam for Astronomy, which covers weeks 9 through 11.
Dr. Ruch
Study Guide
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This 3 page Study Guide was uploaded by Sophie Stella on Wednesday April 27, 2016. The Study Guide belongs to PHYS 104-01 at University of St. Thomas taught by Dr. Ruch in Winter 2016. Since its upload, it has received 25 views. For similar materials see Astronomy in Astronomy at University of St. Thomas.


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Date Created: 04/27/16
PHYS 104: Astronomy Study Guide for test 3, weeks 9-11 Definitions Fusion: The process of changing an element into a heavier element to release energy. Fission: The process of changing an element into a lighter element to release energy. Nutrinos: A neutrally charged subatomic particle, used in the process of fusion. Incandescent: Emits light as a result of being hot. Photosphere: Protons that escape from the visible surface of the sun. (Radius is 696,000 km, and Temperature is 5,800 K) Corona: The hot, thin atmosphere of the Sun. (It has a low density and is about 1,000,000 degrees Convection Granules: Convection cells appear on the surface of boiling substances. There are convection cells on the sun, about 600 miles across each, which we call granules. Luminosity: The total amount of light emitted by an object in space. Apparent Brightness: A measurement of the light that reaches Earth from objects in space. Do not get Luminosity and Apparent Brightness confused. Absolute Magnitude: The intrinsic brightness of a star based on the magnitude system. Apparent Magnitude: The magnitude of a star as viewed from Earth on the magnitude scale. Parallax: Measuring the distance between stars with respect to the Earth after the Earth moves. HR Diagram: A diagram depicting the main sequence as a correlation between the luminosity and temperature of stars. Main Sequence: A correlation between stars that are fusing hydrogen into helium in their core. Protostar: Stars which have not yet come to settle on the main sequence. Degeneracy Pressure: The gravity that holds an object together at equilibrium with the pressure of electrons being squeezed together. Event Horizon: The boundary of a black hole from out perspective. Pulsar: A spinning Neutron Star Standard Candle: An object whose true luminosity is known. Enclosed Mass: The mass of an object around which other objects are attracted gravitationally. Dark Matter: Matter in the universe that can't be observed, and has not yet been detected. I. The Sun A. A mass of incandescent gas. B. Primarily made of hydrogen (which we know because of its emission lines). 1. 98% hydrogen and helium and around 1% of all of the other stuff on the periodic table. C. Evidence from pictures show convection cells on the sun's surface (which means the sun is boiling). D. The sun shines because it's hot. 1. 5800ºF 2. Everything with a temperature emits light. E. Pressure from the internal temperature is what keeps the sun inflated, while internal gravity keeps it contained. This is called Hydrostatic Equilibrium. 1. Without an internal energy source maintaining the temperature, the Sun would shrink. 2. The Sun is always losing radiation into space, which means it must have an internal energy source maintaining it. F. Structure (from inside to outside) 1. Hot, dense core a. This is where fusion reactions happen b. Only about 10 percent of the Sun’s mass will be converted to Helium. 2. Radiative Zone a. Energy carried by photons b. Does not absorb photons, but contains most of them within the sun. 3. Convective Zone a. Energy carried by convection b. Very good at absorbing light photons II. 3 Ways of Calculating Distances A. Kepler's 3 Law P^2=a^3 (P is the orbital period, a is the semi-major axis, both in A.U.) 1. One is able to calculate the orbital radius, given the period, and from there can find other distances within the solar system. B. Parallax 1. One can use the parallax effect to calculate the distance to stars within 100 light years. a. The parallax effect is when closer objects seem to move faster than further objects. b. If one calculates the exact distance a closer object with a known distance appears to move with respect to another further object, than one would be able to find the distance to the further object. C. The Spectroscopic Sequence / HR Daigram 1. The Spectroscopic Sequence is a way of labeling stars based on the emission lines they give off. a. Temperature of an object affects its spectrum. (Stars that give off less emission lines are labeled M stars, and are determined to be colder stars. Stars with more emission lines are O stars, which are found to be hotter.) 2. HR Diagram a. This is a diagram which shows the correlation between temperature and luminosity of stars. 3. To measure distances greater than 100 ly a. Get the temperature from the spectra (1) b. Get the luminosity from the temperature-luminosity correlation on the graph (2) c. From the luminosity, one is able to calculate the distance. III. Stellar Evolution A. Birth 1. Stars are born in clusters from clouds of dust and gas. 2. Protostars a. Are not fusing Helium yet (this begins when they hit the main sequence). b. Get energy from their gravitational potential. B. Main Sequence 1. Brighter main sequence stars are more massive. a. Massive stars die more quickly. 2. A star's lifetime on the main sequence depends only on the mass. 3. Represents mass temperature, and luminosity. 4. The main sequence is different than the evolutionary track. C. Further fusion 1. As a star uses up hydrogen, it gets bigger and redder (cooler, and with less reactions) 2. When it gets heavy enough, helium begins to fuse into carbon. 3. This process will continue until the star begins fusing Iron, but in order to fuse anything heavier, the star would have to be putting in energy, and would no longer be getting energy out. D. Supernovas occur when a star's core collapses and detonates. 1. These cause showers of neutrons throughout the elements created during fusion, creating other elements. E. White Dwarf 1. The leftover core of a star after detonation. F. Neutron Star 1. A white Dwarf held up by neutron degeneracy pressure (basically a giant ball of neutrons). G. Black Hole 1. We don't really know. This is what happens when a neutron star collapses. 2. It occupies a single point in space, without volume. 3. They don't suck things in, they just have gravity. 4. One can only find black hole by looking at their effects. The Chanrasekhar Limit: Electron degeneracy fails for objects with a larger mass. This means there has to be a limit to the size of a white dwarf. This limit is found to be 1.4 Solar Masses. IV. Gravity A. Einstein's theory 1. Gravity is actually warped, curved space-time. 2. The more massive an object, the greater the gravity. 3. The stronger the gravity, the more space-time is warped. B. Gravitational Lensing 1. Not only are objects captured by gravity, so is light. a. Light is bent and warped towards objects with a strong gravitational force. 2. Gravitational lensing makes objects appear to be in different positions than they actually are. V. Measuring distances within the Milky Way A. Using Photons 1. Using the speed of light, measure the amount of time it takes a photon to reach an object. B. Parallax 1. Knowing the distance to the sun, calculate the angular shift between a foreground star and a background star. 2. Use this to calculate the distance. C. Standard Candles 1. Knowing the luminosity of an object, calculate the apparent brightness. 2. Use the apparent brightness to calculate the distance. 3. The Main Sequence as a Standard Candle: a. Measure the temperature of an object. b. Infer from this the object's luminosity. c. Use the luminosity to calculate the distance. VI. The Shape of the Universe A. Gallileo assumed that the Milky Way is composed of innumerable stars. 1. The Milky Way is approximately 100,000 ly wide x 1,000 ly thick. 2. We are positioned about 28,000 ly from the center. B. William Herschel's view of the galaxy 1. He assumed that the luminosity is the same for all stars. 2. He approximated that the “universe” is a flat disk 5 times wider than thick. 3. He also thought the solar system is near the center. C. Kapteyn's veiw of the galaxy 1. He assumed that the universe is 40,000 ly wide x 14,000 thick. 2. He too assumed that the sun is near the center. 3. It turns out that Kapteyn was looking at distant stars in the galaxy through dust. Frequently missed questions from the last tests. 1. Over time spans of a few hours, stars in the northern sky, as seen from St. Paul: -Travel in concentric circles with Polaris at the center. 2. Over time spans of a few hours, stars in the eastern sky, as seen from St. Paul: -Rise from the horizon at a 45 degree angle. 3. The true value of a scientific model is: -Its ability to make accurate predictions. 4. Why wasn't Copernicus's heliocentric model of the solar system an immediate smash hit? -The predictions made by this model were no better than the predictions made by the Ptolomaic model. 5. Gallileo's run in with the church -Was largely a political issue. 6. My car is going 60 mph in a circle. Its acceleration is: -Not Zero 7. Newton showed us that the shape of the trajectory of a baseball thrown near the surface of the Earth: -Is really the very tip of a long elliptical orbit whose focus is at the center of the Earth. 8. The pre-solar cloud contained Hydrogen gasses, rocky compounds, Iron, and icy compounds. Which is the correct list in order of abundance, from least to most? -Iron, rocks, ice, Hydrogen 9. As the temperature of a blackbody emitter is decreased: -The peak of the blackbody shifts towards red and the total emission decreases. 10. What causes light from a star to doppler shift towards the blue? -The star is moving towards us 11. Consider the spectrum pictured. If the two stars were the same size, which would be brighter? -Star A 12. The greenhouse effect warms a planet because: -The atmosphere passes visible light from the sun but blocks infra-red light from the planetary surface. 13. The Earth's atmosphere: -Is transparent in the visible and radio and partially transparent in the infrared.


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