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## physnotes_week_11.pdf

by: Sophie Stella

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# physnotes_week_11.pdf PHYS 104-01

Sophie Stella
UST

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Here are the notes for week 11 of PHYS 104: Astronomy with Dr. Ruch.
COURSE
Astronomy
PROF.
Dr. Ruch
TYPE
Class Notes
PAGES
2
WORDS
KARMA
25 ?

## Popular in Astronomy

This 2 page Class Notes was uploaded by Sophie Stella on Wednesday April 27, 2016. The Class Notes belongs to PHYS 104-01 at University of St. Thomas taught by Dr. Ruch in Winter 2016. Since its upload, it has received 14 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 Week 11 Course Notes, 4/18 – 4/22 Definitions Pulsar: Aspinning 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. III. 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. IV. 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. V. 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. a. Looking through a layer of dust makes distant objects appear dimmer and further. D. Shapley's view of the galaxy 1. He realized Kapteyn's mistake, and took into account the dust in the galaxy. 2. Shapley thought that the globular clusters were in a spherical halo around the galaxy. 3. He measured objects using their orbital velocities. 4. He calculated that the solar system is 45,000 ly away from the center, and that the galaxy is 300,000 ly wide x 30,000 ly thick. a. Shapley's calculations are too big, because he didn't take into account the greater luminosity of the globular clusters, and he overestimated the amount of dust in the galaxy. VI. The Milky Way A. Basic structure of a spiral galaxy 1. Disk: flat and circular, in the mid-plane 2. Bulge: sperical central region 3. Halo: a sphere surrounding the disk and bulge B. The age of the galaxy is about 10 billion years C. The disk contains lots of gas and dust, while the Halo contains almost no gas or dust. D. Orbital Motions 1. The disk has a circular orbit around the center of the galaxy. 2. The halo and bulge are composed of objects which orbit the center, but are not coplanar. 3. The velocity of an object increases as the distance from the center increases. E. Orbital Velocity 1. Even as the orbital radius of the solar system increases, its enclosed mass remains constant. 2. As the orbital radius increases for a galaxy, the enclosed mass also increases. a. This means that, when we add up all the mass in the galaxy, it is less than expected when calculated using the orbital velocity. 3. This velocity discrepancy implies that the galaxy contains more mass than we can see. F. Dark Matter 1. This is what we call the matter that the galaxy must contain, but that is unobservable. 2. Dark matter would make up the majority of the mass in the universe. G. The spiral arms 1. The spiral arms of the galaxy are density waves, in which new stars form. 2. The density waves move faster than the disk spins.

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