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Midterm

# Astronomy 181 Before Mid Term.docx

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School
Department
Physics
Course
PCS 181
Professor
Margaret Buckby
Semester
Fall

Description
The Scientific Method 1. Observation 2. Hypothesis 3. Experiment 4. Conclusion 5. Reproducibility Unit 1: The electromagnetic spectrum (em) - Light is the only way we can get astronomical information, by observing it Starlight gives: 1. The star’s temperature 2. Chemical composition 3. Motion 4. Pressure 5. Magnetic fields, etc. Light: mutually perpendicular oscillating electric and magnetic fields. It moves in wavelike pattern at a speed of c= 3x10^8 m/s The speed of light is the fastest speed possible. Wavelength, λ (lambda): distance between peaks of waves. Tachyons: theoretical faster-than-light particles. Certain wavelength are perceived by the brain as colors: - 1 nanometer= 1nm= 1/1x10^9m= 1x10^-9= 10^-9 - λred= 700nm= 700x10^-9m= 7x10^-7m - λviolet= 400nm= 400x10^-9m= 4x10^-7m - λgreen/yellow= 550x10^-9m The orders of the visible colors are R.O.Y.G.B.I.V Photons: bundles of energy -> light as a particle Frequency= f - Number of wave peaks passing a given point each second - Number of waves/second - Complete cycles/second - Cycles/s or ->Hz f.red= 3x10^8m/s / 700x10^-9m= 4.3x10^14Hz Plank curve To calculate the temperature of the surface of a star: 1. Get star in telescope 2. Sequence of colored filters at eyepiece end of telescope 3. Starlight passes through filter and then computer 4. Computer gives numerical value of light intensity passing through each colored filter. Temperature on the surface of a star: 0.0029mk/ λdom (ex.500x10^-9) T= 5272.7 or 5300k. The surface temperature of the sun is about 5800k. Energy of light E= hf E.red= (6.625x10^-34js)(4.3x10^14cycles/s) E.red= 2.85x10^-19 Note: shorter wavelength= higher frequencies and energie. The Bohr model of the hydrogen atom - Proton carries positive charge - Electron carries negative charge - 1913: Electron is in circular orbit around the proton. Particles can pop into existence and out of existence for a short period of time. The Doppler Effect in astronomy Starlight waves… take spectrum. - If the entire pattern of spectral lines is: Shifted to the blue end of the spectrum (blue-shifted) Shorter distance between wave crests -> Object is moving toward you Procedure: - Obtain spectrum of a stationary (=not moving) source (e.g. hydrogen) in the laboratory - Obtain spectrum of a moving object (e.g. a star) - Compare the two spectrums. If an electron is closer to a proton, it will have a greater percentage of energy. ∆E= E.Final- E.Initial E.Photon= hc/λ Doppler Effect to determine velocity C= speed of light= 3x10^8m/s ∆ λ= wavelength difference between lab and stellar spectra λlab=wavelength on lab spectrum Vr= radical velocity of star Then Vr= c∆ λ/ λlab Question: H spectral line is λ= 656.2850nm in the lab. Same line is λ= 656.2927nm in the spectrum of Regulus. Is Regulus approaching or receding? Radial velocity? This H.a line has a longer wavelength in the spectrum of Regulus - Spectrum is red shifted -> moving away from us. Vr= c∆ λ/ λlab= (3x10^8m/s)(656.2927-656.2850)nm/656.2850nm Vr= 3500m/s= 3.5km/s Unit 2: The Universe Distance units in astronomy: - Lightyear: ly: Distance that light travels in one year: 1ly= 9.5x10^15m - Astronomical unit: Au: Average distance between the Earth and Sun - Parsec: pc: 1pc=3.26ly The Hubble law A galaxy is a large assemblage of stars which orbit a common center, all held together by gravity and which contain from a few thousand to trillions of stars. Hubble and Humason determined the distances and radial velocities of faraway galaxies. - The farther a galaxy is to us, the faster it photon energies have declined enough such that Hydrogen and Helium atoms can survive 1x10^9years Galaxies form 2x10^9years Stars form The age of the universe= Hubble time Recall the Hubbe law: - Vr=Hod where we use a Hubble parameter of 75km/s /MPC - Since d=vt we replace d in the above equation: V=HoVT 1=HoT - T=1/Ho= Hubble time= Age of the Universe - T= 1/75km/s /MPC T=MPC/75km/s T=13.1x10^9years The Universe is 13.7 billion years. Types of Universe Open Universe: Density of matter is low= gravity is weak= Universe expands forever Closed Universe: Density of matter is high= gravity will halt the expansion= Universe will re- contract. If this happens repeatedly, we have an oscillating Universe Flat Universe: Universe contains the so-called critical density of matter= Galaxies barely manage to avoid re-collapse Future of an open or flat Universe Time after the Big Bang Situation 1. 1x10^12 years Hydrogen and helium are depleted-> no new stars can form 2. 1x10^14 years Existing stars run out of fuel 3. 1x 10^17 years All stars will have experienced about 100 disruptive encounters, flinging their planets into space 4. 1x10^18 years Evaporation of stars and collapse of galaxies into the central Black Holes 5. 1x10^30 years Protons decay? p+^n->Ѵ (a positron and a neutrino) e^++e^- (a photon= a burst of radioactive energy 6. 1x10^100 years Hawking-> Black holes decay 7. Far future The Big Fizzle= an ever expanding gas of electrons, protons, neutrons, photons Future of a closed or oscillating Universe 1. The expansion phase= statements 1 through 5 above 2. At some unknown future time, re-contraction begins. The density of the Universe increases, the numbers of Black Holes increases, they feast on everything within reach. 3. Black Holes will meet and coalesce (merge) into One Black Hole= The Universe= the new singularity -> The Big Crunch How to decide among these futures? Is there sufficient matter to exert a strong gravitational force? About or more than 5 protons per m^3 of space -gravity will dominate and
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