Astronomy Week 7 Notes
Astronomy Week 7 Notes EESC1150
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This 7 page Class Notes was uploaded by Erin Bleck on Tuesday July 19, 2016. The Class Notes belongs to EESC1150 at Boston College taught by Dr. Thomas Kuchar in Summer 2016. Since its upload, it has received 32 views. For similar materials see Astronomy in Earth and Environmental Sciences at Boston College.
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Date Created: 07/19/16
Notes March 1, 2016 - Orbit around the Sun takes 365 days 5 hours 48 minutes and 46 seconds o This is why we have a leap year - Newton’s Laws and Gravitation o All projectiles behave the same way! o o F = ma There is no change in mass, but there is a change in acceleration (because there is a constant change in vertical velocity) Gravity pulls it down - Net Forces: Sum of all the forces acting o A force acts in a specific direction For Gravity: that’s always toward the center of the planet, moon, star, etc. More than one force: add (or subtract) the forces to get the Net Force Net Force: F1 – F2 F = (G(mM))/R^2 The Electromagnetic Spectrum - Light o Astronomers’ primary tool in learning about the Universe is electromagnetic radiation (i.e., all forms of light) - White light o Isaac Newton showed White light could be split into component colors with a prism And then recombined into white light with a lens - Basic properties of light o Light is radiant energy o Speed – c = 300,000 km/s o Has characteristics of both a wave and a particle - Basic properties: wavelength o The colors are determined by the wavelength of light Denoted by the Greek letter “lambda” o From crest to crest o Visible light measured in Micron Nanometer Angstrom o Wavelengths of white light Visible spectrum (long wavelengths) ROYGBIV (short wavelengths) - Basic properties: frequency o Frequency – how fast successive crests pass by a given point Denoted by the Greek letter “nu” o Measured in Hz (1 cycle/s) - Frequency and wavelength are related o Frequency x wavelength = c - Basic properties: energy carried by light o E = h x frequency = (h x frequency)/wavelength h is Planck’s constant o Different colors of light have different amounts of energy associated with them - Electromagnetic Spectrum o - What is light? Wave or particle? o In the 17 century, Isaac Newton argued that light was composed of little particles while Christian Huygens suggested that light travels in the form of waves o In the 19 and 20 centuries, Maxwell, Young, Einstein, and others were able to show that light behaves like both, a particle and a wave, depending on how you observe it. Notes March 3, 2016 The Electromagnetic Spectrum - Observations at other wavelengths are revealing previously invisible sights o Shorter wavelengths shows more energy (ex. UV) o Infrared (longer wavelengths) shows heat - What is light? Wave or particle? o In the 17 century, Isaac Newton argued that light was composed of little particles while Christian Huygens suggested that light travels in the form of waves th th o In the 19 and 20 centuries, Maxwell, Young, Einstein, and others were able to show that light behaves both like a particle and a wave depending on how you observe it. - Waves interference – 2 extremes o Waves from two sources “in phase” with each other Peaks and valleys are aligned Waves add to each other and make one big wave o Waves from two sources completely “out of phase” with each other Peaks of one line up with valleys of the other Waves wipe each other out (no wave) Ex. Noise-canceling headphones - Thomas Young’s (1803) interference experiment o Look at the ocean waves coming across a barrier Sometimes, the waves would cancel out and the water would be still Sometimes, the waves would add to each other and the waves would be even higher o Did this with light waves Saw that some of the peaks lined up (so the waves would be even higher) Saw that some of the peaks interfered with each other and sometimes precisely canceled each other out Saw an interference pattern of bright and dim bands This would only work if light were a wave - Scottish physicist James Clerk Maxwell showed mathematically in the 1860s that light must be a combination of electric and magnetic fields o Particle would generate an electric field oscillating wave o Electric field oscillating wave generated a magnetic field oscillating wave o All of these were at different angles to each other o Electric particles that are oscillating will generate light waves! - The nature of light o As a particle (photons) Photons have energies related to frequency E = h x frequency Wave theory (incorrectly) says its energy is a measure of intensity (brightness for light) Photons strike the light sensors (or your eye) like a very small bullet – how your phone’s camera works - Photoelectric effect: on certain metals, shining light causes the atoms to eject electrons o In 1905, Einstein explained it using light as particles (photons) and correctly predicted the energy of the ejected electrons based on the photon E of photon = h x frequency or E of photon = (h x speed of light)/wavelength - Light is a particle or a wave (depending on what experiments you run) Interaction of light, atoms, and energy - Atoms: historical perspective o Atoms first proposed by Greeks ~500BC Not our concept of an atom No protons, neutrons, electrons More philosophical than scientific “Atoms” smallest constituent of matter that cannot be broken down any further o JJ Thompson Discovered electron – (NobethPrize 1906) “Plum pudding” model – 19 century Electrons floating around in a positively charged soup o Ernest Rutherford In early 1909 bombarded gold foil with helium nuclei (aka alpha particles) and watched them ricochet Demonstrated atoms had a dense, charged cover Also helped develop the orbital theory of the atom His model didn’t explain electron-structure o Classic “solar system” model of an atom *** A small, dense nucleus (containing protons and neutrons) surrounded by electrons Proposed by Niels Bohr in 1913 Atoms are mostly empty space The size of the nucleus is about 10^-15m The first electron orbits out at 10^-10m from the center of the atom o Size of the electron orbit is 100,000x greater than the size of the nucleus o Today’s view of the atom The electron should be thought of as a distribution or “cloud of probability” around the nucleus that on average behave like a point particle on a fixed circular path o *** The Bohr Model Experiments in early 20 century found electrons can only orbit atoms at precisely defined levels: orbitals Not like planets, which can orbit at any distance from Sun Electrons can be only specific orbits with specific energies o Referred as “discrete” or “quantized” energies Beginning of quantum physics - The Hydrogen Atom – 1 proton, 1 electron o Electron orbitals are “quantized” – that is they exist only at very specific energies (specific levels) o The lowest energy orbital is called the ground state Minimum energy (an electron is at its lowest energy state) o Electronic transitions – move an electron from one orbital to another To move up an orbital – electron needs to gain energy (excited state) To move down an orbital – electron needs to lose energy o Ionization – the electron is ejected from the atom Atom is no longer neutral but carries a charge (it’s now an ion) o As you increase in orbitals, the spacing between them decreases - Photons (light-waves) are emitted from an atom when an electron moves from a higher energy level to a lower energy level (releases energy) - What determines the energy levels of the electron orbitals? o The number of protons (atomic number) in a nucleus determines what element a substance is Also the number of electrons as protons o Since each element has a unique number of protons, electron energy levels are also unique o Since the energy (light) emitted or absorbed by an atom during an electronic transition is unique, it indicates wat element the atom belongs to, even from millions of light years away! Ex. Absorption and emission of a photon in a Hydrogen Atom An electron absorbs a photon of the exact energy needed to raise it an orbital If the photon energy does not match the transition energy, nothing happens to the electron If an electron drops from one orbital to a lower one, it must first emit a photon with the same amount of energy as the orbital energy difference Spectroscopy - Spectroscopy o The study of light emitted or absorbed by an object at various wavelengths to determine its composition and physical state (e.g., temperature) - How a spectroscope works o A narrow slit focuses the light o A grating or a prism splits the light into its component colors o Ex. Emission spectrum of hydrogen (on slide) We see specific colors unique to hydrogen based on the energy each electron emits to go down an orbital Like a barcode Then you can get a spectrum Also unique to hydrogen The red line (the first right line) is called “H alpha” - Different atom, different spectrum o Every element has its own spectrum o Note the differences between hydrogen and helium spectra below (on slide) - Absorption spectra o What if, instead of hot hydrogen gas, we had a cloud of cool hydrogen gas between us and a star Missing light is absorbed by hydrogen atoms in gas (can see missing light in spectrum) The rest of the light are visible (all colors) - Summary – types of spectra o Continuous spectrum – the source emits light that is continuous and all colors are present o Emission-line spectrum – a hot, thin gas will emit characteristic frequencies of light o Absorption line spectrum – a cool gas will absorb light behind it at a characteristic frequency o Kirchoff’s laws (1859) A hot solid object produces light with a continuous spectrum A hot, tenuous gas produces light with spectral lines at discrete wavelengths (i.e. specific colors) which depend on the energy levels of the atoms in the gas A hot solid object surrounded by a cool tenuous gas (i.e. cooler than the hot object) produces an almost continuous spectrum with gaps at discrete wavelengths depending on the energy levels of the atoms in the gas ALL STARS (INCLUDING THE SUN)!
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