Midterm 2 Notes

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University of Toronto St. George
Astronomy & Astrophysics
Michael Reid

Chapter 5: Light and Matter Three Basic Types of Spectra: 1. Continuous Spectrum: spectrum of ordinary light bulb; rainbow spans broad range of wavelengths without interruption 2. Emission Line Spectrum: thin or low-density cloud because it emits light only at specific wavelengths that depend on composition and temp.; consists of bright emission line against a black background  Graph shows upward spike at the wavelength of each emission line - Atom in any cloud of gas are constantly colliding with one another, exchanging energy in each collision; some of the collisions result in the right amount of energy to bump an e from lower to higher levels - When the e falls back to lower level, it releases a photon of light (the emitted photon must have the same amount of energy that the electron loses – specific wavelength/frequency) - The specific set of lines we see depends on the cloud’s temperature and its composition 3. Absorption Line Spectrum: if the cloud of gas lies between us and light bulb; consists of dark absorption lines over the rainbow background  Graph shows a dip in intensity at the wavelength of each absorption line - The light bulb emits light in all wavelengths and the gas can absorb photons that have the right amount of energy needed to raise an electron form a low energy level to a higher one - Two things can happen after an electron absorbs a photon and rises to a higher energy level: a) Electron can quickly return to its original level, emitting a photon with the same energy that it absorbed b) Electron can lose its energy in some other way but either dropping back down to a different level or by transferring its energy to another particle in a subsequent collision Chemical Fingerprints - Each element has a different set of lines - Ions also have different set of lines (Ne++ different from Ne+ and different from Ne) Differences can help us determine the temp of a hot gas or plasma - Molecules also have spectral fingerprints; in addition to producing lines through changes in energy level, molecules can vibrate and rotate, which also requires energy. o Can absorb or emit photons when it changes its rate of vibration or rotation. o Energy changes usually smaller than with atoms => produce lower-energy photons o The energy levels are bunched more closely together than atoms; usually in the infrared portion of spectrum How does Light tell us the temp of stars and planets? Thermal Radiation: aka blackbody radiation; the spectrum of radiation produced by an opaque object that depends only on the temp of the object - Thermal radiation spectrum is the most common type of continuous spectra Laws of Thermodynamics: 1. Stefan Boltzmann law: Each square meter of a hotter object’s surface emits more light at all wavelengths 2. Wien’s law: hotter objects emit photons with a higher average energy (shorter avg wavelength) Because the thermal radiation spectrum depends only on temp, we can use them to measure the temp of distant objects. Hotter = shorter Doppler Effect: the effect that shifts the wavelengths of spectral features in objects that are moving towards or away from the observer; larger the shift, the faster it’s moving Red shift: objects moving away Blue shift: objects moving towards - The shift only tells us the part of an objects motion that is directed towards or away, not across the line of sight Chapter 7: Our Planetary System Comparative planetology: comparing worlds together, seeking to understand their similarities and their differences Mercury - Highly cratered surface, no atmosphere, no active volcanoes, no wind, no rain, no life, no air - Past geological activity such as plains created by ancient lava flow and tall, steep cliffs that may be wrinkles due to planetary shrinking - Huge exterior with a huge iron core (possible impacts that blew away outer layers) - Temp -170 to 425 degrees Celsius (hot enough to melt lead) - No moons - Has a magnetic field, but we don’t know why Venus - Thick CO at2osphere - About same size as earth; has mountains, valleys and shows signs of past or present volcanic activity - No plate tectonics - runaway greenhouse effect makes it even hotter than Mercury (460 degrees all the time) - rains acid - no moons - rotates very slowly and backwards Earth - only planet in solar system with oxygen to breathe, ozone to shield from deadly radiation and abundant surface water - moderate greenhouse effect - abnormally large moon compared to planet; a giant impact, an object the size of Mars, crashed into Earth blasting rock from its outer layers into space; the material collected and formed the Moon) Mars - thin CO a2mosphere - 2 moons: Phobos and Deimos (probably asteroids captured by planet in early solar system) - many huge, extinct volcanoes - probably earth-like in the past - many surface features appeared to be carved out by water; lots of subsurface water Jupiter - largest planet in the solar system - “gas giant” but really a liquid giant because it has a thick atmosphere around a giant ball of liquid hydrogen - Made primarily of hydrogen and helium; no solid surface; increasing gas pressure as you go deeper into planet - Faint rings - Many moons (>60); Io is the most volcanically active body in the solar system because of Jupiter’s tides Saturn - Real gas giant; mostly gas (hydrogen and helium); no solid surface - Spectacular rings made up of countless small particles that orbit Saturn like tiny moons (chunks of ice and rock the size of dust grains to city blocks - Its moon Enceladus has ice fountains spraying out from the southern hemisphere; Titan has a thick atmosphere (has frozen methane or ethane instead of water) Uranus - ice giant; blue, many moons and faint rings - largely of hydrogen, helium and hydrogen compounds; no solid surface - tipped axis: the entire system (moons, rings); probably from cataclysmic collision when it was forming - extreme seasonal variations Neptune - ice giant - twin of Uranus, but bluer, smaller and more dense - its largest moon Triton has an icy surface that look like geysers that spew nitrogen gas into the sky; it orbits the planet backwards; probably orbited the sun at one point but was captured into orbit (If friction reduced a passing planetesimals’ orbital energy enough, it could have become an orbiting moon; captured moons do not necessarily orbit in the same direction as their planet or in its equatorial plane Trans-Neptunian Objects - dwarf planets, small icy bodies like Pluto - Kiuper Belt: comets that orbit the Sun in same direction as planets - Oort Cloud: a spherical cloud of comets that surround the solar system Four major features of our solar system: 1. Patterns of motion among large bodies: generally rotate and orbit prograde, nearly circular, and same plane 2. Two major types of planets: small, rocky planets close to Sun (few moons), large gassy (mainly hydrogen, helium and hydrogen compounds) planets farther apart and farther from Sun (many moons) 3. Asteroids and Comets: between and beyond planets, asteroids and comets orbit the Sun (asteroid belt, Kiuper Belt, Oort Cloud) the location, orbit and composition follow distinct patterns 4. Exceptions to the rule: Earth’s large moon, Uranus’ tipped rotation, Venus’ backward rotation Four Types of Robotic Spacecrafts 1. Flyby - Uses gravitational slingshot technique to change trajectory without wasting fuel and to speed up the space craft 2. Orbiter 3. Lander or probe 4. Sample Return missions Chapter 8: Formation of the Solar System Nebular Theory: the theory that describes how the solar system formed form a cloud of interstellar gas and dust. As it shrank in size, 3 important processes occurred: 1. Heating: the temp of the nebula increased as it collapsed because gravitational energy => kinetic energy of individual gas particles falling inward. Kinetic => thermal as the particles crashed into one another. Sun formed in the center where it was warmest 2. Spinning: rotated faster as it shrank because of conservation of angular momentum 3. Flattening: natural consequence of collisions between particles of a spinning cloud; particles’ random motions eventually become more orderly and the orbits become more circular Recap: 1. The original cloud is large and diffuse, and its rotation is imperceptibly slow. The cloud begins to collapse. 2. Because of conservation of energy, the cloud heats up as it collapses. Because of conservation of angular momentum, the cloud spins faster as it contracts. 3. Collisions between particles flatten the cloud into a disk 4. The result is a spinning, flattened disk with mass concentrated near the center and he temperature highest near the center. Formation of Planets The key to why terrestrial planets are so different in composition from Jovian planets is their location - The temperature of the places they formed played a direct role. - Condensation (gas  liquid/solid) of particles. These particles started out microscopic but grew over time - Within frost line, rocks and metals condense but hydrogen compounds stay gaseous - Beyond the frost line, hydrogen compounds, rocks and metals condense - Within the solar nebula 98% of material is hydrogen and helium gas that doesn’t condense anywhere Hydrogen and Helium Gas => hydrogen compounds => rock => metal Abundance: highest  lowest Condensation temp: lowest  highest How did Terrestrial planets form? Accretion: small seeds grew into planets through collisions (electrostatic energy stuck them together) - At first they were more like gentle touches, but as they got bigger, more gravity = harder collisions that helped particles stay together => faster growth into planetesimals (pieces of planets) - The planetesimals grew rapidly at first, but once it reached a relatively large size, further growth was difficult; the gravitational encounters between planetesimals altered their orbits and they would collide with so much force that fragmentation occurred more frequently than accretion. How did Jovian planets form? Accretion should have occurred similarly beyond frost line, but with more solid materials (due to condensation of ice) - The planets formed as gravity drew gas around the ice-rich planetesimals much more massive than Earth; because of large mass, they had stronger gravity which meat they could capture and hold hydrogen and helium gas. More gas = more mass = more gravity = more gas etc. - This explains the formation of their moons because Jovian planets came to be surrounded by its own disk of gas, spinning in the same direction that the planet rotated. (formed similarly to planets around Sun) - They stopped growing because solar winds swept remaining hydrogen and helium gas into interstellar space (younger start = stronger winds) Sun’s Rotation - Used to spin very fast because of the law of angular momentum (spinning disk should have spun fastest near the center) - The sun used to have a strong magnetic field that made the Sun more active (more solar flares and winds) - Charged particles = magnetic fields; as the sun rotated, its magnetic field dragged the charged particles along faster than the rest of the nebula, adding to their angular momentum. - As the particles gained angular momentum, Sun was losing; the particles were eventually blown away by solar winds making the Sun with less angular moment and slower rotation today Asteroids and Comets Asteroids are rocky leftover planetesimals of the inner solar system; asteroid belt must have contained enough planetesimals to form a planet at one point but most crashed into terrestrial planets or were ejected out of the system Comets are ice rich leftover planetesimals of the outer solar system. - Those that orbited Neptune should have been able to grow fairly large without disruption of Jovian planets; some grew into dwarf planets. These orbit the Sun like other objects and are now a part of the Kiuper Belt - Those that roamed between Jovian planets were ultimately kicked out due to gravitational encounters and are now a part of the Oort Cloud Chapter 9: Planetary Geology What are terrestrial planets like on the inside? Core: the highest density material; consists primarily of metals like nickel and iron - Two distinct regions: solid core and molten (liquid) outer core Mantle: moderately dense material; mainly minerals like silicon, oxygen and other elements; surrounds core Crust: lowest density rock like granite or basalt  The interiors are layered due to differentiation where gravity pulls denser materials to bottom, driving less dense material to the surface 3 sources of energy that explain interior heat: 1. Accretion: energy brought from colliding planetesimals. Gravitational potential energy => kinetic => heat 2. Differentiation: when the dense material sinks to the bottom and less dense material rises to the surface. Gravitational energy => thermal energy from friction 3. Radioactive decay: when radioactive nuclei decay, subatomic particles fly off at high speeds, colliding with neighbouring atoms and heating them; mass energy => thermal energy 3 basic cooling processes: 1. Convection: hot material expands and rises while cooler material contracts and falls; transfer heat upwards 2. Conduction: transfer of heat from hot material to cooler material through contact 3. Radiation: objects emit thermal radiation characteristics of their temperatures; this radiation carries energy away and therefore cools object. Low temp = infrared Rate of cooling is proportional to surface area/volume Cooling rate = 3/r Size is the primary factor in determining geological activity; the larger the planet, the longer it takes to cool. Why do some planetary interiors create magnetic fields? 3 requirements: 1. Interior region of electrically conducting fluid, such as molten metal 2. Convection in that layer of fluid 3. At least moderately rapid rotation 4 Processes that reshape a planet’s surface: 1. Impact cratering - Craters are perfectly circular because impactors hit the surface faster than the speed of sound and explode - Large impacts can produce shockwaves that rebound off the planet’s interior layers and thrust the center of the crater up, producing a central peak - More craters = older 2. Volcanism - Refers to any eruption of molten lava (from volcano or from cracks in the lithosphere) magma finds its way to the surface - Molten lava rises for three reasons: a) Molten lava is less dense than solid rock so it tends to rise b) Most of the interior is not molten so the solid rock surrounding the chamber can squeeze the molten rock upward under pressure c) Molten rock contains trapped gases that expand as it rises - Magma when underground, lava when above - Depending on the consistency of the lava, they can form different features: volcanic plains, shield volcanoes and stratovolcanoes (runniest to thickest); thicker = solidify before flattening out - Lava plains and shield volcanoes are made of basalt (mixture many different minerals that erupts from volcanoes as a high-density but runny lava) - Volcanoes explain the existence of atmospheres and oceans due to outgassing; when molten rock erupts and expels the trapped gases 3. Plate tectonics - Direct or indirect result of mantle convection 4. Erosion (water, sand, wind, etc.) - Breakdown or transport of materials through the action of ice, liquid or gas; can also build things like sand dunes, river deltas etc. Impact Craters Reveal a Surface’s Geological Age - More craters = older surface; this means that geological processes haven’t erased them over time - On the moon, the Lunar highlands are covered heavily by craters; Luna Maria is generally smooth - The degree of crater crowding allows us to estimate the geological age within a few hundr
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