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PH 104H

Jim Ketter The Sun Star: huge ball of gas held together by gravity Generates light, heat, energy, through nuclear reactions Source of energy 4 billion years old 100x wider than the earth, 1,000,000x larger, 300,000x as massive Astronomical Unit 1 AU = average distance between the Earth and Sun ≈ 150 million km ≈ 93 million miles Light Year → distance (ly) 10 trillion km in one year “look-back” time; it‟s how far back in time we are seeing The Celestial Sphere • Model of the sky • Terms: • Zenith: the point of the sky that is straight overhead. • Nadir: the point on the sphere directly below you and opposite the zenith. • Celestial Poles” two points that do not move - lie exactly above the North and South oles of the Earth. • Polaris: “North Star” - lies close to the direction of true north. • Celestial Equator: lies directly above the Earth’s equator. • Stars on the celestial equator rise due east and set due west. • The pattern of stars is NOT a constellation - asterisms • 88 constellations • Constellations - stars • Celestial Sphere divided by: Lines of declination (East → West) and Lines of ascension (North → South) The Earth • Astronomy → study of the universe • Planet → Earth → size defined the fundamental unit of the metric system: the meter • Moon = Earth’s satellite • 1/25 the size of Earth • Surface = craters • Less internal heat than Earth • ✷No atmosphere → not protected from impacting objects • ↳ no erosion or geological activity = no surface change ever Planets • Mercury → Venus → Earth → Mars → Jupiter → Saturn → Uranus → Neptune • Merc: craters, airless surface • Ven: clouds of sulfuric acid droplets • Ear: white clouds, blue oceans, green jungles, red deserts • Mar: canyons, deserts • Jup: atmospheric storms with lightning • Sat: icy fragments orbit, form bright rings • Ura: tipped axis, blue • Nep: methane clouds, deep blue Beyond the Solar System • Gravity: a force that attracts every object toward every other object across space • Milky Way Galaxy: cloud of several hundred billion stars with a flattened disk-like shape. • 100x the diameter of the solar system. • 2 largest galaxy in the Local Group after Andromeda Galaxy • ↑ • Collection of galaxies • Dark Energy: all-persuasive energy The Light Year (ly) Light Year → distance (ly) 10 trillion km in one year “look-back” time; it‟s how far back in time we are seeing The Parsec (pc) “PARallax SECond” Distance to an object whose parallax motion measures 1 sec of arc in 6 months of observation time on Earth. 1 pc = 3.26 ly Parallax - apparent shift in position of some object because of change in position of observer. Scientific Method General procedures scientists use to construct their ideas about how nature (everything) works. ↓ Scientific Theory/ Model/ Law Explains observable things Must be testable & disprovable Concerns observable and measurable things Protons (positively charged) and neutrons (uncharged) make up the nucleus at the center of an atom. Electrons (negatively charged particles) are found relatively far from the nucleus. Forces in Nature Gravitational Force Force between objects with mass Infinite in range, but weakens with distance Weakest force Electromagnetic Force Force between charged bodies Infinite in range, but weakens with distance Like charges repel Strong Force Holds atomic nuclei together Very short range - 10^-15 meters Weak Force Radioactive decay Short range Annual Motion of the Sun As the Earth revolves around (orbits) the Sun, the Sun appears to move through 13 constellations on a belt around the Celestial Sphere called the Ecliptic. When the sun in the sky is in “front” of a particular constellation, we say that the Sun is “in” that constellation. The Ecliptic The Ecliptic “belt” on the Celestial Sphere is tipped relative to the Celestial Equator due to the 23.5∘inclination of the Earth‟s rotational axis. Analemma - pattern formed by taking a picture of the Sun at the same time on different days of the year. The Earth‟s inclination is ultimately responsible for the change in seasons. Precision I The Sun‟s gravity adds a pull that causes it to “wobble” Wobble means the axis of the Earth is rotating or precessing with a 26,000 year period Nutation - extra wobble Pull by moon Precision II Because of this precession, Polaris has not always been and will not always be “The North Star” Sidereal Time One Solar Day is 24 hours - takes 24 hours for the Sun to return to the same spot overhead on successive days. Our clocks are based on the Solar Day One Sidereal Day is 23 hours and 56 minutes Phases of the Moon 29.5 day cycle Half of it‟s surface is always lit We (n Earth) only see the illuminated portion Full Moon: Earth is between the Moon & Sun ← see illuminated side New Moon: Moon is between Earth & Sun ← see none of illuminated Harvest Moon: first full moon following the equinox Waxing - brighter Waning - darker Same side of the moon always faces us thanks to Tidal Lock: Tidal locking (or captured rotation) occurs when the gravitational gradient makes one side of an astronomical body always face another, an effect known as synchronous rotation. For example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. This causes one hemisphere constantly to face the partner body. Usually, at any given time only the satellite is tidally locked around the larger body, but if the difference in mass between the two bodies and their physical separation is small, each may be tidally locked to the other, as is the case between Pluto and Charon. This effect is employed to stabilize some artificial satellites. Solar Eclipses: At new moon, the moon is between the Earth and the Sun. Sometimes, the alignment is just right allowing the Moon to block the light from the Sun on the surface of the Earth → Solar Eclipse Umbra: darkest part of the shadow, directly behind the body of the Moon. Penumbra: part of the shadow where the light from the Sun is only partially blocked Lunar Eclipse:As the moon passes behind the Earth relative to the Sun, the Earth can cast a shadow on the surface of the Moon. Midterm 7:00-8:20 → Gilbert 224 Kepler‟s First Law Planets move in Elliptical orbits with the Sun at one focus of the Ellipse Sun-centered model (heliocentric) Shape of the Ellipse is described by it‟s semi-major and semi-minor axis Circular Orbit Is possible Elliptical is more likely Kepler‟s Second Law Orbital speed of a planet varies so that a line joining the Sun and planet will sweep out equal areas in equal time intervals. Planets move faster when near the Sun and slower when farther from the Sun. Kepler‟s Third Law The amount of time a planet takes to orbit the Sun (it‟s period) P is related to its orbit‟s size, a, by P² = a³ Describes the shape of a planets orbit, it‟s orbital period, and how far from the Sun it is. Galileo did not invent telescope - improved it Newton discovered laws that govern the motion of all (large) bodies Invented calculus to do it Vector - a physical quantity that requires both a magnitude (size) and a direction to fully define it. Scaler - a physical quantity that can be fully defined by magnitude (size) only. A state of free fall is equivalent to a state of weightlessness. Newton‟s Law of Gravitation - Every mass exerts a force of attraction on every other mass. Larger masses have larger gravity. Objects close together pull more on each other than objects farther apart. For planets, the accelerating (2 Law) force that keeps the planets moving on an elliptical path is the gravitational force. Tides Moon exerts a gravitational force on the Earth, and on the water in the oceans. The effect of the various forces acting on the ocean‟s water causes the water to create tidal bulges both beneath the Moon and on the opposite side of the Earth (from the Moon) at the same time. When the Sun and the Moon line up, higher tides, called “spring tides” are formed. When the Sun and the Moon are at right angles to each other smaller “neap tides” result. Nothing to do with seasons Law of Conservation of Energy The energy in a closed system may change form, but the total amount of energy does not change as a result of any process. Energy can be neither created nor destroyed but only changed in form. Kinetic energy The energy of motion Mass and speed contribute to it. Thermal Energy Energy associated with heat. Is really just a form of kinetic energy. Potential Energy Stored energy - energy due to position Gravitational potential energy - the energy an object has due to its position in a gravitational field. GPE is transformed when the object is allowed to fall, becoming the kinetic energy of the falling motion. Angular Momentum Linear Momentum: p = m x V. If no external forces are acting on an object, then its linear momentum is conserved. Angular Momentum: the rotational equivalent of linear momentum. If no external forces (torques) are actin on an object, then its angular momentum is conserved. L = m x V x r = constant Tidal Lock Epicycle model Ptolmey Opposition of mars Phases of the Moon Light Primary tool in learning about the universe is from the electromagnetic radiation - light. Is radiant energy Travels very fast - 300,000 km/sec, 186,000 miles/sec Has dual nature - can be described either as a wave or as a particle traveling through space. Has 0 mass As a wave: A disturbance in an electric field creates a magnetic field, which in turn creates an electric field, and so on, a self-propagating electromagnetic wave. Light waves can constructively or destructively interfere Color of light is determined by its frequency The energy is also determined by frequency As a particle: Particles of light (photons) travel through space These photons have very specific energies, that is, light is quantized. Photons strike your eye (or other sensors) like very small massless “balls” (maybe bb‟s), and are detected. Light as a Wave Versus Mechanical Waves Wave - transfer of energy without the transfer of matter. Wave phenomena - refraction, diffraction, constructive and destructive interference, super positioning, Doppler shift. Measurable wave characteristics - amplitude, wavelength, frequency, period. Mechanical Waves - water, sound - must have some physical matter - a medium - in which to exist and travel. Light exhibits all wave phenomena and has all the measurable wave characteristics (as a mechanical wave). BUT, light does not require any physical matter for its transfer. Light can exist and travel through the vacuum of space. Wavelength Colors are determined by wavelength of light Wavelength is the distance between successive crests (or troughs) in an electromagnetic wave. Similar to the distance between the crests in ocean waves. Symbol lambda. Frequency How fast successive crests pas by a given point. Frequency has units of Hz (hertz) and is symbol v. 1 Hz = 1 cycle/sec. Long wavelength light has a low frequency, and short wavelength light has a high frequency. Lambda x v = c „c‟ is the speed of light. White Light Light from the Sun arrives with nearly all wavelengths, and we perceive this mixture of colors as white. Is all the colors The Electromagnetic Spectrum I There is much more to light than just visible light. Radio waves ave very long wavelengths, as much as a meter and more. Microwaves are at the upper end of the radio part of the spectrum. Infrared wavelengths are longer in wavelength than visible light. The Electromagnetic Spectrum II Ultraviolet waves are shorter in wavelength than visible waves. X-rays come mostly from stellar sources in nature, and can penetrate many materials, like skin, muscle and bone. Gamma rays have the shortest wavelengths. Energy Carried by Photons A photon carries energy with it that is related to its wavelength or frequency: E = h x c = h x v ƛ Long wavelength (low frequency) photons carry less energy than short wavelength (high frequency) ones. This is why UV waves give us a sunburn, and X-rays let us look through skin and muscles. The Nature of Matter An atom has a nucleus at its center containing protons and neutrons. Outside of the nucleus, electrons move in “clouds” called orbitals. Orbitals are quantized - exist only at very specific energies. Lowest energy orbital is called the ground state. To move an electron from one orbital to the next higher one, a specific amount of energy must be added. Likewise, a specific amount of energy must be released for an electron to move to a lower orbital. The are called electronic transitions. The Chemical Elements The number of protons (atomic number) in a nucleus determines what element a substance is. Atom that is neutrally charged has a number of electrons equal to the number of protons. Electron orbitals are different for each element, and the energy differences between the orbitals are unique as well. Means that if we can detect the energy emitted or absorbed by an atom during an electronic transition, we can tell what element the atom belongs to, even from millions of light years away! Absorption If a photon of exactly the right energy (equal to the energy difference between orbitals) strikes an electron, that electron will absorb the photon and move into the higher orbital. The atom is now in an excited state. If the photon energy doesn‟t match any of the orbital - energy differences it can not be absorbed – it will pass through. We say the element is transparent to those frequencies or colors. Absorption. If the electron gains enough energy to leave the atom entirely, we say the atom is now ionized, or is an ion. Emission If an electron drops from one orbital to a lower one, it must emit a photon with the same amount of energy as the orbital-energy difference. •This is called emission. Emission Spectra Imagine that we have hot hydrogen gas. Collisions among the hydrogen atoms cause electrons to jump up to higher orbitals, or energy levels. Electrons can jump back to lower levels, and emit a photon of energy h x f. If the electron falls from orbital 3 to orbital 2, the emitted photon will have a wavelength of 656 nm. If the electron falls from orbital 4 to orbital 2, the emitted photon will have a wavelength of 486 nm. We can monitor the light emitted, and measure the amount of light of each wavelength we see. Seeing Spectra Seeing the Sun‟s spectrum is not difficult. A narrow slit only lets a little light pass. Either a grating or a prism splits the light into its component colors. If we look closely at the spectrum, we can see dark lines. These correspond to wavelengths of light that were absorbed. Emission Spectrum of Hydrogen This spectrum is unique to hydrogen. Different Atom, Different Spectrum! Every element has its own spectrum. Spectrum is like a chemical fingerprint! Absorption Spectra What if we had a cloud of cool hydrogen gas between us and a star? Photons of energies that correspond to the electronic transitions in hydrogen will be absorbed by electrons in the gas. The light from those photons is effectively removed from the spectrum. The spectrum will have dark lines where the missing light would be. This is an absorption spectrum. Also unique for each element. Types of Spectra - Summary If the source emits light that is continuous, and all colors are present we say that this is a continuous spectrum. If the molecules in the gas are well-separated and moving rapidly (have a high temperature), the atoms will emit characteristic frequencies of light. This is an emission- line spectrum. If the molecules of gas are well-separated, but cool, they will absorb light of a characteristic frequency as it passes through. This is an absorption line spectrum. Measuring Temperature It is useful to think of temperature in a slightly different way than we are accustomed to. Temperature is a measure of the motion of atoms in an object. Objects with low temperatures have atoms that are not moving much. Objects with high temperatures have atoms that are moving around very rapidly. The Blackbody Spectrum As an object (piece of iron for example, or the gas in a star) is heated, the atoms in it start to move faster and faster. When they collide, they emit photons with energy proportional to how hard they hit. Collide lightly - produce long-wavelength radiation. Collide very hard - produce short-wavelength radiation. Most are somewhere in between. Results of More Collisions Additional collisions mean that more photons are emitted, so the object gets brighter. Additional hard collisions means that more photons of higher energy are emitted, so the object appears to shift in color from red, to orange, to yellow, and so on. Of course we have physical laws to describe these effects. Wien‟s Law Hotter bodies emit more strongly at shorter wavelengths. The hotter it is, the shorter the wavelength. Lets us estimate the temperature of stars easily and fairly accurately. Stefan-Boltzmann Law Th luminosity of a hot body rises rapidly with temperature. If we know an object‟s temperature (T), we can calculate how much energy the object is emitting using the SB law. Distance on Light Light from a distant source seems very dim Light is spreading out as it travels from its source to its destination. The farther from the source you are, the dimmer the light seems. The objects brightness, or amount of light received from source, decreases with increased distance. The relationship is mathematical. Brightness = Total Light Output 4pid^2 This is an inverse-square law - the brightness decreases as the square of the distance (d) from the source. Doppler Shift in Sound As a car approaches, the sound from a car seems to have a higher pitch - this is due to shorter wavelengths. As the car passes, the sound shifts to lower pitch due to the longer wavelengths. Police radar guns work on the same principle. The waves reflected off the car will be shifted by an amount that corresponds to the car‟s speed. Doppler Shift in Light If an object emitting light is moving toward you, the light you see will be shifted to shorter wavelengths - toward the blue end of the spectrum. The light is blue-shifted. If an object is moving away from you, the light will be red-shifted. If we detect a wavelength shift of Δλ away from the expected wavelength λ, the radial (line-of-sight) velocity of the object is: Doppler Shift in Space When we look at most every object in deep space,we see that the light from those objects is redshifted. That means that most everything is moving away from us (and everything else too). Therefore the universe is EXPANDING! Components of the Solar System Vast majority of the Solar System‟s mass resides in the Sun. The rocky inner planets (Mercury, Venus, Earth and Mars) are called the terrestrial planets. The gaseous outer planets (Jupiter, Saturn, Uranus and Neptune) are the Jovian planets. An asteroid belt lies between the inner and outer planets. The outer most icy planet, Pluto, is in a class called Trans-Neptunian Objects (TNO). It‟s a dwarf planet. The Kuiper Belt Outside the orbit of Neptune lies the Kuiper Belt. Located about 40 AU from the Sun. Home of TNO‟s Hundreds of objects smaller and larger than Pluto have been found here. How to Be a Planet Once upon a time, be a wanderer in the night sky. When Ceres and Vesta (asteroids) were discovered, we had 10 planets. Pluto made 11. Ceres and Vesta were demoted to asteroids. Since 2006, Be massive enough that your own gravity pulls you into a spheroid shape. Be the dominant mass in your orbital neighborhood. Pluto makes the cut in the first category but not the second. Meet the first criterion, you can be a dwarf planet. The Oort Cloud The Solar System is surrounded by a cloud of cometary bodies. Located about 50,000 AU from the Sun, out to perhaps a light-year. Gravitational influences from passing stars occasionally send comets into the Solar System. Possibly containing more than a trillion comet-like bodies. Rotation and Revolution in the Solar System Because of the conservation of angular momentum, all planets revolve around the Sun in the same direction and in more or less the same plane. Mercury‟s orbit is tipped by 7 degrees. Pluto‟s is tipped by 17 degrees. Most of the planets rotate in the same direction. Counterclockwise as viewed from above. Venus rotates clockwise as viewed from above. Uranus and Pluto‟s rotational axes are tipped significantly. Composition of the Solar System Objects Spectrum analysis shows us the Sun is 71% hydrogen, 27% helium, 2% everything else. Jovian planets have similar composition to our Sun although so cold that the matter is liquefied or frozen. Also, frozen methane, ammonia, and water. Inner planets are rocky, silicon oxide, iron, aluminum, etc. Spectroscopy tells us surface composition. Calculating a Planet‟s Density Calculate the planet‟s mass (M) by observing its satellite‟s orbital distance (d) and period (P). rd Uses Newtons modified form of Kepler‟s 3 law: Average Density tells us a lot Inner planets have high average densities (~5 kg/liter) Small bodies Mostly rock and iron Outer planets have lower densities (~1 kg/liter) Larger bodies Gasses, ices and other volatile gasses The Role of Mass and Radius Mass and size of a planet help determine its environment. Small planets cooled quickly, leading to dead worlds with little seismic activity. Small planets also have trouble holding an atmosphere. (low gravity). Larger planets hold on to their heat, and have active interiors and surfaces. Mars is right in the middle, not too large, and not too small. Once had water and an active surface. Now is cold and dead. The Role of Water and Biological Processes The presence or absence of water helps determine the nature of the atmosphere. Water acts as a sink for carbon dioxide, removing it from the atmosphere. Water helps lock CO2 into rocks as well. Too much CO2 can lead to a runaway greenhouse effect (as with Venus). Too little CO2 can lead to cooling (as on Mars). Biological activity impacts the environment, too. Animals remove oxygen from the atmosphere (and get carbon from plants), and release CO2 (and methane). Plants remove CO2 from the atmosphere, and with sunlight and water, convert it into our food, and release oxygen. Burning (wood, fossil fuels) releases CO2 into the air. The Role of Sunlight A planet‟s distance form the Sun determines how much sunlight it receives. Venus receives ¼ of the energy per square meter that Mercury does. Planets in eccentric orbits receive varying amounts of sunlight. The axial tilt of a planet determines its seasons. Sunlight warms a planet, but the atmosphere has an impact, too Venus‟s atmosphere warms the surface to 750 K, but it would be very warm even without the CO2 Mercury is closer to the Sun, but still cooler than Venus. The Moon is cooer than the Earth, even though they are at the same distance from the Sun. Sunlight also determines the makeup fo the planets. Inner planets are rocky. (iron) Outer planets are gaseous. The Outer Planets Far from the Sun, temperatures are cold enough that water vapor can condense into ice. Beyond the frost line, planets are primarily composed of hydrogen and various ice. The low temperatures allowed the outer planets to capture hydrogen and helium gas, and to grow to immense sizes. High Rotational Speed The outer planets rotate much faster than the terrestrial planets. These high rotational speeds: Make the outer planets much wider at the equator. Create dramatic Coriolis Effects. Create strong magnetic fields. Other Differences All the gas giants have a ring system Some are easy to see like Saturn‟s Others are faint and not visible form Earth. The gas giants have many more moons Coriolis Effect Due to rotation of planet (or moon, or sun). Due to differential rotation speed at different latitudes. E.g. Higher latitudes are moving at slower speed. Causes counterclockwise rotation of weather systems in Northern Hemisphere (incl. Tornadoes, hurricanes) Coriolis Forces effect aircraft and long-range projectiles (missiles, cannon shells.) An insignificant effect on a small system like the water in your tub or toilet. Any “common” demo of such is faked. Winds Rapid rotation gives rise to strong Coriolis forces, and very high winds. Measured max wind speeds of 500 km/hr at Jupiter, and faster at Saturn. Bands of clouds move in opposite directions, creating very large wind shears, vortexes, strong turbulence. The Great Red Spot On Jupiter, these wind shears give rise to enormous vortices, or storms, seen as white, brown or red ovals in its clouds. The Great Red Spot on Jupiter is one such vortex Has lasted for at least 300 years Storms on Saturn Has turbulent storms as well as Jupiter. Higher wind speeds than Jupiter Storms are deeper in its atmosphere. Magnetic Fields The liquid metallic hydrogen in Jupiter and Saturn can carry electrical currents, similar to the liquid core of the Earth. These currents generate very large magnetic fields. Jupiter‟s is 20,000 times as strong as Earth‟s, and if it were visible, would appear larger than the full Moon in our sky. Saturn‟s field is 500 times as strong as Earth‟s. Both Jupiter and Saturn experience auroras (like the Northern or Southern Lights on Earth) Discovery of Uranus 1782 discovered by W. Herschel Originally thought to be a comet Discovery of Neptune Uranus was not following its calculated orbit. Another planet, further out, must be effecting its orbit. Scientists calculated where the unseen planet should be. (1840‟s) Astronomers searched in this location and found Neptune. Others (Galileo) had seen Neptune but didn‟t realize they were seeing a (new) planet. As of 2011, Neptune has made ONE orbit since it was discovered in 1846. The Atmospheres of Uranus and Neptune The atmospheres of both planets are rich in hydrogen and helium. Both have larger amounts of methane, giving hem their blue color. Methane crystals scatter blue light, and methane gas absorbs red light. Their interiors are believed to be ordinary water mixed with methane and ammonia, surrounds a core of rock and iron-rich material. Rotational speeds create similar atmospheric conditions - storms, winds - as on Jupiter and Saturn. Densities: Uranus: 1.3 kg/liter Neptune: 1.6 kg/liter Both planets are very cold Uranus: 80K Neptune: 75K Uranus‟s Axial Tilt Uranus is tipped almost 90 degrees to the ecliptic plane. Possible that a collision early in its history tipped the axis, and broke out material that formed its moons. This inclination means that for one part of Uranus‟ orbit, one hemisphere is in uninterrupted daylight, while the other hemisphere is in darkness. Odd Magnetic Fields Both Uranus and Neptune have strong magnetic fields. Uranus: 47x Earth Neptune: 25x Earth Possibly generated by currents in the liquid water in their interiors. Not centered on the center of the planet and tipped in odd directions. Earth‟s Magnetic Field Earth‟s magnetic north pole and the “north pole” (i.e. North end of axis) are not in the same location. Earth‟s magnetic north (and south) pole aren‟t fixed but change over time. The poles have “flipped” throughout history. Last one,780,000 years ago. We may be “due” for a flip again. Results not likely to be catastrophic but could be interesting if so.. The Galilean Satellites The four largest moons of Jupiter are called the Galilean satellites, in honor of their discoverer. Visible through even a small telescope or binoculars. Their positions change rapidly; orbital periods ranging from 2 to 17 days. One can see relative motion in even one evening‟s viewing. The Galilean satellites would be considered planets if they orbited the Sun independent of Jupiter. Temperatures rang from 110-130 K. The Origin of Planetary Rings If a body held together only by gravity gets too close to a planet, tidal forces pull it apart. This distance is called the Roche Limit. Solid bodies are safe, (chunks of rock or ice, or even the space station) as they are held together by forces other than gravity. The fragments of the broken-up satellite go into orbit around the planet, f
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