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as101 Final Notes.docx

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Shohini Ghose

Nature of Light The warmth of sunlight tells us that light is a form of energy: radiative energy • We can measure the flow of energy in lightin units of watts: 1 watt = 1 joule/s Properties of Waves Wavelength is the distance between two wave peaks Frequency is the number of times per second that a wave vibrates up and down wave speed = wavelength x frequency Wavelength, Frequency, and Energy  x f = c  = wavelength , f = frequency c = 3.00 x 108 m/s = speed of light E = h x f = photon energy h = 6.626 x 10-34 joule x s = Planck’s constant The electromagnetic spectrum Interactions of Light With Matter  Emission: Energy in matter can be converted into light that is emitted  Absorption: Matter can absorb energy in the form of light and convert it to another form or re-emit it  Transmission o Transparent objects transmit light o Opaque objects block (absorb) light  Reflection or Scattering: Light can bounce off objects inone direction (reflection) or random directions (scattering) We see objects that emit light directly We see others by light reflecting off these objects. Reflection and Scattering Mirror reflects light in a particular direction Movie screen scatters light in all directions The structure of matter Atoms are the building blocks of matter. Every element is made up of a different type of atom All atoms are made of - protons with positive charge of +1 - neutrons with 0 charge - electrons with negative charge of -1 Atoms of different elements have a different number of protons Chemical properties depend on number of electrons in the atom Atomic Terminology • Atomic Number = # of protons in nucleus • Atomic Mass Number (weight) = # of protons + neutrons Neutral atoms have no net charge: number of protons = number ofelectrons Ions are either positively charged (number of electrons < number of protons) or negatively charged (number of electrons > number of protons). Atomic Terminology • Isotope: same # of protons but different # of neutrons. (4He, 3He) • Molecules: consist of two or more atoms (H2O, CO2) Summary • What is light? – Light can behave like either a wave or a particle. – A light wave is a vibration of electric and magnetic fields. It does not need a medium to propagate. – The wavelength/frequency of light determines colour. – Speed of light is constant. – Photons are particles of light. • What is the electromagnetic spectrum? – Human eyes cannot see most forms of light. – The entire range of wavelengths of light is knownmas the electromagnetic spectrum. • How does light interact with matter? – Matter can emit light, absorb light, transmit light, and reflect (or scatter) light. – Interactions between light and matterdetermine the appearance of everything we see. Light is the cosmic messenger • Light travels to us from all parts of the universe • Matter in the universe interacting with light leaves its fingerprints in the light • Spectroscopy is the process of dispersing light into its spectrum (different wavelengths) and deciphering the information in each part of the spectrum • A spectral graph shows the intensity or energy (number of photons) at each wavelength/frequency. • Spectroscopy can tell us about the composition, temperature and motion of objects in the universe Three basic types of spectra Spectra of astrophysical objects are usually combinations of these three basic types Continuous (Thermal) Spectrum • The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption • Continuous spectra are observed from hot, dense objects. Properties of Thermal Radiation 1. Hotter objects emit more light at all frequencies per unit area. 2. Hotter objects emit photons with a higher average energy. Emission Line Spectrum • A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines Absorption Line Spectrum • A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the continuous spectrum Chemical Fingerprints in Light • Each type of atom, molecule and ion has a unique spectral fingerprint of absorption or emission lines How does light tell us the speed of a distant object? The Doppler Effect: The frequency of waves measured by an observer changes if the source of the waves is moving. Ex: Change in sound of siren as ambulance passes. Doppler Effect for Light • We generally measure the Doppler Effect from shifts in the wavelengths of spectral lines • Red shift: object moving away • Blue shift: Object moving towards us The larger the shift the faster the object is moving How does light tell us the rotation rate of an object? • Different Doppler shifts from different sides of a rotating object spread out its spectral lines Spectrum of a Rotating Object • Spectral lines are wider when an object rotates faster Summary What are the three basic type of spectra? – Continuous spectrum, emission line spectrum, absorption line spectrum How does light tell us what things are made of ? – Each atom has a unique fingerprint. – We can determine which atoms something is made of by looking for their fingerprints in the spectrum. How does light tell us the temperatures of planets and stars? – Nearly all large or dense objects emit a continuous spectrum that depends on temperature. – The spectrum of that thermal radiation tells us the object’s temperature. How does light tell us the speed of a distant object? – The Doppler effect tells us how fast an object is moving toward or away from us. • Blueshift:objects moving toward us • Redshift: objects moving away from us How does light tell us the rotation rate of an object? – The width of an object’s spectral lines can tell us how fast it is rotating The eye • Refraction can cause parallel light rays to converge to a focus • The focal plane is where light from different directions comes into focus. • The image behind a single (convex) lens is actually upsidedown! Your brain flips the image. Camera Digital cameras detect light with charge-coupled devices (CCDs) • A camera focuses light like an eye and captures the image with a detector (film or CCDs) • The CCD detectors in digital cameras are similar to those used in modern telescopes Camera • What are the advantages of a camera over the eye? – Image can be reliably stored for later analysis. – Image has more details – Exposure time (amount of light hitting detector) can be controlled. Faint objects can be observed with long exposure times). • What are the advantages of CCDs over film? – More sensitive to light. – Broader dynamic range: bright and faint objects can be recorded at the same time. – Image stored as digital data that can be processed on a computer. Basic designs of telescopes • Refracting telescope: Focuses light with lenses • Reflecting telescope: Focuses light with mirrors 30 Refracting Telescope Angular magnification M is large when f i1 much greater than f 2 Refracting Telescope • Need to be very long, with large, heavy lenses Reflecting Telescope • Reflecting telescopes can have much greater diameters • Most modern telescopes are reflectors • What are the advantages of a reflecting telescope over a refracting telescope ? – Only the reflecting surface of mirrors in a reflecting telescope have to be perfectly shaped. In a lens the entire shape of the lens and both surfaces are important. – Objective lenses are heavy and difficult to stabilize at the top of the telescope. Heavy mirrors at the bottom of the telescope are less problematic – Lenses have chromatic aberrations that must be corrected. What are the two most important properties of a telescope? 1. Light-collecting area: Telescopes with a larger collecting area can gather a greater amount of light in a shorter time. 2. Angular resolution: Telescopes that are larger are capable of taking images with greater detail. Light Collecting Area • A telescope’s diameter tells us its light collecting area: Area = π(diameter/2)2 • The largest telescopes currently in use have a diameter of about 10 meters Angular Resolution • The minimum angular separation that the telescope can distinguish. Recall that Angular separation= actual separation x 360 degrees/(2π x distance) • Ultimate limit to resolution comes from interference of light waves within a telescope. • Larger telescopes are capable of greater resolution because there’s less interference • The rings in this image of a star come from interference of light wave. • This limit on angular resolution is known as the diffraction limit • Diffraction limit depends on the wavelength of light and diameter of the telescope What do astronomers do with telescopes? • Imaging: Taking pictures of the sky • Spectroscopy: Breaking light into spectra • Timing: Measuring how light output varies with time Imaging • Astronomical detectors generally record only one colour of light at a time • Several images must be combined to make fullcolour pictures • Astronomical detectors can record forms of light our eyes can’t see • False-colour or colour coded images use colour to represent - different energies of nonvisible light - different atoms in the object Image from Chandra X-ray telescope Spectroscopy • A spectrograph separates the different wavelengths of light before they hit the detector • Since the light is separated out, more total light (longer exposure times) is required for the same telescope to make a spectrum than to make an image. Timing • A light curve represents a series of brightness measurements made over a period of time • Timing observations study how some property of an object (ex: brightness, shape, spectrum) changes over time. How does Earth’s atmosphere affect ground-based observations? • The best ground-based sites for astronomical observing are – Calm (not too windy) – High (less atmosphere to see through) – Dark (far from city lights) – Dry (few cloudy nights) Light Pollution • Scattering of human-made light in the atmosphere is a growing problem for astronomy Twinkling and Turbulence Turbulent air flow in Earth’s atmosphere distorts our view, causing stars to appear to twinkle Transmission in Atmosphere • Only radio and visible light pass easily through Earth’s atmosphere • We need telescopes in space to observe other forms • Space telescopes also avoid the problems of light pollution and atmospheric turbulence Interferometry • Interferometery is a technique for linking two or more telescopes so that they have the angular resolution of a single large one Summary How does light propagate through different materials? – Light rays travel in straight lines that can be reflected or refracted (bent) at an interface between two materials • How does your eye form an image? – It uses refraction to bend parallel light rays so that they form an image. – The image is in focus if the focal plane is at the retina. • How do we record images? – Cameras focus light like your eye and record the image with a detector. – The detectors (CCDs) in digital cameras are like those used on modern telescopes • How does a telescope work? – Refracting telescopes focus light with lenses – Reflecting telescopes focus light with mirrors – The vast majority of professional telescopes are reflectors – Collecting area determines how much light a telescope can gather – Angular resolution is the minimum angular separation a telescope can distinguish • What are telescopes used for? – Imaging – Spectroscopy – Timing • How does Earth’s atmosphere affect groundbased observations? – Telescope sites are chosen to minimize the problems of light pollution, atmospheric turbulence, and bad weather. • Why do we put telescopes into space? – Forms of light other than radio and visible do not pass through Earth’s atmosphere. – Also, much sharper images are possible because there is no turbulence. What does the solar system look like? • Eight major planets with nearly circular orbits • 4 inner planets have orbits that are relatively closely spaced • All large bodies in the solar system orbit in the same direction and in nearly the same plane • Pluto is smaller than the major planets and has a more elliptical orbit The Sun • Radius: 695,000,000 m = 108 x radius of Earth • Mass: 333,000 x mass of Earth • Over 99.9% of solar system’s mass • Surface temperature 5800 K • Composition: 98% hydrogen and helium, 2% other elements • Sun’s gravity and radiation influences motion, visibility and temperature of all the objects in the Solar System • Solar wind (emission of charged particles) influences planetary magnetic fields and atmospheres Two Main Planet Types • Terrestrial planets are rocky, relatively small, and close to the Sun • Jovian planets are gaseous, larger, and farther from Sun Mecury • Made of metal and rock; no atmosphere • Most rich in metals • Very hot and very cold: 425°C (day), –170°C (night) Venus • Extreme greenhouse effect: Hotter than Mercury: 470°C, day and night • Atmospheric pressure like 1km underwater! • No oxygen or water • Roughly same size as Earth Earth • An oasis of life • The only surface liquid water in the solar system • A surprisingly large moon Earth and Moon to scale Mars • Looks almost Earth-like, but don’t go without a spacesuit! • Giant volcanoes, a huge canyon, polar caps, more… • Water flowed in the distant past; could there have been life? Jupiter • Much farther from Sun than inner planets • Mostly H/He; no solid surface • Many moons, rings Saturn • Giant and gaseous like Jupiter • Spectacular rings Rings are NOT solid; they are made of countless small chunks of ice and rock, each orbiting like a tiny moon. • Many moons, including cloudy Titan Uranus • Smaller than Jupiter/Saturn; much larger than Earth • Made of H/He gas & hydrogen compounds (H O, NH2, CH ) 3 4 • Extreme axis tilt • Moons & rings Neptune • Similar to Uranus (except for axis tilt) • Looks blue • Many moons (including Triton) • Has rings Pluto: A Dwarf Planet • Much smaller than other planets: Dwarf planet • Icy, comet-like composition, not like other planets • Density similar to Jovian planets • Its moon Charon is similar in size • The plane of its orbit is tilted • Other dwarf planets: Eris, Ceres Formation of the Solar System What properties of our solar system must a formation theory explain? 1. Patterns of motion of the large bodies • Orbit in same direction and plane 2. Existence of two types of planets • Terrestrial and jovian 3. Existence of smaller bodies • Asteroids and comets 4. Notable exceptions to usual patterns • Rotation of Uranus, Earth’s moon, etc. What theory best explains the features of our solar system? • The nebular theory states that our solar system formed from the gravitational collapse of a giant interstellar gas cloud—the solar nebula (Nebula is the Latin word for cloud) • Kant and Laplace proposed the nebular hypothesis over two centuries ago • A large amount of evidence now supports this idea • Predictions of this theory have been accurate Galactic Recycling • When the solar system formed 4.6 billion years ago, about 2% of the original hydrogen and helium in the universe had been converted to other elements. • The solar system was born from a cloud of gas (solar nebula) consisting of 98% hydrogen and helium and 2% of other elements Conservation of Angular Momentum • Initial slow rotation speed of the cloud increased as the cloud contracted Recall conservation of angular momentum Heating • Collapse of the cloud caused gravitational potential energy to be converted to kinetic energy of gas particles. • Collisions of these fast moving particles converted some kinetic energy to thermal energy (heat). • The sun formed at the centre where temperatures and densities were highest. Flattening • Collisions between gas particles in cloud gradually reduce random motions leaving only the net spin • Collisions between gas particles also reduce up and down motions • Spinning cloud flattens as it shrinks Motion in the Solar System • The spinning disk explains the uniform motions observed in the Solar System today: • Planets all orbit in the same direction of spin of the disk they were formed from • Planets orbit in the same place because of the flattening of the disk they were formed from. Why are there two types of planets? Inner parts of disk are hotter than outer parts. Rock can be solid at much higher temperatures than ice. Inside the frost line: Too hot for hydrogen compounds to form ices. Only rocks and metals can be solid Outside the frost line: Cold enough for ices to form. How did terrestrial planets form? • Small particles of rock and metal (seeds) were present inside the frost line • Planetesimals of rock and metal built up as these particles collided. This process is called accretion • Gravity eventually assembled these planetesimals into terrestrial planets over a few million years. • Collisions between planetisimals destroyed all but the largest, which grew into planets. How did Jovian planets form? • Ice could also form small particles outside the frost line. • Larger planetesimals and planets were able to form. • Gravity of these larger planets was able to draw in surrounding H and He gases. • Moons of jovian planets form in miniature disks around Jovian planets Solar Wind and Solar Rotation •Outflowing matter from the Sun -- the solar wind – blew away the leftover gases and ended planet formation •In nebular theory, young Sun was spinning much faster than now •Friction between solar magnetic field and solar nebula probably slowed the rotation over time Asteroids and Comets • Leftovers from the accretion process • When solar wind blew away remaining gas, leftovers outside Neptune’s orbit could not form a planet • Rocky asteroids inside frost line • Icy comets outside frost line • Water may have come to Earth by way of icy planetesimals from outer solar system How do we explain ―exceptions to the rules‖? Heavy Bombardment • Leftover planetesimals bombarded other objects in the late stages of solar system formation Evidence supporting this theory: Moon’s composition is similar to outer layers of the Earth. •Giant impacts might also explain the different rotation axes of some planets Radioactive Decay • Some (parent) isotopes in a rock decay into other (daughter) nuclei • Example: Potassium- 40 decays into Argon- 40 • A half-life is the time for half the nuclei in a substance to decay The amount of parent isotopes and daughter isotopes in a rock allows us to calculate the age of a rock. • Example: The half-life of Potassium-40 is 1.25 billion yrs. • Thus a rock with equal amounts of Potassium-40 and Argon-40 is 1.25 billion years old. • After 2 half-lives (2.5 billion years) half of the remainingPotassium will be converted to Argon • Thus after 2.5 billion years the amount of Argon will be three times the amount of Potassium • A rock with 3:1 ratio of Argon- 40 to Potassium-40 is 2.5 billion yrs old. When did the planets form? • Radiometric dating tells us that oldest moon rocks are 4.4 billion years old • Oldest meteorites are 4.55 billion years old • Planets probably formed 4.5 billion years ago • Therefore the solar system is roughly 4.5-4.6 billion years old. Earth’s Interior • Core: Highest density; nickel and iron • Mantle: Moderate density; silicon, oxygen, etc. • Crust: Lowest density; granite, basalt, etc. • A planet’s outer layer of cool, rigid rock is called the lithosphere. It ―floats‖ on the warmer, softer rock that lies beneath Terrestrial Planet Interiors • Applying what we have learned about Earth’s interior to other planets tells us what their interiors are probably like Reason for layers: Differentiation • Gravity pulls high-density material to center • Lower-density material rises to surface • Material ends up separated by density Heating of Interior • Accretion and differentiation when planets were young • Radioactive decay is most important heat source today • Interior heat drives geological activity: volcanism, tectonics Cooling of Interior • Convection transports heat as hot material rises and cool material falls • Conduction transfers heat from hot material to cool material • Radiation sends energy into space Role of Size in Heating and Cooling • Smaller worlds cool off faster and harden earlier • Moon and Mercury are now geologically ―dead‖ since their interiors have cooled down. Sources of Magnetic Fields • A world can have a magnetic field if charged particles are moving inside • 3 requirements: – Molten interior – Convection – Moderately rapid rotation Processes that Shape Planetary Surfaces • Impact cratering – Impacts by asteroids or comets • Volcanism – Eruption of molten rock onto surface • Tectonics – Disruption of a planet’s surface by internal stresses • Erosion – Surface changes made by wind, water, or ice Comparison of Planetary Surfaces • Mercury & the Moon • heavily cratered {scars from the heavy bombardment} • some volcanic plains • Venus • volcanoes and bizarre bulges • entire surface seems to have been ―repaved‖ 750 million years ago • Mars • volcanoes and canyons • apparently dry riverbeds {evidence for running water?} • Earth • all of the above plus liquid water and life Geology of Venus • Has a thick, cloudy atmosphere -- you can not visually see the surface • we must image the surface using radar • smooth plains with few mountain ranges • few craters • many volcanoes and domes of lava (corona) • Venus is very active with tect
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