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Transcript
Lecture 4 Ast 1001 6/6/07 Energy in Atoms • Electrons can only exist in specific states called Energy Levels • Electron volts (eV) are often used to measure the energy of the levels • Ground state is the lowest energy level • Excited states are energy levels with more energy than the ground state Transitions • Electrons can change energy levels by gaining energy • If electrons gain too much energy, then the atom will become ionized Spectroscopy Basics • Reading the information from a spectrum is called Spectroscopy • Usually done graphically • Amount of radiation, or Intensity on y axis • Energy, wavelength, or frequency of light on the x axis. Types of Spectra • Continuous spectra are those that show all of the colors in the rainbow – Incandescent lightbulb is the primary example • Emission Spectrum only has discrete bright lines – Fluorescent lightbulbs • Absorption Spectrum has most of the rainbow, but dark lines have been removed Emission Absorption How Do You Get the Spectra? • Continuous spectra come from “blackbody” sources • Emission spectra come from something like a cloud of gas that is getting energy from somewhere • Absorption spectra is when light from a continuous source is absorbed Why Spectroscopy is Important • Each atom (or molecule) has unique lines – Thus, we can figure out what things are made out of by analyzing the light • If the source is continuous (or mostly so) we can tell what temperature it is – Hotter things peak at higher energies Some Math • Stefan-Boltzmann law tells you how much energy per unit area is coming off of a source – Emitted power = σ*T4 • Wien’s Law is the relationship between temperature and spectroscopic peak – λmax = 2,900,000/Temperature The Doppler Shift • Common example: a siren moving towards and then away from you • If its moving away from you, light is redshifted • If its moving towards you, light is blueshifted • Can also be used to determine rotation rates Detectors • Astronomical detectors are usually CCDs • Consist of a number (millions?) of pixels – When light hits the pixel, electrical charge builds up • Far superior to film for a number of reasons – Much more sensitive to light – Can work at many different wavelengths Why Use Telescopes? • To collect light – Ability to collect light depends on the area of the aperture • To increase angular resolution – Angular resolution is the ability to tell that two nearby dots are distinct – Telescopes can be 60x better at resolving angles than the human eye The Kinds of Telescopes • Refracting Telescopes – Uses lenses – The earliest telescopes were refracting – Largest refractor is 1 meter in size • Reflecting Telescopes – Uses mirrors – Professional telescopes are reflecting – Largest reflectors are 10 meters in size What Astronomers Do • Imaging – Basically taking pictures – Usually the images are in black and white – Filters are heavily used • Spectroscopy – Use a diffraction grating to split light into parts – Spectral resolution is how much information we can get from the spectral lines What Astronomers Do cont. • Timing – Many objects vary with time – Measure brightness over time • Getting time to observe is difficult – Most astronomers only observe a couple of times a year – Time on telescopes is very competitive The Atmosphere • Light pollution • Twinkling (turbulence) – Air in the atmosphere moves, modifies light – Can put telescopes above the atmosphere – Use adaptive optics Non Visible Light • Radio telescopes – Basically reflecting telescopes – Can be very large • Infrared telescopes – Similar to visible light – Atmospheric problems are greater for IR than visible light More Non Visible Light • Ultraviolet Telescopes – Must be above atmosphere – Currently somewhat unpopular • X-Ray/Gamma Ray astronomy – Must be above atmosphere – Fairly recent kind of astronomy Arrays • Interferometry greatly increases angular results – Not nearly as efficient for increasing light collecting ability Solar System Properties • Patterns of motion already discussed – Planets revolves, orbits Sun • Two kinds of planets – Small, rocky, terrestrial planets – Large, gassy, jovian planets • Lots of little rocky objects – Asteroid belt, Kuiper belt, Oort cloud Nebular Theory • First proposed by Kant (1755), Laplace (1795) • Involves gravitational collapse of a cloud of gas • Most of the gas formed the Sun, leftover gas formed the planets Where Did the Gas Come From? • Stars die, spew out their guts – Supernovae, novae • Our Sun is a second generation star • We can see other gas clouds forming – Can also see stars forming within gas clouds – Best example: the Orion Nebula From Gas to the Solar System • Initially gas was spread out over several light-years • Gas starts to collapse – Temperature rises – Cloud begins to rotate more and more quickly – Flattens Planet Formation • Problem: solar nebula was 98% hydrogen/helium, planets aren’t • Planets formed via condensation • Early solar system breakdown: – – – – 98% hydrogen/helium Hydrogen compounds (1.4%) Rock (.4%) Metals (.2%) • Frost Line dictates where hydrogen compound things form (jovian planets) and where rocky things form (terrestrial planets) More Planet Formation • Accretion is the primary mechanism for planet growth – Small particles build up planetesimals – Planetesimals combine to form planets • Jovian planets got big enough that their gravity was great enough to capture hydrogen and helium gas The End of Planet Formation • Eventually the solar wind pushed all of the gas out into interstellar space • Sun was spinning much more quickly • Eventually the Sun’s magnetic field dispersed its angular momentum After Planet Formation • Lots of planetesimals left over – Became comets, asteroids • Bombardment Phase – Planets knocked around – Water (probably) brought to Earth – Moon formed • Moons captured by big planets The Age of the Solar System • The Solar System is about 4.6 billion years old. How do we know this? • Atoms are identical: young atoms are indistinguishable from old atoms • Atoms can undergo spontaneous radioactive decay and turn into other elements or isotopes Radioactive Dating • Example: potassium-40 can decay into argon-40 – Potassium is the parent isotope, argon is the daughter isotope • Rate at which atoms decay is characterized by the half-life • If you start out with a given amount of potassium40, and no argon-40, you can look at how much argon you have now and figure out how old the material is Group Work • Lets say that you know a rock is 3 billion years old and measure that it currently has .75 units of potassium-40. How much argon-40 would you predict that the rock should have? (Hint: follow the example on the bottom of page 241)