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Lecture 4

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

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Lecture 4

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  1. Lecture 4 Ast 1001 6/6/07

  2. 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

  3. Transitions • Electrons can change energy levels by gaining energy • If electrons gain too much energy, then the atom will become ionized

  4. 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.

  5. 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

  6. Emission

  7. Absorption

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. The Atmosphere • Light pollution • Twinkling (turbulence) • Air in the atmosphere moves, modifies light • Can put telescopes above the atmosphere • Use adaptive optics

  18. 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

  19. 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

  20. Arrays • Interferometry greatly increases angular results • Not nearly as efficient for increasing light collecting ability

  21. 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

  22. 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

  23. 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

  24. 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

  25. 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)

  26. 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

  27. 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

  28. 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

  29. 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

  30. 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 potassium-40, and no argon-40, you can look at how much argon you have now and figure out how old the material is

  31. 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)

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