11ASTR 1020 Introductory Astronomy 2: Stars & GalaxiesJanuary 25, 2008Professor Jack BurnsNewcomers - All class info is at website:http://solo.colorado.edu/~jaburns/Astr1020Sp08/index.html2Astronomy In the News: CU-BOULDER SPACE SCIENTISTS READY FOR MESSENGER MISSION FLYBY OF MERCURY• For more information on the MESSENGER mission, including images, photos, animation and videos, visit the Web at: http://messenger.jhuapl.edu/. For more information about LASP, visit the Web at: http://lasp.colorado.edu/.3Homework • Reading: Chapter 5, sections 5.3- 5.5; summary of key concepts.• MasteringAstronomy Tutorials & Exercises – Scales of the Universe (complete by Jan. 28). For calculation problems, use at least 3 significant digits – example: 1.34 or 134. Correct answer has to be within 2%.• Clicker points start on Monday!4Today’s ClassChapter 5: Matter and Light• Atoms and Molecules• Light Waves andParticles• The ElectromagneticSpectrum5Matter: a Material WorldAtoms: nucleus made of protons and neutronsA surrounding cloud made of electrons (please try to get rid of the “solar system” vision of atoms!)Electrons are held onto the atom by electric force.Electrons have negative electric charge, protons are positive. Neutrons are neutral.6Atomnucleuselectrone-protonsneutronsp+n27Hydrogen: simplest and most commone-p+atomic number = 1atomic mass number = 18Heliume-p+ne-np+atomic number = 2atomic mass number = 49Periodic Table of the Elementsatomic number = #protonsatomic mass no. = #protons + #neutrons10p+ne-np+atomic number = 2atomic mass number = 4What if an electron is missing?ionHe+111What if two or more atoms combine to form a particle?p+p+8p+8nmoleculeH2O (water)12Wave Nature of Light“Speed of light” = c= 300,000 km/sec.Wavelength of light is the distance from crest to crest = λ (“lambda”), measuredin meters.Frequency, f, is how many wave peaks pass by in 1 second (cycles per second),f = c / λ.313Particle Nature of Light• Light can also be thought of as a particleÆ “photon”A photon is a mass-less particle of electromagnetic radiation energy????? Dual nature of light ?????“wave-particle duality”Seen in other particles (e.g., electrons) as well14Photons can have any energy from tiny to giganticThe greater the energy of the photon (E), the higher the frequency, smaller the wavelength:E ~ f E ~ 1 / λ15The Electromagnetic spectrumvideo16Spectra and spectral analysis• A spectrum shows the intensity of light versus wavelength, frequency OR energyCan be created by passing light through a prism or other opticsSpectral analysis uses what we know about how light is emitted by and interacts with matter- by looking at the light’s spectrum, we can tell something about its sourceNote: rainbow is only accurate for Visible light; IR and UV are invisible and have no color!Note dark bands where light is absent and areas where light is more intense17Clicker QuestionFrom the spectrum shown here:A) Most light is being emitted in the infraredB) The color of the object to our eyes would be blue-ishC) The color to our eyes would be very dark redD) The object would have faint stripes E) None of the above18• E: None of the above. The color in visible light would be yellow/orange, with some invisible emission in the infrared. The fine features would not be discernable to our eyes.419• Objects can look very different depending on the wavelength of light you are detecting:• Sun as seen in visible, UV, X-ray and radio light20Emission from Atoms• IF electrons are in a high energy state, they can transition to a lower energy state by emitting a photon of the according energy. Energy is conserved!21• Each atom has a different set of energy levels Ædifferent emission/absorption spectrum• Examples: mercury, sodium, neon, hydrogen, mercury…. • Demo: diffraction grating spectroscopes22Most common visible light emission line:• “Hydrogen Alpha”• n=3 to n=2 energy jump at 656.3 nm• The universe is mostly