1. Cool, invisible galactic gas (60 K, f peak in low radio frequencies) Dim, young star (600K, f peak in infrared) The Sun’s surface (6000K, f peak in visible) Hot stars in Omega Centauri (60,000K, f peak in ultraviolet) The higher the temperature of an object, the higher its I peak and f peak

2. Wien’s Law The peak of the intensity curve will move with temperature, this is Wien’s law: Temperature * wavelength = constant = 0.0029 K*m So: the higher the temperature T, the smaller the wavelength, i.e. the higher the energy of the electromagnetic wave

3. Example Peak wavelength of the Sun is 500nm, so T = (0.0029 K*m)/(5 x 10 -7 m) = 5800 K Instructor temperature: roughly 100 F = 37C = 310 K, so wavelength = (0.0029K*m)/310 K = 9.35 * 10 -6 m = 9350 nm  infrared radiation

4. Measuring Temperatures Find maximal intensity  Temperature (Wien’s law) Identify spectral lines of ionized elements  Temperature

5. Color of a radiating blackbody as a function of temperature Think of heating an iron bar in the fire: red glowing to white to bluish glowing

6. Activity: Black Body Radiation Pick up a worksheet Form a group of 3-4 people Work on the questions on the sheet Fill out the sheet and put your name on top Hold on to the sheet until we’ve talked about the correct answers Hand them in at the end of the lecture or during the break I’ll come around to help out !

7. Kirchhoff’s Laws: Dark Lines Cool gas absorbs light at specific frequencies  “the negative fingerprints of the elements”

8. Kirchhoff’s Laws: Bright lines Heated Gas emits light at specific frequencies  “the positive fingerprints of the elements”

9. Kirchhoff’s Laws 1.A luminous solid or liquid (or a sufficiently dense gas) emits light of all wavelengths: the black body spectrum 2.Light of a low density hot gas consists of a series of discrete bright emission lines: the positive “fingerprints” of its chemical elements! 3.A cool, thin gas absorbs certain wavelengths from a continuous spectrum  dark absorption ( “Fraunhofer”) lines in continuous spectrum: negative “fingerprints” of its chemical elements, precisely at the same wavelengths as emission lines.

10. Demonstration Gas Lamps Which one is He, which is H? Combined, you are looking at 99% of the (non-dark) matter content of the universe!

11. Spectral Lines – Fingerprints of the Elements Can use this to identify elements on distant objects! Different elements yield different emission spectra

12. Spectral Lines Origin of discrete spectral lines: atomic structure of matter Atoms are made up of electrons and nuclei –Nuclei themselves are made up of protons and neutrons Electrons orbit the nuclei, as planets orbit the sun Only certain orbits allowed  Quantum jumps!

13. The energy of the electron depends on orbit When an electron jumps from one orbital to another, it emits (emission line) or absorbs (absorption line) a photon of a certain energy The frequency of emitted or absorbed photon is related to its energy E = h f (h is called Planck’s constant, f is frequency)