# Astronomy 1020 Stellar Astronomy Spring_2015 Day-17.

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Astronomy 1020 Stellar Astronomy Spring_2015 Day-17

Course Announcements Smartworks Chapters 5: Due Mon. 3/2 Smartworks Chapters 6: Due Mon. 3/16 Exam-2 – Fri. 3/6 – Chapters 5 & 6(??) APSU Research and Creativity Forum April 11-12, 2014 Abstracts are due: 4:30pm Fri., March 21 Mar. 2 – Last day to drop with an automatic “W” Apr. 2 – Last day to drop a class.

1931-2015 Live Long and Prosper

 The wavelengths at which atoms emit and absorb radiation form unique spectral fingerprints for each atom.  They help determine a star’s composition, temperature, and more.

 The motion of a light source toward or away from us changes our perception of the wavelength of the waves reaching us.  Doppler effect.

 If you know the wavelength of light you are observing as well as the wavelength of light the object would be emitting if it were at rest, you can find the speed of the object using the Doppler effect. MATH TOOLS 5.2

 Light from approaching objects is blueshifted; the waves crowd together.  Light from receding objects is redshifted; the waves are spaced farther apart.

Lecture Tutorial Doppler Shift: (pg. 75) Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group.

Concept Quiz—Doppler Shift Hydrogen emits light at = 656 nm. You see a distant galaxy in which the light from hydrogen has = 696 nm. This galaxy is A. moving toward us. B. moving away from us.

 Temperature is a measure of the average speed of the motions of atoms.  Kelvin scale: Water freezes/boils at 273 K / 373 K.  Absolute zero is when thermal motion stops.

Emitted Light  Luminosity: amount of light leaving a source.  The amount and type of light leaving a source changes as an object heats up or cools down.  The hotter an object is, the more luminous it is.  The hotter an object is, the bluer it is.

 Dense objects emit a blackbody (or Planck) spectrum.  Continuous.  Gives light at all wavelengths.  Example: incandescent light bulb.

 For two objects of the same size, the hotter one will: Emit more total light at all wavelengths. Emit more total energy every second. Emit light at shorter wavelengths, on average.

Stefan’s Law  Flux is the total amount of energy emitted per square meter every second (the luminosity per area).  Then: where T is the temperature, F is the flux, and  (sigma) is called the Stefan- Boltzmann constant.  Hotter objects emit much more energy (per square meter per second) than cool objects.

Wien’s Law  The peak wavelength of a blackbody is inversely proportional to its temperature.  Peak wavelength peak : the wavelength of light of a blackbody that is emitted the most.  Here the wavelength is in nanometers and the temperature is in kelvin.  “Hotter means bluer.”

 With the Stefan-Boltzmann law, you can find Earth’s flux using its average temperature of 288 K.  Using Wien’s law, you can find the Sun’s surface temperature using the fact that its peak wavelength is around 500 nm. MATH TOOLS 5.3

Luminosity is the total energy (light) emitted by an object in each second. Stefan-Boltzmann law Luminosity depends on an surface area (A), and its temperature (T 4 ); Surface Area ∝ R 2 Luminosity = 4π R 2  T 4 Big and Hot objects have greater luminosity than small cool objects

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