Astronomy 1010 Planetary Astronomy Fall_2015 Day-19
Course Announcements Dark Sky nights – Wed. 10/7 starting at 7:30pm – at the Observatory. Exam-2 will be Friday, Oct. 9; Ch. 3, 4, & 5 SW-chapter 5 posted: due Fri. Oct. 9 No lab next week – Fall Break CHECK YOUR LAB GRADES IN D2L. If you are missing grades, and have gotten the lab back, return it so we can enter the grade. (mostly Scientific Methods, but might be others as well.)
Doppler Shifts Redshift (to longer wavelengths): The source is moving away from the observer Blueshift (to shorter wavelengths): The source is moving towards the observer = wavelength shift o = wavelength if source is not moving v = velocity of source c = speed of light
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
i_Clicker Question Light & Spectra: Hydrogen Doppler Shift Light & Spectra: Racetrack Radar
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.”
i_Clicker Question Light & Spectra: Blackbody Curve Laws Light & Spectra: Blackbody Curve Temperatures
Brightness is the amount of light arriving at a particular place. Decreases as the distance from a light source increases, obeying an inverse square law. The light spreads out over a greater area.
i_Clicker Question Light & Spectra: Inverse Square Flux
Equilibrium Temperature Balance between absorbed and radiated energy. Albedo (reflectance) of a planet. a = 1: 100% reflection a = 0 : 100% absorption Energy absorbed = R 2 * L/4 d 2 * (1 – a) Energy Radiated = 4 R 2 * T 4
Radiation laws help figure out the equilibrium temperatures of the planets. Distant planets are cold mainly because of the inverse square law of light. Balance of heating and cooling.
A stable equilibrium is a balance that is unlikely to change, while an unstable equilibrium can easily be nudged away from its balance point. Static equilibrium involves a situation where forces are balanced and opposing each other, so nothing changes. Dynamic equilibrium involves a situation that is always changing, but remains in balance. CONNECTIONS 5.1
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
The equilibrium temperature of a planet depends on the energy it receives and its albedo (a), its reflectivity. MATH TOOLS 5.4
The equilibrium temperature of a planet depends on the energy it receives and its albedo (a), its reflectivity. Simplifying: MATH TOOLS 5.4
PROCESS OF SCIENCE Confirmation of an idea from different fields of science can be a strong indication of the truth of that idea.
Exam-2 To Here!