1. Blackbody Radiation A blackbody is something that absorbs all radiation that shines on it Are all blackbodies black? - no!! - imagine a box full of lava A constant temperature blackbody (a.k.a. a very precise oven) Order your own blackbody online

2. Blackbodies and Astronomy Stars are very similar to blackbodies  emit a continuous spectrum of radiation Why aren’t they black?

3. Blackbodies emit light at all wavelengths Cooler blackbodies emit more red than blue light.

4. Planck Function I ν (T) is the specific intensity, depends on T and ν T is temp in Kelvin (Kelvin = Celsius + 273.2) h is Planck’s constant: 6.636 x 10 -34 J s k is Boltzmann’s constant 1.38 × 10 -23 m 2 kg s -2 K -1 c is the speed of light

5. Rayleigh Jeans Limit At low frequencies, hν << kT so the Planck function can be approximated as

6. Properties of the Planck Law Wien Displacement Law: m  T = b, where m  is the wavelength at which I peaks, and b (=0.0029 m K) is called the Wien displacement law constant. Alternatively, ν m / T = 5.88 x 10 10 Hz K -1, where ν m  is the frequency at which I ν peaks. Note that

7. Wien’s Law The peak wavelength of a blackbody spectrum is inversely proportional to temperature: Temperatures of stars and planets are measured using Wien’s law.

8. Star B Star C Star D Which star is the hottest? Red Blue Wavelength

9. Properties of the Planck Law Stefan-Boltzmann law: F =  T 4, where F is the total radiated power per unit area (W per square m) and  is called the Stefan-Boltzmann constant: 5.67 x 10 -8 W m -2 K -4 

10. Stefan-Boltzmann Law: Hotter blackbodies emit more total energy  notice the area under the blackbody curve: 5000 K 4000 K

11. A perfect blackbody produces a continuous spectrum: Dark lines in solar spectrum are from absorption by Sun’s outer atmosphere

12. Emission Line Spectra Take a thin cloud of gas composed of a pure element (e.g. hydrogen) and heat it to high temperature  It does not emit a continuous spectrum. It emits light at specific wavelengths:  the exact same ones at which it absorbs

13. The role of density If thin gases produce only emission lines, and the Sun is made of gas, why does the Sun’s spectrum look continuous (like a rainbow?) Solar spectrum

14. Low-density A high-density gas cloud produces a continuous spectrum http://astro.unl.edu/animationsLinks.html