Atmospheric Transparency General rule: radiation interacts with objects ~λ in size … UV and other short-λ : ozone (Freon problem) IR : CO 2 (global warming/greenhouse effect) Lack of non-optical windows: why NASA does astronomy!

Blackbody (“thermal”) radiation: Qualitative characteristics Some energy at all λ’s Hotter BB emits more energy at all λ’s Maximum occurs at shorter wavelength for hotter BBs

Temperature scales…

Blackbody radiation: Quantitative characteristics Planck Curve equation

Blackbody radiation: Wien’s Law (From differentiating Planck curve and setting = 0) Example: light bulb ~3,000 K λ max = 2.9x10 7 /3x10 3 = 1x10 3 Å = 10,000 Å Where in spectrum? (Pasachoff, p.25)

Eye response We see what is there to be seen Centered on blue-green like sun Thus sun (and similar stars appear white, not green But, cool stars appear red, hot blue-white

Blackbody radiation: Stefan-Boltzmann law Found by integrating (summing up) all energy at all wavelengths for a given temperature. Result: E = σ T 4 (energy/area of the BB) So, energy/area of a BB proportional to T 4 Total brightness or luminosity of a blackbody, such as a star, is product of E and the total area: L = 4πR 2 σ T 4 Some sample calculations…

Spectral Lines Fraunhofer discovered lines in solar spectrum in the 1800s Absorption lines are where light is missing.

Emission Lines In artificial lighting this is how more energy is put into the visible spectrum (compared to the thermal blackbody radiation of incandescent bulbs). Some objects show bright emission lines, the spectrum of neon is shown above. In astronomy …

Stellar and nebular spectra What we see from different angles. For a star, gas is its cooler atmosphere layers What’s going on? …. (Paint/demo)