1	Astr 1050 Fri. Oct. 3, 2003 Today: Finish Chapter 6 –Starlight and Atoms
2	Homework #3 Question 1 RADIO photons have the lowest energy. Question 2 The thickness of a plastic freezer storage bag is roughly 0.1 mm. How many wavelengths of red light is this? 140 = 0.1mm/7000Angstroms. Question 3 ALL statements are true. Question 4 Compared to gamma rays, radio waves travel at the same speed! Question 5 Calculate wavelength/diameter for each, pick the smallest value. a.A 2.3 meter telescope operating at 2 microns (like WIRO): 9x10^-7 b.The Hubble Space Telescope (2.5 meter) at 150 nanometers: 6x10^-8 c.The Very Large Array (36 km) at 43 GHz (0.65 cm): 1.8x10^-7 d.The 10-meter Keck telescope at 500 nanometers: 5x10^-8 Question 6 All statements are true. Question 7 The correct order of increasing photon energy: Radio -- Infrared - optical -- Ultraviolet -- X-ray -- Gamma Rays
3	Planck and other Formulae Planck formula gives intensity of light at each wavelength It is complicated. We’ll use two simpler formulae which can be derived from it. Wien’s law tells us what wavelength has maximum intensity Stefan-Boltzmann law tells us total radiated energy per unit area
4	Example of Wien’s law What is wavelength at which you glow? Room T = 300 K so This wavelength is about 20 times longer than what your eye can see. What is temperature of the sun – which has maximum intensity at roughly 0.5 mm?
5	Example of the Stefan-Boltzmann law Suppose a brown-out causes the temperature of a lamp filament to drop to 0.9 of its original value. By what factor does the light output of the lamp drop? Using the Stefan-Boltzmann law (with the numerical value of s) we could have calculated how big (in m²) a light filament would have to be to emit 100 W of light, at any given temperature. We could also find the size of a star, if we know how much energy that star emitted
6	Kirchoff’s laws Hot solids emit continuous spectra Hot gasses try to do this, but can only emit discrete wavelengths Cold gasses try to absorb these same discrete wavelengths In stars we see absorption lines – what does that tell us? Stars have “atmospheres” of gasses Stars must be colder on the outside, hotter on the inside
7	Funny?!
8	Hydrogen Lines Energy absorbed/emitted depends on upper and lower levels Higher energy levels are close together Above a certain energy, electron can escape (ionization) Series of lines named for bottom level To get absorption, lower level must be occupied Depends upon temperature of atoms To get emission, upper level must be occupied Can get down-ward cascade through many levels
9	Which levels will be occupied? The higher the temperature, the higher the typical level Collisions can knock electrons to higher levels, if moving atoms have enough kinetic energy At T ~ 300 K (room T) almost all H in ground state (n=1) At T ~ 10,000 K many H are in first excited state (n=2) At T ~ 15,000 K many H are ionized Because you have highest n=2 population at ~10,000K you also have highest Balmer line strength there. This gives us another way to estimate temperatures of stars
10	Sense larger T range using many atoms Different atoms hold on to electrons with different force Use weakly held electrons to see low temperatures (Fe, Ca, TiO) TiO molecule is destroyed above 4000K Ca has lost 1 electron by ~5000K, but still has others to give lines
11	Sense larger T range using many atoms Different atoms hold on to electrons with different force Use moderately held electrons to sense middle temperatures (H) Below 6000 K most H electrons in lowest state – can’t cause Balmer lines Above 15,000K most H electrons completely lost (ionized)
12	Sense larger T range using many atoms Different atoms hold on to electrons with different force Use tightly held electrons to sense high temperatures (He, ionized He) Below 10,000K most He electrons in ground state – just like H, no visible absorption lines Above 15,000K most H has lost one electron, but still has a second one to cause absorptions
13	Classification of stars O B A F G K M scheme Originally in order of H strength – A,B,etc Above order is for decreasing temperature Standard mnemonic: Oh, Be A Fine Girl (Guy), Kiss Me Use numbers for finer divisions: A0, A1, ... A9, F0, F1, ... F9, G0, G1, ...
14	Composition of Stars Somewhat complicated – we must correct for temperature effects Regular pattern: More of the simplest atoms: H, then He, ... Subtle patterns later – related to nuclear fusion in stars
15	Doppler effect Effect similar in light and sound Waves compressed with source moving toward you Sound pitch is higher, light wavelength is smaller (bluer) Waves stretched with source moving away from you Sound pitch is lower, light wavelength is longer (redder) v = velocity of source c = velocity of light (or sound) l = apparent wavelength of light l_o = original wavelength of light
16	Doppler effect examples Car with horn blowing, moving away from you at 70 MPH. Speed of sound is ~700 MPH = 1000 ft/sec Original horn pitch is 200 cycles/sec Þ l_o ~ 5 ft
17	Doppler effect examples Car with horn blowing, moving away from you at 70 MPH. Speed of sound is ~700 MPH = 1000 ft/sec Original horn pitch is 200 cycles/sec Þ l_o ~ 5 ft
18	Doppler effect examples Star moving toward you at 200 km/sec = 2.0´10⁵ m/s Speed of light c = 3.00 ´ 10⁸ m/s Original Ha l_o= 0.65647 mm
19	Doppler effect examples Star moving toward you at 200 km/sec = 2.0´10⁵ m/s Speed of light c = 3.00 ´ 10⁸ m/s Original Ha l_o= 0.65647 mm
20	For Next time Chapter 7, the Sun. Homework #4 is up on WebCT, due Friday Friday mornings, rest of the semester: bring in an astronomy article for extra credit!