Spectra PHYS390 (Astrophysics) Professor Lee Carkner Lecture 4

Questions 1) Consider violet photons at 400 nm, red photons at 700nm and green photons at 550 nm. If a star peaks at 550 nm will it emit more violet or red photons? Answer: red Explain: The blackbody curve falls off more rapidly to short wavelength than to long 2) Consider two stars at the same distance one red and one blue. If the red star looks brighter, which star is larger? Answer: red Explain: The only way a cool object can emit more light than a hot object is if it is larger

Spectroscopy   Measuring the brightness (magnitude) of stars is called photometry   Measuring the intensity of each wavelength of light from the star

Spectral Lines  If you pass pure blackbody light through a grating, you get a rainbow   Each element has a unique spectral signature  pattern of lines

Spectrograph  Diffraction and interference can separate light into wavelengths   Use more slits, get better separation by wavelength   The resolving power is a measure of the smallest wavelength separation (  ) the grating can resolve R = ave /  = nN  where n is the order and N is the number of rulings on the grating and ave is the average of the two wavelengths

Kirchhoff’s Laws 1)A hot, dense gas or object produces a continuous spectrum  2)A hot, thin gas produces emission lines  3)A cool, thin gas in front of a blackbody produces absorption lines 

Stellar Spectra  Stars show absorption spectra   Different stars show different lines at different strength

Bohr Atom  In the early 20 th century Niels Bohr tried to model the hydrogen atom as a proton orbited by an electron with quantized angular momentum  Angular momentum =  Coulomb energy =  Kinetic energy =  Can solve for energy  E = eV (1/n 2 )  n is an integer (1, 2, 3 …)  Note that hc = 1240 eV nm

Hydrogen Series  Transitions  n=1  n=2  n=3  Transition from next level up is called , from 2 levels up , etc.   The Balmer series is the only one at visible wavelengths

Consequences of Bohr Atom  Atoms can only absorb or emit certain photons   For movement between energy levels E photon = eV [(1/n 2 high )-(1/n 2 low )]   This is only a classical approximation of the quantum reality  Good for hydrogen only

Kirchhoff’s Laws Explained 1)Atoms in a dense state have the quantum states blurred into a continuum 2) 3)Atoms in a cool, thin gas will absorb only the right photons removing them from the spectrum

Doppler Effect   For low speeds (v<<c)  rest = v/c   Note that Doppler shifts only give us velocity along the line of sight  radial velocity

Photoelectric Effect   Will only eject electrons if the incident photons have photon energy (h ) greater than the work function    The maximum kinetic energy is thus K max =h –  

Next Time  Read: 2.1, 2.3, 2.4, 7.1  Homework: 5.2, 5.10, 5.11, 2.7, 2.8