Spectra

PHYS390 (Astrophysics)

Professor Lee Carkner

Lecture 4

Questions1) 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: redExplain: 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: redExplain: 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 Laws1) 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 20th 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 = -13.6 eV (1/n2) 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

Ephoton = -13.6 eV [(1/n2high)-(1/n2

low)] This is only a classical approximation of the quantum

reality Good for hydrogen only

Kirchhoff’s Laws Explained1) 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

Kmax =h –

Next Time

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