Astronomical Spectroscopy

The Electromagnetic Spectrum

● Newton first held a prism up to white light and saw the visible spectrum ● Blue light is higher frequency and shorter wavelength ~ 400nm ● Red light is lower frequency and longer wavelength ~700nm ● 1 nanometer (nm) = 1x10 -9 meters (m) The Visible Spectrum

● Prisms refract light of different wavelengths different amounts resulting in dispersion of the colors ● Diffraction gratings diffract light of different wavelengths different amounts creating interference patterns which disperse the colors ● Grating spectrometers use a diffraction grating to disperse the light, then assign a scale to measure its wavelength The Solar Spectrum ● The Sun contains a continuous source below the surface ● Cooler gas in front absorbs at specific wavelengths producing a rainbow spectrum with dark lines ● Dark lines were first noticed by Wollaston in 1802 ● Fraunhofer identified over 575 of these lines in the sun in They are now named after him

Kirchof's Laws 1. A heated source emits a continuous spectrum of radiation: Continuous Spectrum 2. A glowing gas emits light at particular wavelengths corresponding to its composition: Emission Line Spectrum 3. A cooler gas with a continuous source behind it emits a continuous spectrum with dark lines at specific wavelengths corresponding to its composition: Absorption Line Spectrum

Blackbody Spectrum Emission from a continuous source reveals the temperature of the emitting body based on the shape of the Blackbody Curve Different temperatures have different intensities at different wavelengths: The Gator Star

Bohr Atom ● Electrons reside in “discrete” energy levels around the nucleus ● Electrons can be excited by a photon to higher energies ● Electrons at higher energies soon drop down to lower energies, and emit a photon with an energy equal to the difference in levels ● Energy is proportional to wavelength ● This photon always has the same energy i.e. the same wavelength! ● Hydrogen Balmer lines come from electrons falling from higher energy levels to the level n=2, or the first excited state of the atom.

● Every element has a unique set of energies levels in its atoms which correspond to a unique set of emission or absorption lines in the gas ● Emission and absorption occur at the exact same wavelength. ● Emission when the electron loses energy and releases a photon ● Absorption when the same electron gains energy by absorbing a photon with the same energy and wavelength ● Astronomers use this to determine the presence of elements in stars, galaxies, planets and nebula

Doppler Effect Astronomers use the Doppler effect when looking at stellar spectra to find shifts in the location of the expected lines due to a second body such as a binary star or even extra solar planets!

This Experiment ● Please DO NOT touch the glass tubes ● Turn off the power supplies when not in use-they will eventually burn out! ● DO NOT adjust the slit width as mentioned in the lab ● Start the part B. with the scale light OFF: the scale will produce a secondary spectrum which you should ignore, so find the real spectrum first

Hydrogen Balmer Lines H-alpha: nm H-beta: nm H-gamma: nm H-delta: nm