Stellar Spectra

Ay16 Lecture 2

Feb 5, 2008

The Nearest Star

SOHO UV Image

1. What is a Spectrum.

2. The Atmospheres of stars.

3. Spectral Lines & Quantum Mechanics (the Bohr atom)

4. Ionization

5. Spectral Types

6. The Hertzsprung-Russell Diagram (intro to the evolution of stars)

Diffraction/Refraction

Prism refracts

Grating diffracts

The Solar Spectrum

Fraunhofer Lines in the spectrum of the Sun

Fraunhofer Lines in the Solar Spectrum (~1817)

Designation Wavelength Origin A 759.4 nm terrestrial oxygen B 686.7 terrestrial oxygen C 656.3 hydrogen (H alpha) D 589.0 neutral sodium (Na I) E 527.0 neutral iron (Fe I) F 486.1 hydrogen (H beta) G 430.3 metal blend H 396.8 ionized Calcium (Ca II) K 393.4 ionized Calcium (Ca II)

Where do these features come from?

Blackbody

Emission by hot

gas

Absorption by

cold gas in front

of a hot source

The Bohr

Atom

•

Hydrogen Atom Energy Levels

Hydrogen Ionization Fraction

IONIZATION ENERGIES (in electron volts = eV) Atom 1st 2nd 3rd 4th 5thHydrogen H 13.60Helium He 24.59 54.42Lithium Li 5.39 75.64 122.45Beryllium Be 9.32 18.21 153.90 217.72Boron B 8.30 25.15 37.93 259.38 340.23Carbon C 11.26 24.38 47.89 64.49 392.09Nitrogen N 14.53 29.60 47.45 77.47 97.89Oxygen O 13.62 35.12 54.94 77.41 113.90Fluorine F 17.42 34.97 62.70 87.14 114.24Neon Ne 21.56 40.96 63.45 97.12 126.21Sodium Na 5.14 47.29 71.62 98.91 138.40Magnesium Mg 7.65 15.04 80.14 109.27 141.27Aluminum Al 5.99 18.83 28.45 119.99 153.83

Ionization Energies

Stellar Spectral Classification

• Spectroscopy (photographic!) started in the late 19th century.

• Classifiers originally arranged them alphabetically, A-O

• A. J. Cannon figured out the right order from 200,000 objective prism spectra

• O B A F G K M • And astronomers realized it was a well

defined sequence in line strength.

Digital spectral atlas

(the modern version of the photographic

atlas!)

Hertzsprung-Russell Diagram

Multicolor photography --- Blue and Visual Plates allowed “colors”

Objective Prism Spectra

Realization that Spectral Type related to

Color related to

Surface Temperature

Hipparcos

HR Diagram

4907 nearby stars with distances to

< 5%.

HR Diagram in

Luminosity vs

Temperature

Solar Spectrum

NOAO Atlas, N. Sharp

Hydrogen Atom Energy Levels