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 5th
Hydrogen H 13.60
Helium He 24.59 54.42
Lithium Li 5.39 75.64 122.45
Beryllium Be 9.32 18.21 153.90 217.72
Boron B 8.30 25.15 37.93 259.38 340.23
Carbon C 11.26 24.38 47.89 64.49 392.09
Nitrogen N 14.53 29.60 47.45 77.47 97.89
Oxygen O 13.62 35.12 54.94 77.41 113.90
Fluorine F 17.42 34.97 62.70 87.14 114.24
Neon Ne 21.56 40.96 63.45 97.12 126.21
Sodium Na 5.14 47.29 71.62 98.91 138.40
Magnesium Mg 7.65 15.04 80.14 109.27 141.27
Aluminum 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