Transcript Stellar Spectra - www.cfa.harvard.edu/

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
Grating diffracts
Prism refracts
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
• OBAFGKM
• 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