classification of stellar spectra essentially all stars appear as point sources. only differences...
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Classification of Stellar Spectra
• Essentially all stars appear as point sources. Only differences are brightness and spectra.
• Many differences in spectra due to temperature– Pseudo blackbody (with chromosphere outside
photosphere)– Relative strength of absorption lines (due to
different stages of ionization and energy states)
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Hydrogen Absorption Lines in Visible• Classification originally based on strength of Balmer
absorption lines (2 -> n). A type stars have strongest lines– (OBAFGKM) Harvard Classification obafgkmrns.html
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http://astronomy.swin.edu.au/cosmos/h/harvard+spectral+classification
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Hydrogen Balmer Series
• Balmer series due to transitions from n=2 to n>2
• Strength of absorption lines depends on fraction of atoms that are neutral and in the n=2 state
• Maxwell-Boltzmann Statistics:
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Saha Equation• Relative number of atoms in ith ionization state
also determined using Maxwell-Boltzmann statistics– Derivation complicated due to the continua of
possible states for free electrons– Result is the Saha equation:
• Partition function (effective number of states):
• Ionization energy:
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Hydrogen mostly ionized above 12,000 K
Thus the fraction of hydrogen atoms in the n=2 energy state is given by fractionof neutral atoms times the fraction of neutral atoms in n=2.
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Calcium Lines
• Abundance of calcium is 1/500,000 that of hydrogen
• Lower ionization energy (6.1 eV) makes calcium essentially all singly ionized (Ca II)
• Roughly 265/266 Ca II atoms in ground state– 1s22s22p63s23p64s1
– 4s to 3d produce calcium H & K lines– Even though abundance is low, the high fraction of
calcium in ground state produce large absorption lines
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Abundances: H – 91%He – 8.9%O - .05%C – 0.03N – 0.01Ne - .01Si - .003Mg - .003Fe – .003
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Solar Spectrum
D: SodiumE & G: IronF: Hydrogen (H beta)C: Hydrogen (H alpha)H & K: Calcium II
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Hertzsprung-Russell Diagram
Hipparcos catalog 2010: 16,631 stars with relative distance precision better than 10%
Luminosity vs Spectral Classification
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Hertzspring-Russell Diagram
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HR Diagram for Nearest Stars
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Stellar Radii
Along constant radius line
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• Sirius A
• Sirius B
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• Betelgeuse
• d = 640 ly• L = 105 Lsun
• R = 1,200 Rsun=4.5 AU
• M = 19 MSun
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Correlation between mass, luminosity, and position in MS branch:
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