Stellar Spectra ASTR 2120 Sarazin. Solar Spectrum

Stellar Spectra ASTR 2120 Sarazin Solar Spectrum

Solar Prominence Sep. 14, 1999

Solar Activity Due to rotation, convection, and magnetic field (Section 7.2 review)

Charged Particles in Magnetic Fields Helical motion F = q c v B Work = ( ) q = charge F d r = F v dt = 0 E = constant, thus KE = constant v = constant In plane B, circle orbit r g = mv c qb gyro radius v = constant, v = constant Helical motion B

Bulk Properties of Plasma with Magnetic Field Faraday s Law: changing magnetic field electric field current (if conductor) opposite sign Ampere s Law: current magnetic field Acts to prevent change in magnetic field B

Bulk Properties of Plasma with Magnetic Field Can t pull wire from B field Plasma = like wires in all directions Frozen-In Condition Plasma and Magnetic Field are locked together B B wire plasma

Bulk Properties of Plasma with Magnetic Field Frozen-In Condition plasma and magnetic field tied Who is master and who is slave? Bigger pressure wins. Gas pressure P gas = n k T Magnetic pressure P B = B 2 /(8p)

Stellar Spectra ASTR 2120 Sarazin Solar Spectrum

Theory of Stellar Atmospheres Divide stars into Atmosphere Narrow outer layer, 1 mean free path, t» 1 Makes light we see Interior Not directly observable

Theory of Stellar Atmospheres Assume 1. Very thin Dr << R Treat as flat plane

Theory of Stellar Atmospheres Assume 1. Very thin Dr << R Treat as flat plane 2. No energy sources L in = L out = L 3. Static Forces balance, hydrostatic equilibrium

Theory of Stellar Atmospheres dp dr = GM(r)ρ Hydrostatic equilibrium r 2 M(r) mass interior to r = M * ρ mass density = mass/volume r R * constant, thin atmosphere Review Sec. 9.2 g * GM * R * 2 surface gravity, constant dp dr = g *ρ Hydrostatic equilibrium P = ρkt µm p Pressure, ideal gas law Review Sec. 7.2

Theory of Stellar Atmospheres L = constant, thin atmosphere, flux important L = 4π R *2 σt 4 eff, F = L / (4π R 2 * ) = σt 4 eff constant Equation of Radiative Transfer Review Sec. 5.4 di ν dx = κ νρi ν +ε ν ρ absorption reduces, emission increases ε ν,κ ν emissivity, opacity, depend on T,ρ, composition

Theory of Stellar Atmospheres dp dr = g *ρ Hydrostatic equilibrium P = ρkt µm p Pressure, ideal gas law F = L / (4π R 2 * ) = σt 4 eff constant di ν dx = κ νρi ν +ε ν ρ Eqn. Radiative Transfer

Theory of Stellar Atmospheres dp dr = g *ρ Hydrostatic equilibrium P = ρkt µm p Pressure, ideal gas law F = L / (4π R 2 * ) = σt 4 eff constant di ν dx = κ νρi ν +ε ν ρ Eqn. Radiative Transfer Four unknowns: P, ρ, T, I ν vs. r Four equations solvable

Inputs: F = L / 4π R 2 * = σt 4 eff = constant g * κ ν,ε ν,µ depend on ρ, T, composition R * but only scales the fluxes

Stellar Spectra and Stellar Atmospheres Determined by (in decreasing order of importance) 1. T eff 2. g * 3. Composition

Solar Spectrum

Stellar Spectra General result Stellar spectra = continuum emission + absorption lines ~blackbody hotter brighter, bluer hotter molecular lines atomic lines ions (more and more ionized)

Stellar Spectra hotter cooler

Stellar Spectra ions hotter molecules cooler

Stellar Spectra ions molecules hotter 50000 25000 10000 8000 6000 5000 4000 3000 Temperature cooler

Spectral Classification ~1900, done by Annie Jump Cannon, assistant to Prof. E. C. Pickering at Harvard

Annie Jump Cannon

Spectral Classification ~1900, before atomic theory, spectral lines hard to understand Use Balmer lines of hydrogen

Stellar Spectra Hb hotter Ha cooler

Spectral Classification ~1900, before atomic theory, spectral lines hard to understand Use Balmer lines of hydrogen From excited states Ha Hb n = 4 n = 3 n = 2 n = 1

Stellar Spectra ions molecules hotter 50000 25000 10000 8000 6000 5000 4000 3000 Temperature cooler

Stellar Spectra Ha strength 3000 4000 5000 6000 8000 10000 25000 50000 Temperature

Stellar Spectra Ha strength 3000 4000 5000 6000 8000 10000 25000 50000 Temperature Excitation Ionization to n=2 of hydrogen n 2 /n 1 = exp(- E / kt) Ha Hb n = 4 n = 3 n = 2 n = 1

Spectral Classification ~1900, before atomic theory, spectral lines hard to understand Use Balmer lines of hydrogen Alphabetical system (A - P) based mainly on strength of hydrgen Balmer lines A = strongest

Stellar Spectra Ha strength K G F A B C D E M O 3000 4000 5000 6000 8000 10000 25000 50000 Temperature

Stellar Spectral Classes T eff 50,000 K 2,000 K O B A F G K M L T Oh, be a fine { } guy girl kiss me Memorize L, T = brown dwarfs

Stellar Spectra ions molecules hotter cooler

Spectral Types

Stellar Spectra and Stellar Atmospheres Determined by (in decreasing order of importance) 1. T eff 2. g * 3. Composition

Spectral Luminosity Classes g * = G M * / R * 2 R * µ M * 0.75 for normal (main sequence) stars g * doesn t vary too much Giants, supergiants big R * White dwarfs small R * g * mainly determined by R * Fixed T eff, L = 4p R *2 s T eff 4 R * changes L g * gives L or R *

Spectral Luminosity Classes

Stellar Spectra and Stellar Atmospheres Determined by (in decreasing order of importance) 1. T eff 2. g * 3. Composition

Stellar Composition Mainly hydrogen and helium

Solar Composition Element Abundance by mass Hydrogen 73.5% Helium 24.8% Oxygen 0.788% Carbon 0.326% Nitrogen 0.118% Iron 0.162%

Solar Composition log Abundance by number Atomic number

Stellar Composition Mainly hydrogen and helium X = mass fraction of hydrogen ~ 0.74 (90% of atoms) Y = mass fraction of helium ~ 0.24 (10% of atoms) Z = mass fraction of heavier elements ~ 0.02 in Sun (0.1 % of atoms) Fraction of heavy elements varies Population I = like Sun, Z ~ 0.01 Population II = low abundances, Z ~ 0.001