Stellar Spectra ASTR 2120 Sarazin. Solar Spectrum

Transcription

1 Stellar Spectra ASTR 2120 Sarazin Solar Spectrum

2 Solar Prominence Sep. 14, 1999

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

4 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

5 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

6 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

7 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)

8 Stellar Spectra ASTR 2120 Sarazin Solar Spectrum

9 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

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

11 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

12 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

13 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

14 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

15 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

16 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

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

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

19 Solar Spectrum

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

21 Stellar Spectra hotter cooler

22 Stellar Spectra hotter cooler

23 Stellar Spectra ions hotter molecules cooler

24 Stellar Spectra ions molecules hotter Temperature cooler

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

26 Annie Jump Cannon

27 Annie Jump Cannon

28 Annie Jump Cannon

29 Annie Jump Cannon

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

31 Stellar Spectra Hb hotter Ha cooler

32 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

33 Stellar Spectra ions molecules hotter Temperature cooler

34 Stellar Spectra Ha strength Temperature

35 Stellar Spectra Ha strength 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

36 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

37 Stellar Spectra Ha strength K G F A B C D E M O Temperature

38 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

39 Stellar Spectra ions molecules hotter cooler

40 Spectral Types

41 Spectral Types

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

43 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 *

44 Spectral Luminosity Classes

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

46 Stellar Composition Mainly hydrogen and helium

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

48 Solar Composition log Abundance by number Atomic number

49 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)

50 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