Sun. Sirius. Tuesday, February 21, 2012

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1 Spectral Classification of Stars Sun Sirius

2 Stellar Classification Spectral Lines H Fe Na H Ca H

3 Spectral Classification of Stars Timeline: 1890s Edward C. Pickering ( ) and Williamina P. Fleming ( ) label spectra alphabetically according to strength of Hydrogen (Balmer) lines, beginning with A (strongest). 1890s Antonia Maury ( ) developed a classification scheme based on the width of spectral lines. Would place B stars before A stars Annie Cannon ( ), brilliantly combined the above. Rearranged sequence, O before B before A, added decimal divisions (A0...A9) and consolidated classes. Led to classification scheme still used by astronomers today! OBAFGKM (Oh Be A Fine Guy/Girl, Kiss Me) Early Type Stars Late Type Stars

4 Timeline: Spectral Classification of Stars 1901 Annie Cannon ( ), brilliantly combined the above. Rearranged sequence, O before B before A, added decimal divisions (A0...A9) and consolidated classes. Led to classification scheme still used by astronomers today! OBAFGKM (Oh Be A Fine Guy/Girl, Kiss Me) Early Type Stars : Stars near the beginning of Sequence Late Type Stars : Stars near the end of the Sequence. One can mix the definitions: K0 star is an early-type K star. B9 is a late-type B star During 1990s Annie Cannon classified 200,000 spectra, listed in the Henry Draper Catalog. Catalog ID s are HD (ID for Betelgeuse in the constellation Orion). Two new letters added to Sequence for very cool, Brown-Dwarf stars. L spectral types (T= K) and T types (T < 1300 K). OBAFGKMLT (Oh Be A Fine Guy/Girl, Kiss Me - Less Talk!)

5 Spectral Classification of Stars Hotter Cooler Spectral Type O B A F G K M L T Characteristics Hottest blue-white stars, few lines. Strong He II (He + ) absorption lines. He I (neutral helium) stronger). Hot blue-white. He I (neutral Helium), strongest at B2. H I (neutral Hydrogen) stronger. White stars. Balmer absorption lines strongest at A0 (Vega), weaker in later-type A stars. Strong Ca II (Ca + ) lines. Yellow-white stars. Ca II lines strengthen to later types. F-stars. Balmer lines strengthen to earlier type F-stars. Yellow stars (Sun is a G5 star). Ca II lines become stronger. Fe I (neutral iron) lines become strong. Cool orange stars. Ca II (H and K) lines strongest at K0, becoming weaker in later stars. Spectra dominated by metal absorption lines. Cool red stars. Spectra dominated by molecular absorption bands, e.g., TiO (titanium oxide). Neutral metal lines strong. Very cool, dark red (brown dwarfs). Brighter in Infrared than visible. Strong molecular absorption bands, e.g., CrH, FeH, water, CO. TiO weakening. Coolest stars. Strong methane (CH4), weakening CO bands.

6 Spectral Classification of Stars

7 Spectral Classification of Stars

8 Spectral Classification of Stars

9 Spectral Classification of Stars

10 Spectral Classification of Stars

11 Stellar Classification Spectral Lines H Fe Na H Ca H

12 Spectral Classification of Stars Physical Description Hydrogen when T < 9900 K Majority of electrons in ground state, n=1 Hydrogen when T = 9900 K Majority of electrons in first excited, n=2 state, and capable of producing Balmer lines Hydrogen when T > 9900 K Majority of electrons unbound, ionized hydrogen.

13 Spectral Classification of Stars Physical Description Stars are not composed of pure hydrogen, but nearly all atoms (mostly H, He, and metals = anything not H or He). Typically 1 He atom for every 10 H atoms (and even fewer metals). Helium (and metals) provide more electrons, which can recombine with ionized H. So, it takes higher temperatures to achieve same degree of H ionization when He and metals are present. Abundance is = log10(nelement/nh) I.e., Abudance of Oxygen = 8.83, which means: 8.83 = log10(no/nh) + 12 NO/NH = = /1480 There is one Oxygen atom for every 1480 H atoms! Most Abundant Elements in the Solar Photosphere. Element Atomic # Log Relative Abundance H He ± O ± 0.06 C ± 0.06 Ne ± 0.06 N ± 0.06 Mg ± 0.05 Si ± 0.05 Fe ± 0.05 S ± 0.11 Al ± 0.07 Ar ± 0.06 Ca ± 0.02 Na ± 0.03 Ni ± 0.04

14 Spectral Classification of Stars Physical Description In 1925, Cecilia Payne ( ) calculate the relative abundances of 18 elements in stellar atmospheres (one of the most brilliant PhD theses ever in astronomy).

15 Spectral Classification of Stars Hertzsprung-Russell Diagram How would you measure a star s mass?

16 Spectral Classification of Stars Hertzsprung-Russell Diagram How would you measure a star s mass? Answer: Kepler s Laws. Works well for binary stars. m1 r1 + = r2 m2 R = m1r1 + m2r2 m1 + m2 = 0 m1 m2 r2 = = r1 a2 a1

17 Animations of binary Stars:

18 Binary Stars Term binary was first used by Sir Williams Herschel in "If, on the contrary, two stars should really be situated very near each other, and at the same time so far insulated as not to be materially affected by the attractions of neighbouring stars, they will then compose a separate system, and remain united by the bond of their own mutual gravitation towards each other. This should be called a real double star; and any two stars that are thus mutually connected, form the binary sidereal system which we are now to consider."

19 Binary Stars Two Stars in Albireo system.

20 Binary Stars Sirius A brightest star in the sky m = In 1844, Friedrich Bessel deduced it was a binary. In 1862 Alvan Graham Clark discovered the companion. Sirius B m = 8.30

21 Spectral Classification of Stars Hertzsprung-Russell Diagram How would you measure a star s mass? angle of inclination For circular orbits, v1 = 2πa1/P m1 = v2 Plane of Sky Orbital Plane m2 m2 Determine radial component of velocities, vr, using doppler-shifted spectral lines. v1r=v1 sin(i) & v2r=v2 sin(i) v1 To Earth m1 m2 = v2r / sin(i) v1r / sin(i) = v2r v1r Therefore, we can determine the mass ratio without knowing the angle of inclination! m1

22 Spectral Classification of Stars Hertzsprung-Russell Diagram How would you measure a star s mass? Need one other equation to relate m1 and m2. Comes from Kepler s 3rd law, replace a = a1 + a2 a = a1 + a2 = (P/2π) v1 + (P/2π)v2 = (P/2π) (v1+v2) Kepler s 3rd law, (P 2 = 4π 2 a 3 / GM ), becomes m1 + m2 = (P/2πG) (v1+v2) 3 Substituting v1 = v1r /sin(i), the angle of inclination. m1 + m2 = (P/2πG) (v 1r+v2r) 3 sin 3 (i)

23 Spectral Classification of Stars Hertzsprung-Russell Diagram How would you measure a star s mass? Combine our two formula: m1 m2 = v2r / sin(i) v1r / sin(i) = v2r v1r m1 + m2 = (P/2πG) (v 1r+v2r) 3 sin 3 (i) To get (rearranging terms): m2 3 (m1 + m2) 2 sin 3 (i) P = 2πG v1r 3 This is the mass function, depends only on the period, P, and radial velocity, v1r. In practice, only a spectrum for one star in binary pair is available. Mass function sets a lower limit on m2. In rare cases of eclipsing spectroscopic binaries, i 90 o and both masses can be measured directly.

24 Eclipsing Binaries: java/eclipse/eclipse.htm

25 Spectral Classification of Stars Mass-Luminosity relation

26 Spectral Classification of Stars Hertzsprung-Russell Diagram This is the Hertzprung- Russell (HR) diagram, which is a stellar classification system developed by Ejnar Hertzprung and Henry Norris Russel in Denmark around Ejnar Hertzsprung Henry Norris Russell The HR diagram relates the magnitudes and colors of stars as a function of their temperature and luminosity.

27 Henry Norris Russell s first diagram

28 Spectral Classification of Stars Enormous Range in Stellar Radii! If stars cool over time as they contract, there should be a relation between their temperatures and luminosities. R = 1 T 2 L 4πσ Hertzsprung ( ) found that stars of Late type (G and later) have a large range in luminosity. If two stars of the same spectral type (same Temperature) then more luminous star is larger. Giants: Stars with big radii & Dwarfs: Stars with small radii. Our Sun is a G0 dwarf. Similar Conclusions reached by Henry Russell ( )

29 Spectral Classification of Stars Hertzsprung-Russell Diagram

30 Spectral Classification of Stars Hertzsprung-Russell Diagram

31 Brighter 30,000 K 10,000 K 7500 K 6000 K 5000 K 4000 K 3000 K Luminosity Text HR diagram where data points show measurements from 22,000 real stars from the Hipparcos satellite. (Lines are Theoretical, expected luminosities and temperatures of stars) Temperature: Hotter Color Index: B-V

32 Brighter HR diagram Absolute Magnitude (M) (Luminosity) Spectral Type Hotter Temperature (Color, B-V)

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