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Download Lecture 10-11 - OSU Astronomy
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Stellar Spectra Colors of Stars Stars are hot, dense balls of gas • Continuous spectrum from the lowest visible layers (“photosphere”). • Approximates a blackbody spectrum with a single temperature. From Wien’s Law, we expect: • hotter stars appear BLUE (T=10,000-50,000K) • middle stars appear YELLOW (T~6000K) • cool stars appear RED (T~3000K) U-B-V Photometry bU bB For a cool star: bV For a Sun like star: bU < bB > bV bU < bB < bV For a hot star: bU > bB > bV Spectra of Stars Hot, dense lower photosphere of a star is surrounded by thinner (but still fairly hot) atmosphere. Flux Hydrogen • Produces an Absorption Line spectrum. • Lines come from the elements in the stellar atmosphere. Continuum Absorption Lines 4000 Can we use stellar spectra to distinguish among different types of stars? 5000 6000 Wavelength 7000 Objective Prism Spectra Spectral Classification of Stars 1866: Angelo Secchi observed the spectra of ~4000 stars. • Divided them into 4 broad classes by common spectral absorption features. 1886: Draper Memorial Survey at Harvard • Objective Prism Photography • Obtained spectra of >220,000 stars Harvard Classification (1890) Edward Pickering & Williamina Fleming made a first attempt to classify ~10,000 stars by their spectra: • Sorted by Hydrogen absorption-line strength • Spectral Type “A” = strongest Hydrogen lines • followed by types B, C, D, etc. (weaker) Problem: Edward Pickering Harvard “Computers” (c. 1900) The other lines didn’t fit into this sequence. Annie Jump Cannon In 1901, Annie Jump Cannon • Re-ordered the types using strengths of many absorption lines. • Many classes thrown out as redundant. • Left with 7 primary classes: O B A F G K M Annie Jump Cannon Stellar Spectral Sequence Henry Draper Catalog of Stars Annie Cannon further refined the spectral classification system by dividing the classes into numbered subclasses: For example, A was divided into A0 A1 A2 A3 ... A9 Between 1911 and 1924, she classified about 220,000 stars, published as the Henry Draper Catalog. From O0………….to………………..M9 Spectral Sequence Mnemonics Harvard (1920s): Oh Be A Fine Girl, Kiss Me Berkeley (late `60s): Oh Buy A Fine Green Kilo Man Caltech (early `80s): On Bad Afternoons Fermented Grapes Keep Mrs Richard Nixon Smiling OSU (2009): OSU Buckeys Are Fine Gridiron Kicking Machines Cecilia Payne Gaposhkin and Meghnad Saha • First comprehensive theoretical interpretation of stellar spectra. • Based on the then new atomic physics. Showed that stars are mostly Hydrogen and Helium and small traces of all of the other metals. The Spectral Sequence O B A F G K M Hottest 50,000K L Coolest 1300K Bluest Reddest Spectral Sequence is a Temperature Sequence The Spectral Sequence is a Temperature Sequence Gross differences among the spectral types are due to differences in Temperature. Differences in chemical composition are minor at best. Why? What lines you see depends primarily on the state of excitation and ionization of the gas. Line Strengths Example: Hydrogen Lines Visible Hydrogen absorption lines come from the second excited state (n=2). B Stars (11-30,000 K): Most of H is ionized, so only very weak H lines. A Stars (7500-11,000 K): Ideal excitation conditions, strongest H lines. G Stars (5200-5900 K): Too cool, little excited H, so only weak H lines. O Stars • Hottest Stars: T=30,000 K—50,000K • Strong lines of ionized Helium • No lines of Hydrogen B Stars • T=11,000 - 30,000 K • Strong lines of Helium • Very weak lines of Hydrogen F Stars • T = 6000 - 7500 K • weakening lines of Hydrogen • stronger lines of ionized Calcium A Stars • T = 7,500—11,000 K • Strongest lines of Hydrogen • Weak lines of ionized Calcium (Ca+ or Ca II) G Stars • T = 5000 - 6000 K • Strong lines of ionized Calcium, ionized Iron, & other metals (metals mean everything except H and He) • Much weaker Hydrogen lines The Sun is a G-type Star (G2) • • • • K Stars M Stars Cool Stars: T = 3500 - 5000 K Strongest metal lines Hydrogen lines practically gone First weak CH & CN molecular bands • Cool stars: T ~3500 – 2000 K • Strong absorption bands from metal-oxide molecules (e.g. TiO) • Weak neutral metal lines • No lines of Hydrogen New Spectral Types: L & T Coolest stars (<2500K) discovered by new digital infrared surveys. L stars: • Temperatures ~1300-2500K • Strong lines of metal hydrides & neutral metals. T dwarfs: • Strong Methane (CH4) bands, like Jupiter. • May be failed stars (“Brown Dwarfs”) Which of the following spectral classifications represents the hottest stellar surface temperature? 1) 2) 3) 4) A G K B In the spectral classification of stars, absorption lines/bands by which of the following would indicate very low surface temperature? A) B) C) D) Mg II Fe II TiO He I A star was found to be brightest in U, less bright in B and faintest in V. What conclusion can be drawn from this information? A) The star has high surface T B) The star has low surface T C) The star has intermediate surface T, close to that of the Sun D) This information is insufficient to draw any conclusion about the star surface T The Sun is a G2 star while Enif is K2. This tells us that Enif is: A) B) C) D) Hotter than the Sun Brighter than the Sun Fainter than the Sun Cooler than the Sun