The Sun and the Stars

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3 Classification of stellar spectra Potted History : 1802 William Wallaston showed that the spectrum of the sun is not simply a continuous spectrum, but is broken up by a series of dark lines (absorption lines) Joseph Fraunhofer identified ~600 lines in the solar spectrum and measured wavelengths for approx half. that number. Lines now known as Fraunhofer lines Angelo Secchi crude spectral typing 1864 William Huygens matched some of the Fraunhofer lines to absorption lines seen in the spectra of other stars. 1890s Edward Pickering (Harvard College Observatory) starts project to obtain spectra of all stars observed down to 8 th magnitude Subsequently shown that : small number of distinct patterns of absorption lines seen in stellar spectra Spectral classification

4 Spectral Classification Harvard Classification scheme Work originally started by Henry Draper, continued by Annie Jump Cannon in 1910s, results published in the Henry Draper Catalogue. Extended version of HD catalogue contains spectra of 225,000 stars down to 9th magnitude. Annie Jump Cannon spectra are dominated by lines of Hydrogen and Helium and small amounts of metals. Original scheme had classes OBAFGKM (now extended to L, T and Y dwarfs) Most important lines are Balmer series of H, neutral and singly ionised He, Fe, Ca H and K doublet, the G band (CH molecule), neutral Ca, metal lines and Ti O. excitation-ionisation sequence Scheme is based on (i) absence of lines (ii) line strengths (W(λ)) (iii) line ratios

5 The Hertzsprung-Russell Diagram (E. Hertzsprung and H.N. Russell) Plot of surface temperature versus luminosity, or colour (e.g. B-V) versus absolute magnitude M, and various other combinations bright Can distinguish between early type and late-type stars dim Hot Cool

6 HCS is a temperature classification scheme why? Once a star has formed, there is little mixing of the core and surface material, and few chemical reactions spectral differences reflect primarily differences in the surface temperature O 50,000 K strong He+ lines, no H lines B 20,000 K strong neutral He lines, v. weak H lines A 10,000 K Strongest H lines, weak Ca+ emerge F 7,500 K H grows weaker,ca+ becomes stronger, metals G 6,000 K Strong Ca+,Fe+ and other metals, H weaker K 4,000 K Strong metal lines, weak CH,CN bands emerge M 3,500 K strong TiO and VO emerge and strengthen Cannon - further subdivided classes from 0 to 9 (0 hotter, 9 cooler), so that F5, lies halfway between F0 and G0. In this scheme the Sun is classed as G2

7 Harvard classification scheme Type Colour Surface Temp. Characteristics Examples O Blue >25,000 K singly ionised He in emission or absorption. Strong UV continuum 10 Lac B Blue 11,000-25,000 neutral He lines in absorption Rigel, Spica A Blue 7,500-11,000 H lines at max strength for AO, decreasing from AO-A9 Sirius, Vega F Blue to White metallic lines emerge and strengthen Canopus, Procyon G White to Yellow lines of neutral metals, Ca+ Fe+ Sun, Capella K Orange to Red metallic lines dominate, weak blue continuum Arcturus, Aldebaran M Red <3,500 molecular bands of TiO noticeable Betelgeuse

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9 Examples spectral classification sequences

10 1943 Yerkes SCS (or MKK named after Morgan, Keenan and Kellman) Extends Harvard classification by using line shapes to estimate surface gravities GM g s = R * 2 * MKK luminosity classification Ia luminous Supergiants Ib less luminous Supergiants II luminous Giants III normal Giants IV sub Giants V main sequence stars (dwarfs) VI sub Dwarfs In this extended scheme the sun is classified as G2V

11 dwarf stars : smaller radii higher surface gravity c.f. giants therefore higher surface density and higher surface pressure under higher pressure atoms collide more frequently, collisions shorten lifetime of excited state. smaller Δt, larger ΔE (ΔE Δt h/2π) => lines are broader! also at a given temperature, T, atoms are thermalised more easily at lower pressure

12 MKK luminosity classification

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14 Additional classes Wolf Rayet, W or WR mostly He atmos. - dying supergiants with fast stellar winds subtypes WC, WN or WO (for Carbon, Nitrogen and Oxygen) (Possible GRB/Hypernovae progenitors) Carbon stars, C (originally R and N classification), red-giants/supergiants near end of lives with excess, Carbon in atmosphere WDs, D stars are no longer undergoing thermonuclear reactions, simply cooling subclasses designated A,B,O,Q,Z,C,X depending on optical lines present (ie atmospheric composition) L stars,t,y dwarfs Cool stars, T and Y dwarfs are not massive enough to undergo nuclear fusion (Brown dwarfs)

15 Since spectral type is essentially a temperature sequence, we can construct a colour-magnitude diagram, a diagram of Absolute magnitude Mv versus colour, e.g. Hyades open cluster Colour magnitude diagrams can be used to estimate distances spectroscopic parallax

16 Spectroscopic parallax From a stars spectrum, we can identify i) Spectral type (or colour) ii) Luminosity class Since, m M = 5log d 5 These fix a position on the HR diagram from which we can measure Mv, the observed apparent magnitude m gives us the distance modulus, and hence the distance to the star

17 Colour-colour diagrams NB a colour is just a flux ratio If stars radiated like blackbodies then a colour-colour plot would be unique and follow a straight line BB The kink in the lazy S Is due to the Balmer discontinuity U-B (BD at 3640 falls into U-band) B-V

18 When we looks at young clusters and globular clusters their colour-magnitude diagrams appear very different Globular clusters consist of ~half a million stars gravitationally bound, and located above the plane of the galaxy. They are old metal poor stars. Show extensive red-giant,agb and horizontal branch As well as main sequence Young open clusters are metal rich pop 1 stars. Dominated by their main sequence stars, they are young so very few stars have evolved off of the main sequence. Age can be determined from the location of the turn-off point, younger clusters will turn off at higher luminosities

19 Globular cluster schematic colour-magnitude diagram