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10/17/2012 Stellar Spectra Lecture 5 A STAR IS A HUGE BALL OF FIRE YOU CAN PUT A MILLION EARTHS INSIDE THE SUN 1 10/17/2012 What is a star? • • • • • Stars “shine” at night (during the day too!). A star is a self-luminous sphere of gas. (A HUGE BALL OF FIRE) (You can put a million earths inside the sun) Stars are held together by gravity. n But what keeps them from collapsing? Thermal energy from nuclear reactions: Hydrogen fusion The Spectra of Stars • We use a telescope with a spectrograph to measure and record the spectrum of a star. • This gives us the brightness of the star at different wavelengths. n Like we discussed with blackbodies. 2 10/17/2012 Spectra • Almost all stars show a “continuum” spectrum with “absorption” lines. • Some stars show “emission” lines. – All stars do not have the same spectrum! • Interstellar clouds show a continuum with emission lines Energy Examples of Stellar Spectra Wavelength OR 3 10/17/2012 Stellar Spectra • The spectral (absorption) lines we see in stars are very important. • The “missing” photons give us info on: – Chemistry – Temperature – Density • Kirchhoff’s laws tell us about the region which gives rise to the spectrum. Formation of Absorption Lines in a Star Photons See this Star Photosphere: “Continuum Source” Outer layers are Cooler -- Absorb Photons Like Kirchhoff’s 3rd law 4 10/17/2012 Example Stellar Spectrum H H H H ... 4000 6000 5000 7000 (A) The Hydrogen-Balmer spectrum (in the visible) is present for most stars. Classification of Stars • In the late 19th century astronomers categorized stars according to the strength of the hydrogen absorption lines in the spectrum. • They labels these A, B, ... from strongest to weakest. • Unfortunately, this was the wrong way to do it! 5 10/17/2012 Classification (cont’d) • Annie Jump Cannon arranged the spectra of stars in a sequence which corresponds to the temperatures. • Classified over 500,000 stars! • The spectral sequence is: O, B, A, F, G, K, M (RNS) hotter ------------> cooler • A temperature sequence. Classifying stars Annie Jump Cannon (1929) Prism spectra of a star cluster 6 10/17/2012 The Harvard GROUP 13 May 1913 Spectral Sequence = Temperature Sequence Cooler Hotter O Violet B A Blue F G Yellow K M RNS Red Henry Norris Russell (1920) “Oh, Be A Fine Girl (Guy), Kiss Me” 7 10/17/2012 ONLY BRIGHT APPLES FALL GROUNDWARD KNOCKING ME REMARKED NEWTON SOLEMNLY OUT BEYOND AMBER FIELDS, GRACEFULLY KNOLLS MUST ROLL, NEVER SUBSIDING 8 10/17/2012 Spectral Classification (cont’d) • Each of these class (O, B, etc.) can be subdivided into tenths, i.e. G0, G1, ... G9, K0, K1, ... K9 (G0 is hotter than G9) • The Sun is a G2 star. 9 10/17/2012 Spectral Sequence O5V B5V A5V F5V G4V K5V M5V 350 nm 750 nm Stellar Spectra Na 10 10/17/2012 Changes to spectral sequence • For the first time in over 100 years the spectral sequence has need for more letters. • Very low temperature stars discovered with infrared surveys of the sky. • Now have L and T stars! OBAFGKMLT Spectral Sequence = Temperature Sequence Cooler Hotter O Violet B A Blue F G Yellow K M L T Red Contest: Make up your own pneumonic for remembering the spectral sequence – winner will be recited in class. (Must be publicly presentable). 11 10/17/2012 Stellar Photometry & Colors • It is not necessary to measure the entire spectrum to determine the spectral peak of a star. • We can use color filters to determine the “colors” of a star. n By this we mean how much flux is seen in each color filter. n A green filter transmits only green photons. The UBV System: A set of color filters which give coarse spectral information. 1 V B Transmission U U at 3500 A B at 4300 A V at 5500 A 0 3000 5000 7000 A Wavelength 12 10/17/2012 Temperature and Colors • • • • U at 3500 A => ultraviolet B at 4300 A => blue. V at 5500 A => visible (yellow) A hot star will have more flux in the U filter than the V filter compared to a cool star. Colors of a hot star vs. a cool star 1 Hot Star Cool Star Energy M U V 0 3000 5000 7000 A O Wavelength at U Flux at U Flux Flux at V Hot Star Flux at V Cool Star 13