Things are not as they seem. Phaedrus, Fables

Assignments: Problem Set #2 due tomorrow 5 pm Read Chapter 21 (pp. 331-343) of your text for Friday's class

In Class: --------------------- review: density makes the difference between blackbody and spectral line emission BOTTOM LINE: high density --> blackbody emission low density --> spectral line emission ----------- So what's with stars? we see absorption spectra must be continuous emission somwhere (ie., high densities) but this emission must pass through a low density gas somewhere on its way to us that's just what happens inner bits of stars have very high densities produce continuous emission via the BB process outer bits are fluffy and less dense emit and absorb in spectral line mode atoms suck up their favorite photons great example of Kirchhoff's Laws in action (BALL DEMO) BB emitter: 5 purple spectral line astronomer: 5 purple 5 cyan absorber 5 cyan 5 green ---> likes green ----> 0 green 5 yellow | 5 yellow 5 red | 5 red | V astronomer 0 purple 0 cyan 1? green 0 yellow 0 red consider a continuous spectrum source and a low density cloud of atoms what will you see? look through the cloud at the source - see continous emission - with absorption lines removed from the spectrum look at the cloud from the side - no continuous spectrum - but those atoms are all absorbing photons electrons are going to higher levels - eventually they will de-excite go back to low energy state when they do, they'll emit of photon where? antwhere sometimes out this way - what will you see spectral line emission the "lost" photons real spectra - What do they tell us? 1) since the vast majority of stars show absorption line spectra - they must be dense on the inside (to generate continuous spectrum) - and fluffy on the outside (to perform spectral line absorption) i.e., not solid surfaces -- stars have "atmospheres" -- they're gassy 2) do lines tell us what stuff is there? - YES - IF you see an absorption line corresponding to a particular atomic species, you know it must be there - and NO - EVEN IF you _don't_ see the absorption line, that species might be there There's more to the story for spectral lines in stars, and it confused people for about 100 years during the 19th century. -> role of temperature in determining spectral line absorption ------------------------- In the 19th cent., stellar spectral lines had been observed, but they weren't understood theoretically until the advent of quantum mechanics in the 20th cent. Joseph Fraunhofer was the first to observe absoprtion lines in the Sun (1811) and later in other stars - noted that different stars had different patterns of absorption lines spectral line emission and absorption had been observed in the lab in the mid-19th century (Gustav Kirchhoff, among many others) - flame spectra (DEMO) - they knew that different atomic species had different spectra - could match them up a bit Fraunhofer's result led many to believe that the composition of the stars was not constant, but differred from star to star - naturally, you'd want to know how many of each kind are up there --> classification sequence based on spectral lines -- really got going in the late 19th century with a bequest from the widow of Henry Draper to create a spectral type catalog for zillions of stars. -- original classification scheme based on strength of H absorption line idea: stronger H absorption, more H i.e., classification based on composition A,B,C,...M,N,O while all of this spectral line classification was going on, others were looking at the broadband spectrum 1866 Father Pietro Angelo Secchi classified stars based on their "color" white, yellow, red, deep red - he guessed (correctly) that color changed with temperature idea: evolutionary sequence -- stars cool off over time (incorrect) white hot -> yellow -> red ... no one knew why stars were hot anyway, so the thought of them just slowly cooling off made sense 1901, Annie Jump Cannon -- working on the HD catalog at Harvard classifying spectra in the alphabet sequence -- noticed that if she adopted Secchi's temperature sequence based on the broadband spectrum -- the spectral line classification also made sense -- i.e., smooth variations from spectral type to spectral type -- no longer ordered with decreasing H absorption line intensity but still smooth in variations. --> RESULT: the sensible sequence was OBAFGKM, not ABCD... -> a connection between the blackbody properties and spectral line properties of stellar spectra NOTE: _not_ based on theoretical understanding pure classification result this happens more often in science than you might think people have no idea what's going on, but just by looking at the data and seeing patterns, they can trip over some fundamental relationships. Cannon's result made some sense in terms of temperature based on BB emission, but the patterns of spectral lines, and how they changed with spectral type was mysterious compositional differences? -- are hot stars deficient in H? -- are cool stars deficient in He? -- perhaps compositional differences are responsible for different temepratures (i.e., T determined by what fuel you've got) these are perfectly reasonable guesses, but they're WRONG it really took until the 1920's to understand the connection between temperature and spectral line emission