Mon. Feb. 22, 2010 n How do stars ? absorption lines tell us n Rotation? Temperature? Surface gravity? Relative motion? n Challenge: how many of our questions about the cosmos can already be answered with the tools we have? Based on a participation point question from last week n How might we tell that a star is moving towards us at 30 km/sec? n It would get measurably brighter n Its color would get noticeably bluer n Its color would get noticeably redder n Its spectrum lines would shift slightly but measurably to the blue n Its spectrum lines would shift slightly but measurably to the red 30 km/sec is .0001 c n Wavelengths are shifted to blue by .01% n Blue (440 nm) light shifted to 439.99 --would you notice that? (no) How about the brightness? n Let ? s say star is 20 million AU away n In 2 months, we move 140 million km closer together n Is that a lot? n Not compared to the 3100 trillion km of its distance! Another quick example: n Venus ? surface T = 740 K n Venus spectrum peaks at 3900 nm (3.9 microns) But we can ? t SEE Venus ? surface n How do we know it ? s spinning backwards? Fig 4.18 (adapted to Venus case) n From receding edge, radar signal red shifted by 6x10 -9 n From approaching side, radar signal blue shifted by 0.002 km/sec Does the sun satisfy these? n Near enough n Large enough (in angular terms) n Not reflective: So, we can use some of these techniques: n We can n Measure the sun ? s peak wavelength ( spectrum shape) => Temperature n Look for Doppler shift at edges n But we can ? t use radar to confirm its distance Review solar T: Peak wavelength 500 nm, so T =5800 K (but let me pose a little issue ? ) n What happens during the day? n Sun is white in color at noon. By sunset, its color is n Same n Yellow n Orange n Red Do you think the sun ? s temperature has changed between noon and 6:00 p.m.? n No, so Wien ? s Law must be wrong n Yes, it must have ? Wien ? s Law is experimentally verified n No, something else must be affecting its color A few more things we can learn about the sun: Sunspots are darker than the surrounding photosphere. So ? n Consider patches of equal area. n Less light is emerging from the darker one n What does that tell you? Why are sunspots darker than the surrounding layers? n They just are. Deal with it. n The sunspot area must be cooler n The sunspot area must be hotter. n Something else is in the way of the sunspot gas. If we get a spectrum of a small patch of the sun ? s outer layer, n We see that its spectrum lines vary: n To bluer wavelengths n Then back to expected l n To longer (redder) wavelengths n All in minutes These changes in wavelength n Are an optical illusion n Result from Earth ? s spinning on its axis n Result from Earth ? s orbital motion n Result from vertical motions of sun ? s gas We can SEE the sun ? s outer layers bubbling What will be different about other stars: n Much, much, much more distant n Cannot see the disk of the star n Star 100 times radius of sun at 20 million AU n Angular diameter 0.02 So, we don ? t have all this information we had for Venus; we see the composite line profiles shown in Fig. 4.18 How can stars ? absorption lines tell us so much? n We have to know a few things: n What elements produce which lines which pattern of lines n True wavelengths n If lines appear at wavelengths NOT the true values, that ? s due to relative motion What about the lines themselves? n A Fe line implies presence of iron n Does the absence of Fe lines imply the absence of iron? n If not, why might they be missing? Our own gedanken experiment n Consider a H atom n Which of its lines do we actually see? n What has to be true for us to see a particular line? n Can the atom be disrupted?
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