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(Joseph von Fraunhofer, 1814) The device that resolves the colors of light: the spectroscope (Joseph von Fraunhofer, 1814) • Light from telescope enters slit (to block off stray light) • Prism (or grating) separates the colors • Produces spectrum on a screen/on a film (Put this device in a box and attach it to the telescope) The spectroscope Surprise: The spectrum of the Sun has black lines: Some colors are missing! (Fraunhofer-lines) Spectrum: each color is a “line”

The spectrum of the Sun Absorption lines (“Frauenhofer-lines”), (Color and b/w version of the same thing.) H H Absorption lines (“Frauenhofer-lines”), many in hydrogen wavelengths Here: colder gas takes out a few colors Solar atmosphere

the chemical composition Types of spectra Continuous spectrum (thermal glow & synchrotron radiation) Absorption spectrum (gas illuminated from behind) Emission spectrum (rarified gas: fluorescence) Each chemical element has its own spectral lines: A good way to tell the chemical composition of a star!

The spectra of stars – the effect of photospheric temperature

Questions coming …

sec 8 9 10 12 11 7 6 1 2 3 4 13 30 35 40 45 14 25 16 15 18 17 19 20 5 Question 4 How do we know that the Sun is made mainly of hydrogen? A Its density equals the density of hydrogen. B Spaceships brought back a samples from the Sun. C Its spectrum contains hydrogen lines. D The gravity of the Sun can be explained only by hydrogen gas. E Only burning hydrogen (and no other gases) can produce enough heat to explain why the Sun is hot. Next question coming …

sec 10 9 11 13 14 8 12 7 2 1 3 4 6 5 15 17 26 25 27 28 30 29 16 24 19 18 23 20 22 21 Question 5 Which one of the following types of spectra indicate the presence of cold gas located between a star and Earth? A Absorption spectra. B Emission spectra. C Continuous spectra. D Line spectra. E Band spectra.

sec 10 9 11 12 15 14 13 8 7 1 2 3 5 4 6 16 26 17 27 28 30 29 24 25 23 19 18 20 21 22 Question 6 The spectrum of the Sun consists of A a few bright lines only. B dark lines over a bright continuum. C dark bands of light with bright centers, over a bright continuum. D a continuous bright spectrum like a rainbow. E a few bright bands with dark centers. Next question coming …

The effect of movement on spectral lines: The Doppler effect • Light source moves towards observer: spectral lines are shifted towards blue • Light source moves away from observer: spectral lines are shifted towards red • Light source moves sideways: no change

The spectrum of another star The Sun H H Polaris Spectral lines shifted towards blue: Polaris is approaching us Measure shift: find radial speed 17 km/s Why are the spectral lines shifted? -- the Doppler effect

Questions coming …

sec 8 9 10 12 11 7 6 1 2 3 4 13 30 35 40 45 14 25 16 15 18 17 19 20 5 Question 7 In what are spectra of other stars different from the Sun’s? A The same spectral lines are there in every star’s spectra, but they may be shifted if the star is moving. B Different spectral lines may be visible, in addition to Doppler-shift. C All stars have the same spectrum because they are all made of gas. D The spectrum is the same, except that the blue part of hotter stars’ spectra is brighter. Next question coming …

sec 8 9 10 12 11 7 6 1 2 3 4 13 30 35 40 45 14 25 16 15 18 17 19 20 5 Question 8 What is the Doppler effect? A Stars move on tiny circles in the sky, once a year. B The stars appear red when they are rising or setting. C Far-away stars appear red because the Universe is expanding. D The spectral lines of a star are shifted towards red, if the star is moving away from us, and towards blue, if the star is approaching us. E A fast moving star’s spectral lines are shifted towards red, independently of the direction of motion. Next question coming …

sec 10 9 11 12 15 14 13 8 7 1 2 3 5 4 6 16 26 17 27 28 30 29 24 25 23 19 18 20 21 22 Question 9 A particular galaxy is moving away from us with large speed. Which of the following happens to its spectrum? A The spectral lines show up shifted towards red. B The spectral lines show up shifted towards blue. C The color of the galaxy becomes redder. D The color of the galaxy becomes bluer. Next question coming …

sec 10 9 11 13 14 8 12 7 2 1 3 4 6 5 15 17 26 25 27 28 30 29 16 24 19 18 23 20 22 21 Question 10 Which of the following happens to the sound of the whistle coming from a train approaching us at high speed? A It becomes lower-pitched. B It becomes higher-pitched. C It becomes less loud. D It becomes more loud.

Example: Rotational line broadening: The details of the spectra tell a lot: • Chemical composition: from color of lines • Rotation: from width of lines • Movement (towards us or away from us): from shift of lines • Strength of magnetic field (if any): from split of lines • Temperature, gravity, wind speed, …: from detailed structure of lines Example: Rotational line broadening: What do spectra tell us?

Example: a rotating galaxy The top of the slit shows blueshift - - stars there are moving towards us The bottom of the slit shows redshift - - stars there are moving away: Example: a rotating galaxy Example: This galaxy rotates!

Questions coming …

sec 10 9 11 12 15 14 13 8 7 1 2 3 5 4 6 16 26 17 27 28 30 29 24 25 23 19 18 20 21 22 Question 11 How do we know that galaxies rotate? A Their spectral lines are broad. B The spectral lines of different parts of the galaxy show different red- and blueshift. C The shape of a galaxy changes in a few years. Next question coming …

sec 10 9 11 13 14 8 12 7 2 1 3 4 6 5 15 17 26 25 27 28 30 29 16 24 19 18 23 20 22 21 Question 12 How will the rotation of a star affect its spectrum? A Spectral lines get blueshifted. B Spectral lines get redshifted. C Spectral lines broaden. D The spectral lines are not affected, but the stars overall color changes.

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