ASTR 100. Lecture 16: Light and spectra, Classifying stars

Transcription

1 ASTR 100 Lecture 16: Light and spectra, Classifying stars

2 Reading: Light and Matter (Ch. 5), The Sun (Ch. 10), and Stars (Ch. 11) Tomorrow: More spectra, start Ex. 6 Wednesday: Stars Friday: Quiz, Ex. 5 due Midterm: Tuesday, February 18 th UW planetarium: Tuesday, March 4 th 3:00pm

3 Summary: Review the Physics and Anatomy of Sun Electromagnetic radiation: Light and spectra Star classification The two ways that light is made: Blackbody and emission/absorbtion lines

4 Main parts of the Sun and what s going on inside of them: 74% Hydrogen 24% Helium 2% Heavier stuff Core: where fusion happens: H->He Radiation zone: up to 1,000,000 year random walk of photons Convection zone: Heat carried by columns of hot plasma Photosphere: Surface of the sun, part we see Atmosphere : Interesting

5 The Sun s different rate of rotation at the poles and equator causes the magnetic field to get tangled.

6 Eventually these tangled field likes bulge and pop out of the surface. A sunspot.

7 Eventually these tangled field likes bulge and pop out of the surface. A sunspot.

8 Been observing Sunspots since Galileo Sunspot activity occurs on an 11-year cycle (with some other behavior associated with with climate on Earth).

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10 Coronal Mass Ejections: The plasma flung off of the Sun August 28 th - Sept. 1 st 1859 The Carrington Event -Telegraphs shock operators -Aurora Borealis visible in Cuba -Estimated would cause ~$3Trillion in damage if it happened now

11 Talk more about the Electromagnetic spectrum X-ray Ultraviolet

12 Start seeing more and more pictures like this:

13 Solar storm in UV Solar storm in X-ray

14 < meters < meters All light is a wave made out of wiggling Electric and Magnetic fields. In space, all light moves at the same speed: c = 300,000 km/sec (the same c in E = mc 2 ) The only difference is what the wavelength of light is, the distance from peak to peak of the wave

15 For visible light different wavelengths = different colors red is 7.9 x 10-7 meters blue is 4.7 x 10-7 meters < meters < meters

16 Going to start doing astronomy in all these wavelengths

17 Art is astronomy false coloring

18 Even sneakier: Composite images Blue: x-ray Green/Red: IR

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21 Spectrum: How much of each type of light an object emits Do you think the spectrum continues?

22 We typically describe spectra with intensity diagrams instead of coloring them: 1) It s more convenient 2) We can represent the spectrum beyond visible light

23 Going to do that Newton trick where we hope and pretend that the physics and theories which we see locally can be extended to the far reaches of the cosmos.

24 Why is our Sun that nice yellowish and other stars red or blue or white?

25 Any guesses?

26 Is it chemical composition?

27 The temperature of something is indicated by the color it glows. Blackbody spectrum

28 Everything has a Blackbody spectrum, most just aren t in the part of the spectrum visible to human beings

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31 Can classify stars based on spectra Harvard spectrum cataloging factory: or so

32 Spectra (and their intensity diagrams) for stars are more complicated than those smooth lines I showed you.

33 Real spectra of a bunch of stars: (Weird gaps in spectra?!)

34 Annie Jump Cannon, personally classified 400,000 stars Temperature-based order OBAFGKM (LT) O hottest (40,000 K) Subdivided: F0 hotter than F5

35 One last mystery in the spectrum of the Sun. The gaps.

36 The presence of the gaps took until the early 20th century to explain. The quantum theory. Erwin Schrodinger: Quantum theory Cecilia Payne-Gaposhkin: Applied to stars

37 For visible light different wavelengths = different colors Instruments produce certain pitches because only certain wavelengths are allowed (for example, by the nut and bridge holding a string in place)

38 Quantum Mechanics: Atoms obey a wave equation, like a musical instrument. Only certain specific wavelengths are allowed to be emitted or absorbed

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40 So you can make light two ways: Emission Spectrum: Diffuse gas of cool atoms or molecules, they only absorb or emit specific wavelengths. (Like a fingerprint - every atom or molecule has a unique spectra.) Blackbody Spectrum: A huge amount of hot atoms or molecules all smooshed together will emit a blackbody spectrum. (No information about the chemical composition - all hot, dense sources at the same temperature have the same blackbody spectrum.)

41 So what s going on in Stars? Blackbody behind a cooler gas, the cooler gas absorbs and scatters away only its allowed wavelengths. Absorption Spectrum

42 Kirchoff s laws of spectroscopy Blackbody Emission Absorbtion = Blackbody - Emission

43 All of this happens in the photosphere. Crazy. Also, cold gas is 5800 degrees.

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45 Key terms: Spectrum, Intensity diagrams, Blackbody (continuous) spectrum, Emission lines, Absorption lines, Energy level transitions, spectroscopy Key Ideas: What are the basic parts of the Electromagnetic spectrum? What is a blackbody spectrum and how can we use it to take something s temperature? What is OBAFGKM(LT)? What is responsible for the gaps in the spectra of stars? Why are the emission/absorption spectra of elements considered fingerprints? How do we know what elements and molecules are present in stars?