Deciphering The Stars: Ch. IV,23,24,25,27 COSMOS: Ch. 2

1. Deciphering The Stars: Ch. IV,23,24,25,27COSMOS: Ch. 2 Fraunhofer’s 1814 drawing of solar spectrum and missing light and on German Postage Stamp Modern Solar Spectrum Notice no American contributions to astronomy yet. Why?

2. What are the Stars?In ~1835 it was assumed we could never know… Huggins: “That they shine, that they are immensely distant, that the motions of some of them show them to be composed of matter endowed with a power of mutual attraction” • Come quickly, I am tasting stars! Dom Perignon, at the moment of his discovery of champagne • Be humble for you are made of dung. Be noble for you are made of stars. --Serbian Proverb • It was almost as if the distant stars had really acquired speech and were able to tell of their constitution and physical condition--Annie Jump Cannon • I ask you to look both ways. For the road to a knowledge of the stars leads through the atom; and important knowledge of the atom has been reached through the stars.--Arthur Eddington Ancients: stars as holes pierced in the screen between us and heaven. By mid 19th century: how they move, that a new one may suddenly appear, that they may change in brightness. But what are they? Need to learn nature of light, how produced, how iencodes the nature of stars.

3. Pass "white light" (ordinary sunlight) through a slit, prism: see rainbow (spectrum). What is light? slit R O Y G B I V white light glass prism Sun "Roy G. Biv" – good mnemonic(I=indigo; obsolete) Discovered by Sir Isaac Newton Light refracts (bends) when it passes through different medium As Snell’s law (shortest path): nisini=nrsinr (slower--> towards normal). But nglass depends on color (wavelength) of light so (any) glass also disperses light into colors.

4. Prism Demo

5. l Q l (Greek "lambda") = wavelength= distance from one crest to the next v (Greek "nu") = frequency= number of times per second a crest passes some fixed point Q [Note: v 1 = 1/v = P = period] c = lv Typical unit of measurement for l: 1 Å (Ångstrom) = 1010 m = 0.1 nanometer (nm) Red: ~6500 Å Yellow: ~5500 Å Green: ~5000 Å Blue: ~4500 Å Light Sometimes Acts Like a Wave(i.e., interferes) Wavelike (“Two Slit Experiment”--interference) http://www.youtube.com/watch?v=5PmnaPvAvQY

6. Particle-Like, photons of discrete energy Wave-acles or Paves  = hc/ l E=h Increasing Energy Light Also Acts Like A Particle (i.e., comes in chunks) http://www.ifae.es/xec/phot2.html Like dislodging ball takes big hit, Lots of tiny hits won’t do it Photoelectric effect. It takes a minimum amount of energy, E=h, to eject electrons from a metal plate. So low energy (frequency) light cannot free electrons no matter how long shined. This is particle-like (i.e., comes in discrete packets) behavior.

7. Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields.

8. Remote Question A difference between gamma rays and radio waves is: gammas travel faster radio waves travel faster gammas have more energy gammas have longer wavelength answer, c

9. Spectrum of visible light Plot brightness (intensity or power) as a function of color (wavelength l) for quantitative analysis of spectrum: b l VIBGY O R V I B G Y O R Visible light is one type of electromagnetic radiation. Different colors correspond to electromagnetic waves having different wavelengths (l). A spectrum turns out to be a very powerful diagnostic!

10. l In 1802 Wollaston then in 1814 Fraunhofer passed sunlight through telescope + spectroscope (slit+ prism)..”ebony threads sewn across a rainbow” Missing lines! What are they? Kind of “fingerprint” of sunlight What’s In Sunlight/Starlight? b Spectrum With more prisms can Split line more finely And see even more lines! Fraunhofer Lines (labeled A,B,C..)

11. Fraunhoher’s Contributions • Also observed Moon, planets through his “spectroscope”--same missing lines as Sun. Must shine by reflection! • Castor, Betelgeuse, Sirius--some lines the same and some different than Sun. Stars have different fingerprints • Discovered that steps or engravings on a transmissive or reflective surface also splits light, “diffraction grating”

12. Announcements • Observing this week, Lab due Monday, March 4 • HW #3 due this Wednesday • Midterm March 13,

13. Spectroscopy: The Fingerprint of Matter 19th c. Chemists had noticed. Bunsen: improved burner to be free of contaminants. Vaporized material emits specific color-bands of light. Key finding: each Chemical element produced a unique and characteristic pattern. Could tell which elements were in a sample from lines (fire on the Rhine). Vaporized material H Fe

14. Atomic Spectroscopy Demo

15. Discharge Lamps

16. Kirchhoff’s Laws • A hot solid, liquid or gas, under pressure, gives off a continuous spectrum • A hot gas under low pressure produces a bright-line or emission line spectrum • A hot object surrounded by a cooler gas results in gaps or an absorption spectrum

17. Emission-line spectrum Continuous spectrum Absorption-line spectrum Thin cloud Cool gas Hot Object Hot Star+cool atmosphere Absorption-line spectrum Putting it Together: Composition of the Stars Two parts to read in a star spectrum: the lines 2) continuum Can read the composition of the stars from absorption spectrum!

18. Question:The Sun Looks Yellow Because • its absorbs blue and red light • its emits red and blue light • it emits more yellow light than blue or red • it absorbs more yellow light than blue or red answer,c

19. Profound Realization Alert • If the stuff on Earth (vaporized in Bunsen’s burner) was in the Sun and in the stars, then the products which make up the Earth, i.e. chemistry is truly Universal. One periodic table to rule them all! • Kirchhoff identified hydrogen, sodium, iron, calcium, magnesium, chromium, barium, copper, zinc, and nickel in the Sun as well as a previously unknown element (later identified as Helium!). In 1863 W. Huggins saw similar lines in Sirius, Betelgeuse, and Aldebaran.

20. Absorption Spectra=Nature’s Barcode 32 Oz of Tide with Bleach made by Proctor-Gamble Betelgeuse- Filled with Ca, H, Fe, Mg, etc

21. For each letter select the matching interpretation for this part of the graph. What is A? • Hotter source • Cooler source • Continuous spectrum • Emission spectrum • Absorption spectrum

22. For each letter select the matching interpretation for this part of the graph. What is B? • Hotter source • Cooler source • Continuous spectrum • Emission spectrum • Absorption spectrum

23. For each letter select the matching interpretation for this part of the graph. What is C? • Hotter source • Cooler source • Continuous spectrum • Emission spectrum • Absorption spectrum

24. For each letter select the matching interpretation for this part of the graph. What is D? • Hotter source • Cooler source • Continuous spectrum • Emission spectrum • Absorption spectrum

25. For each letter select the matching interpretation for this part of the graph. What is E? • Hotter source • Cooler source • Continuous spectrum • Emission spectrum • Absorption spectrum

26. Continuous Spectrum • Let’s return to what the continuous spectrum (the rainbow) can tell us. What do you think it tells us?

27. Red Stars and Blue Stars • Notice that some stars look red, some yellow, and some look blue? Why? Betelgeuse Sirius Let’s return to Kirchhoff’s First law (continuous spectrum)…

28. Temperature Continuous spectrum, not shape, not composition Black Body Radiation “Black Bodies” are dense bodies (solids, liquids, gas under pressure) Metal Rock ~1000 deg lava Y X Well known from laboratory and shown by Max Planck, different material in an oven at same temperature will emit same continuous spectrum of light (color and brightness). So why do objects look different at all? The reason objects look different is reflected spectrum

29. Reflected vs Emitted Light Reflected (visible) Emitted (infrared)

30. 1) Color of any black body depends on temperature lpeak(Å) 7200 5800 4800 4100 Curve 1 2 3 4 T(K) 4000 5000 6000 7000 4 Hotter objects peak at shorter l Cooler objects peak at longer l Wien’s Law 3 2 1 l(Å) Wien’s Law lpeak= 2.9 x 107 [Å-K] T

31. 2) Flux from surface of black body depends on temperature Curve 1 2 3 4 T(K) 4000 5000 6000 7000 Watts/cm2 1500 3500 7400 13700 http://phet.colorado.edu/sims/blackbody-spectrum/blackbody-spectrum_en.html 4 Hotter objects emit more per cm2 Cooler objects emit less per cm2 Wien’s Law 3 area under curve rises with Temperature4 2 1 l(Å) Stefan-Boltzman Law flux = T4 [watts/cm2] ≈ 5.7 x 10-12 watt cm2 K4

32. But different elements seen in red and blue stars! Why? Different Gas Composition? Mystery in mid 19th century. lpeak(Å) 7200 5800 4800 4100 T(K) 4000 5000 6000 7000 Sirius Betelgeuse 9,900o K 3,600o K G A S Color Indicates Temperature BB Betelgeuse Sirius

33. Scenario 2: Color=Expression Skin color too, it’s the expression of melanin Examples: How Are Color and Composition (lines) Related? Scenario 1: Color=Composition This difference in color is based on the atomic structure of the mineral.

34. Early Spectroscopy William Huggins, British astrophysicist 1824-1910, also identified composition of many nebulae lacking bright nucleus • Pioneering, early spectroscopists examined hundreds of stellar spectra, trying to find reason for differences, began placing stars in crude groups • Also discovered element Helium (Helios), in Sun’s spectrum (Norman Lockyer, 1868) “A single line, not aggregate of stars”— purely a cloud of gas Dumbbell Nebula

35. In 1872 Henry Draper photographed First spectrum, Vega Astrophotography: Adding up the Light • The human eye “integrates” for a fraction of a second, only ~0.2 sec in dark, severely limiting the faintest source you can see, even with telescope. Also eye only absorbs ~2% of light • Invention of astrophotography, allowed long exposures (~1 hr) to see much fainter! Multiplexing too. Good as needed many spectra to unravel their meaning! Plates absorb ~10%. Light. John Draper (1840), moon Daguerrotype of Moon 1852 Louis Daguerre (1787-1851)

36. Classifying the Stars • Upon death endowed HCO to produce HD Catalogue of >250,000 stars. The classification was undertaken by “computers”--hired female assistants--0.25 $/hour, Pickerings Girls! (Annie Jump Cannon, Williamina Fleming, Henrietta Leavitt, Antonia Maury, 80 others)

37. Spectral Classification,Putting stars in order Hydrogen A --Sirius M-Betelgeuse In 1881 Williamina Fleming came up with lettering based on how strong hydrogen lines appeared A,B, …O But after > 1000 stars by 1901 Annie Canon discovered a smooth sequence of rising & falling strength of each & every line/element! New order Discovery: Order by temperature same as order by element/isotope strength! Indicated presence of elements determined by temperature not composition

38. Sun=G2 Two Ways to order Stars-same sequence! By color (temperature) By similar strength of elements O B A F G K M blue Sun red Hottest Coolest Oh Be AFine Girl/ Kiss Me Guy

39. Spectral Classification From color of star Like a puzzle, the picture matches the pieces! Implies deep order dictated by temperature!

40. Spectra reveal Temperature But why? In 1900 it was not yet obvious, but as Eddington said, I ask you to look both ways. For the road to a knowledge of the stars leads through the atom; and important knowledge of the atom has been reached through the stars.--Arthur Eddington

41. Stars: What we have discovered motions distance (luminosity) temperature visible composition masses They are enormous bright, hot balls of gas (inner parts dense, outer parts diffuse) like Sun, but light years away But, how are they born, die, and live?

42. Which of the following stellar properties can you estimate simply by looking at a star on a clear night? • a) Distance. • b) Brightness. • c) Surface temperature. • d) Both a and b. • e) Both b and c. Answer,e

43. For two stars of the same apparent brightness, the star closer to the Sun will generally have • a) a higher flux. • b) a hotter temperature. • c) a lower luminosity. • e) identical physical properties. Answer, c