What Are Stars Composed of? Archives: Ch. 29, Cosmos: Stars

1. What Are Stars Composed of?Archives: Ch. 29, Cosmos: Stars Recall that the spectra revealed some elements in the stars. But to conclude stars are composed mostly of those elements would be like assuming cake is made of frosting or that people are made of skin. We want to know what’s inside! Presence vs. Abundance

2. Are some stars made of hydrogen and some of Iron? Recall the big difference in the presence of elements in different color stars. Could they be made of different stuff? Temperature seems to relate to elements seen (expressed), but to understand how we need to get the latest on the atom from Niels Bohr, 1913…

3. Why do elements on Periodic Table have different spectra?

4. = electron (-) = proton (+) = neutron (o) me ≈ mp (mass) 1 1830 The Atom and its “Orbits” Atoms Size of proton or neutron ≈ 10-13 cm 10-8 cm electrons drawn to nucleus by opposite charge have discrete orbits, energy levels (comes from Quantum Mechanics)

5. 1 2 3 4 Electrons have discrete energy levels H (ground state n=1) Bohr Atom Outer "orbits" (levels) have greater energy (less negative) than inner "orbits" (levels).

6. Games with Discrete Moves Quantum Mechanics is a set of rules for the atom with discrete moves, like other games you’ve played… Quantum Mechanics Not for children under 18, PhD not included A specific (discrete) roll of the dice or spin of the wheel to win!

7. 1 1 2 2 3 3 4 4 E = hv (photon) H (ground state) H (excited state) • Reaching outer "orbits" (levels) requires input energy • Electron can only make a quantized transition • from one shell (energy level) to another by absorbing or • emitting a photon of the exact energy

8. H Before absorption After absorption 1 1 2 2 3 3 4 4 hc Ephoton = DE = E4 E2 = hvphoton = lphoton (difference in energy levels) Continuous spectrum from BB

9. 1 2 3 4 Get emission of photon if electron (in excited state, n>1) falls down to a lower energy level Emission occurs in any direction (random). Several drops can occur, to various levels.

10. VIBGY O R V I B G Y O R “Absorption Line” Some green photons are missing b brightness l

11. Before absorption 1 2 3 4 Why So Excited? Normally electron in ground state (1). But if atom is in a warm gas (or in an electric field like discharge tubes) atoms jostle (collisional excitation), electrons move to higher levels H

12. + + - - Incident photon has sufficiently high E, = or > than level Electron is stripped [ I'M AN ION. … ARE YOU SURE? YES, I'M POSITIVE! ] Hydrogen (ground state) Hydrogen (ionized)

13. Question Which element below can be ionized the greatest number of times? H O Fe answer, c

14. a Balmer b e d Lyman g d g b ∆E a n=1 n=2 a 3 b g Paschen 4,5,6,... Hydrogen Each neutral element and ionized element (i.e., one that lost ≥ 1 electron) produces a unique pattern of lines (like a fingerprint). Can identify elements in the Universe!

15. g . . . b a g d Paschen b . . . b IR Balmer (Hydrogen) a g d . . . Lyman Visible UV l Energy Hydrogen spectrum Energy of photon absorbed = hv = hc/l = DE = difference between energy levels

17. No line No line No line weak line weak line strong line Ground state Excited to n=2…ready! ionized stronger line strongest line stronger line It takes enough atoms and at “Goldilocks” temperature! e.g., H (n=2 to 3, =656 nm --green photon) Stellar Atmosphere H,He, Fe… So Why Did Different types of stars show different elements? Cold Star Warm Star Hot Star Line strength Line strength Line strength No H Some H Lots of H

18. Announcements • Turn in lab Wednesday • HW #4 due Monday • Midterm March 13

19. Heavy, Shielded Barer nucleus Harder to excite Need Higher T Shielded nucleus Easier to excite Lower T enough excited (n=2) once ionized, less shielding, harder to excite ionized ground state H Different Atoms,Different Energy Levels “Goldilocks” Temperature Weakly bound Strongly bound Heavy, highly ionized Very Strongly bound

20. At a specific temperature, line strength tells abundance of element strong line More H Less H weak line No line

21. Question: not much hydrogen in an O-star O-star too cold to excite hydrogen O-star too hot for neutral hydrogen we never looked Why don’t we see much hydrogen in an O-star? answer, c

22. In the Sun, the transition from level 4 to level 2 of hydrogen produces photons with a wavelength of 486.1nm. In a star twice as hot as the Sun, this transition would produce photons with • a) half that wavelength. • b) the same wavelength. • c) twice that wavelength. • d) four times that wavelength. Answer, b

23. The number of electrons lost by an atom in a gas (i.e. its ionization state) depends primarily on the • a) velocity of the gas. • b) level of the ground state. • c) temperature of the gas. • d) size of the gas cloud. • e) energy required to strip away all the atom's electrons. Answer, c

24. Atom X has energy levels of 1 and 10. Atom Y has energy levels of 1 and 12. An electron in each atom moves from the upper energy level to the lower energy level, emitting a single photon in the process. Which of the emitted photons has a longer wavelength? • a) the photon from atom X • b) the photon from atom Y • c) the wavelength is independent of the photon energy • d) cannot tell without more information Answer, a

25. Two Ways to order Stars By color By similar strength of elements O B A F G K M blue red Hottest Coolest Oh Be AFine Girl/ Kiss Me Guy

26. At a specific temperature, line strength tells abundance of element strong line More H Less H weak line No line In 1920, if you understood these principles and made use of quantitative descriptions (Saha and Boltzman eqs) and empirical energy levels, you could compare prediction to observations to figure out abundances of elements And in 1920, the composition of all stars was discovered by the lowest member of the academic food chain in 1920, a female graduate student!

27. Cecilia Payne-Gaposhkin 1925 HCO: “most brilliant Ph. D. thesis ever written in astronomy” 1900-1979 Found differences in temperature alone caused differences in elements seen in star spectra, without much change in composition. Stars all have same recipe! 2) Accounting for temperature, derived the relative abundance of elements from the intensity of their absorption lines.

28. Key Page

29. The Prime Element Hydrogen Consider that the solid Earth was understood to be more Iron, Fe than anything else (32%) So conclusion was that H/Fe=1011/104.8~106 So hydrogen in stars was a million times more abundant than Fe and anything else other than Helium!

30. Can you Believe This? Most of Universe? Earth Composition This was a bombshell and if you were a female grad student in 1925 it was risky to drop a bomb!

31. The Rebuke January 14, 1925 My dear Miss Payne:

Here, at last, are your notes on relative abundance which you were so good to send me some time ago....
You have some very striking results which appear to me, in general, to be remarkably consistent. Several of the apparent discrepancies can be easily cleared up… There remains one very much more serious discrepancy, namely, that for hydrogen, helium and oxygen. Here I am convinced that there is something seriously wrong with the present theory.It is clearly impossible that hydrogen should be a million times more abundant than the metals, and I have no doubt that the number of hydrogen atoms in the two quantum state is enormously greater than is indicated by the theory of Fowler and Milne. Compton and I sent a little note to ﾔNatureﾕ about metastable states, which may help to explain the difficulty. . . .

 Very sincerely yours,

Henry Norris Russell The consequence was that Payne edited her thesis to conclude, “[the results on hydrogen and helium being so abundant] are regarded as spurious…almost certainly not real.” Within a few years the correctness of her original conclusion was confirmed by others and finally Russell. Payne went on to become first female tenured professor at Harvard and first female chairwoman of the Department.

32. Cecilia Payne-Gaposhkin “The reward of the young scientist is the emotional thrill of being the first person in the history of the world to see something or to understand something. Nothing can compare with that experience … The reward of the old scientist is the sense of having seen a vague sketch grow into a masterly landscape.”

33. Lots of Line: Spectrum of the Sun as an Image Ref: http://www.noao.edu/image_gallery/images/d5/suny.jpg

34. Spectrum of the Sun in the Blue Ref: ftp://ftp.noao.edu/fts/visatl/cph20700.ps

35. Abundance as we Know It 10 10 10 10 10 10 10 Although H atoms outnumber heavy elements by 105, common heavy elements are ~10-100 times heavier, so stars by mass are: ~90% Hydrogen, ~10% Helium, 0.1% the rest! What sets this abundance pattern…..?

36. Sun vs Solar System Heavy Element Abundance