Understanding Atoms and Light in Astronomy: A Comprehensive Guide

1. Astronomy 130, Lecture 6 Interstellar Medium

2. Outline • Atoms and Light • Radiative processes in the active ISM • Star formation

3. Spectra Light from telescope

4. Atoms • Nucleus + electron cloud ManhattanIsland Ping pong ball

5. The Nucleus • Atomic Number, Z, is the number of protons in the nucleus • Isotope Number, or rough atomic mass, A, is the total number of nucleons • Nucleus ALWAYS positively charged Z=1, A=1 Z=1, A=2 Z=6, A=13 Z=6, A=12 +

6. The Electron Cloud • Electrons are negatively charged • Attracted to the positively charged nucleus by Coulomb force • Like gravity: inverse square law, mutual force according to both charges • orbits?

7. Classical Orbits Fail • Charges radiate when they are accelerated. • An electron is a classical circular orbit would continuously radiate. • For instance, electrons can circle in magnetic fields. When they do, they radiate by cyclotron • The orbital energy would be radiated away, and the orbit would decay rapidly. • Electron orbits simply cannot be classical. Power radiated Acceleration

8. Cyclotron Radiation http://www.gemini.edu/files/docman/press_releases/pr2004-6/Cyclotron_radiationMR.jpg

9. Synchrotron (relativistic cycolotron) http://lifeng.lamost.org/courses/astrotoday/CHAISSON/AT325/IMAGES/BG16FG11.JPG http://www.plasma-universe.com/images/3/39/M87_jet.jpg http://www.daviddarling.info/images/synchrotron_radiation.jpg

10. Electron Orbits • Classically, expect orbits according to Kepler’s laws • Quantum Physics tells a different story • “quantized” orbits at only specific energy levels (Bohr atom) Z=5 Z=1 Z=2 “permitted orbits”

11. Really spatial probability distributions, not orbits, per se. http://guildofscientifictroubadours.com/wp-content/uploads/2010/11/Hydrogen_Density_Plots.png

12. Excitation • Electrons can be “excited” from on orbit to another move from orbit to orbit • Collisions with other atoms • Incoming radiation • Energy change must be exactly right

13. Excitation by Collision • Similar to excitation by light, but collision is with another atom, rather than a photon • In a hot gas (thousands of K) most atoms have collisionally excited electrons

14. Excitation by Light • Allowed energy transitions Allowed energy changes in Hydrogen

15. Absorption and Emission of Light • If energy is just right, atom absorbs photon, and stores energy in electron energy state • Quickly, the atom re-radiates, however • Absorption and emission only at specific energies

16. Spectral Lines Absorption Lines: Light at particular energies is absorbed by atoms in the gas. The atoms become excited. Later they radiate the energy away in a random direction. This leaves a net deficit in those wavelengths.

17. Spectrographs Light from telescope

18. Absorption Lines as Thermometers Hydrogen’s Energy Levels • Consider Hydrogen • If very cold, atoms in ground state: no Balmer (all Lyman) • If very hot, all atoms in highly excited state: no Balmer • Strong Balmer ONLY when temperature is about 10,000 K (atoms in low excited state) Fraction of transitions to different levels tells temperature!

19. Thermometers of Other Elements • Multiple elements work in different ranges

20. Spectra of Stars

21. Stellar Types and Temperature Stellar Type is defined uniquely by temperature

22. Spectrum of the Sun

23. Active Regions of the ISM • Typical 8” view of Orion Nebula • Prettier view of Horsehead region Pretty, but what’s going on?

24. The Ionization Region • Young OB stars in the Trapezium blast nearby gas with radiation • Several processes produce the light we see Brilliant O,B Stars Photo-Ionized Gas

25. Absorption and Re-emission This process generates the light most visible to our eyes. Important species include Hydrogen transitions to the 2nd electron state (blue-green and red), and ionized Oxygen transitions (green). • O star emits a blue photon (Hb=0.486mm) 2. Photon strikes neutral atom in gas, exciting an electron to a higher energy level 3. Atom remains excited only about 10-8 sec. 4. Atom re-emits photon in a random direction and returns to its un-excited state

26. Absorption and Re-emission • A little like fog, but only in specific wavelengths Gas absorbs starlight then radiates it in a random direction Fog scatters light from streetlight in a “random” direction

27. Absorption and Re-emission absorption re-emission excitation Spectrum seen on Line-of-sight to star Spectrum seen off Line-of-sight to star

28. Spectral Lines • Absorption strength depends on amount of material in the way Strong Absorption Weak Absorption

29. Column Density • In any sq. cm., I see some fraction of the star. • In that sq. cm., I see same fraction of total absorbers. • Fraction of light blocked is proportional to the number of absorbers per sq. cm. Extreme zoom in on my view of the star

30. Column Density • Amount of absorbing material is proportional to the density of the gas, and the length of the path through the gas N = “column” density of atoms per cm2 n(x) = density of atoms percm3 x

31. Column Density and Spectral Lines Curve of Growth • Low column density: line is narrow, may not block any wavelength completely • Moderate column density: line is broader, blocks some wavelengths completely Relative flux • Very high column density: line is very broad, with wings; blocks a range of wavelengths completely wavelength

32. Ionization and Recombination Hydrogen’s Energy Levels • Ionizing radiation has energies sufficient to eject the electron completely • 13.6eV (l = 0.0912mm) from ground state to “free” levels • More energetic photons also work, but less likely as energy increases • X-rays don’t work very well

33. Ionization and Recombination This process is so efficient that we can’t even see the star in the UV at these ionizing wavelengths 1. O star emits a UV photon (l < 0.0912mm, E > 13.6eV) 2. UV photon strikes neutral atom in gas, ionizing it 3. Ion now very interactive with electrons + 4. Ion Recombines with electron, and emits a photon in a random direction 4 (alt). Ion Recombines with electron into a different orbits and emits a photon at a different energy in a random direction… still excited

34. Ionization and Recombination • Similar to absorption and re-emission • Photons are effectively sprayed in random directions • Differences • Occurs over a wide range of wavelengths (above the ionization energy), not just a specific wavelength • Likelihood of interaction decreases as photon energies become very large (X-rays don’t ionize much) • Ionization can leave gas nearly completely ionized, which means little neutral absorption and re-emission is possible • Ionized gas remains very hot (around 10,000 K)

35. HII Regions • II (Roman 2) means once ionized • Highly Ionized regions • Usually about 10 atoms/cc • Usually about 5-10,000 K (at this temperature, balance of neutral gas and collisionallyionized gas) • Luminescence from recombinations HII HII

36. HII Regions • These regions tend to stay less than about 10,000K, because at this temperature region, hydrogen emission is very efficient at radiating energy away. • This is the temperature at which there is a balance of neutral and ionized gas. • Neutral gas is mainly transparent, allowing radiation to escape, which permits cooling. • Exchange of ionization and emission is very efficient. • See the Saha Equation

37. Molecular Clouds • Frequently, active regions also have molecular clouds • Regions where gas is denser and cooler: < 10,000 molecules / cc, T > 10-100K • Abundant molecular transitions “self-shield” cloud • Rapid absorption of incoming radiation • Rapid radiational cooling • Principal species are H2, CO, some hydrocarbons, NH3, etc. • Dust… Molecular Cloud

38. Interstellar Dust • Dust • About 1-5m in size (similar to fine room dust) • Larger than wavelength of visible photon • Grain looks like a boulder to photon • Primarily carbon • Extremely efficient absorber

39. Dust Grains • Dust grains absorb light, heat up and radiate thermally 1-5 mm Grain absorbs Photon, causing it to heat up Visible/UV photon strikes grain Grain radiates heat thermally, and cools

40. Visible light image Dust • Dust clouds can extinguish light by up to 50 magnitudes (a factor of 1010) • Infrared light penetrates dust… much larger wavelength (dust not a boulder) Infrared image

41. Dust Column Density • Dust does not generate absorption lines, rather VERY broad absorption regions of the spectrum • Column Density translates to exponential decrease in light Half of light absorbed Half of light absorbed n(x) = density of grains percm3 (Same math)

42. Extinction Law • Optical Depth, t • Proportional to column density, flux decreases exponentially in t • Linear change in magnitude AV is extinction

43. Extinction and Color • Red light is generally less extinguished than blue light • What looks like a boulder to blue light isn’t so big to red (or infrared) light Blue light, scattered or absorbed (Rayleigh Scattering) Red light just passed through

44. Visible light image Dust • So we can see through the dust with infrared light • Also, can estimate total amount of dust by comparing extinction in different colors Infrared image

45. Another Dust Pic scattered blue light reddening

46. Dust in the Milky Way

47. More Scattered Blue Light • Dust remaining from star formation process …

48. Outline • Atoms and Light • Radiative processes in the active ISM • Star formation

49. Active Star Forming Regions Hot, ionized gas M16: The Eagle Nebula Young OB stars Dense Molecular gas, dust

50. Bright OB star Fully Ionized Gas Absorption and Re-emission Partially Ionized Gas Ionization and Recombination “Evaporating” molecular gas Grain absorption, heating and re-radiation Ionization Front Dusty Molecular Gas Molecular Cooling (thermally excited molecule radiates)