Discovering the Universe Ninth Edition

1. Neil F. Comins • William J. Kaufmann III Discovering the Universe Ninth Edition CHAPTER 4 Atomic Physics and Spectra

2. WHAT DO YOU THINK? • Which is hotter, a “red-hot” or a “blue-hot” object? • What color does the Sun emit most brightly? • How can we determine the age of space debris found on Earth?

3. In this chapter you will discover… • the origins of electromagnetic radiation • the structure of atoms • that stars with different surface temperatures emit different intensities of electromagnetic radiation • that astronomers can determine the chemical compositions of stars and interstellar clouds by studying the wavelengths of electromagnetic radiation that they absorb or emit • how to tell whether an object in space is moving toward or away from Earth

4. WHEN FIRST HEATED, THE POKER GLOWS DIMLY RED AS THE TEMPERATURE RISES, IT BECOMES BRIGHTER ORANGE AT HIGHER TEMPERATURES, IT BECOMES BRIGHTER AND YELLOW These stars have roughly the same temperatures as the bars above.

5. Blackbody (Thermal) Radiation • Every object does it • Broad distribution over • wavelength, with one peak • 3. Hotter – peak shifts to • shorter wavelength • 4. Hotter – energy goes up • … a lot more than shown • Rules 3 and 4 are governed • by equations. . .

6. Blackbody (Thermal) Radiation • Every object does it • Broad distribution over • wavelength, with one peak • 3. Hotter – peak shifts to • shorter wavelength • 4. Hotter – energy goes up • … a lot more than shown • The 3000 K object looks red • The 6000 K object looks white • The 12,000 K object looks blue

7. Blackbody (Thermal) Radiation • Every object does it • Broad distribution over • wavelength, with one peak • 3. Hotter – peak shifts to • shorter wavelength • 4. Hotter – energy goes up • … a lot more than shown • Rule 3 is Wien’s Law: • λMAX (nm) = 2.9 x 106 / T (K) • Rule 4 is Stephan’s Law: • P = σT4 • P = power radiated per • square meter of area • σ = Stephan’s constant

8. Stellar surfaces emit light that is close to an ideal blackbody. We can estimate the surface temperature of a star by examining the intensity of emitted light across a wide range of wavelengths.

9. THERMAL RADIATION QUESTIONThe curves A, B, C, and D represent the observed spectra of 4 stars. Which star is the hottest?

10. THERMAL RADIATION QUESTIONThe curves A, B, C, and D represent the observed spectra of 4 stars. Which star is the coolest?

11. THERMAL RADIATION QUESTIONThe curves A, B, C, and D represent the observed spectra of 4 stars. Between Stars B and C, which is hotter? A – Both Equal B – Star B hotter C – Star C hotter D – insufficient information

12. Stellar surfaces emit light that is close to an ideal blackbody. We can estimate the surface temperature of a star by examining the intensity of emitted light across a wide range of wavelengths.

13. Mystery of Thermal Radiation • In the early 1900s the theory of thermal radiation worked at long wavelengths, but held that an infinite amount of energy was radiated as short wavelengths. • “Ultraviolet catastrophe” – for the theory, which was clearly seriously wrong. • Max Planck proposed that radiation was emitted in lumps (photons) . This theory predicted the observed curve exactly. • Planck’s constant “h” of quantum mechanics was born.

14. The combination of lines from the solar spectrum allows us to determine which chemicals are present and in what abundance.

15. Gustav Kirchoff and Robert Bunsen Pioneers in spectroscopy circa 1860 Kirchoff’s Laws of Spectra A high-pressure gas (or a solid or liquid) emits all colors of rainbow. This is a continuous spectrum. A low-pressure gas emits bright lines at specific wavelengths if excited. This is an emission line spectrum. When a source of a continuous spectrum is viewed behind a cool gas, dark lines are seen on the rainbow. This is an absorption line spectrum.

16. This schematic diagram summarizes how different types of spectra are produced. The prisms are added for conceptual clarity, A hot, glowing object emits a continuous spectrum. If this source of light is viewed through a cool gas, dark absorption lines appear in the resulting spectrum. When the same gas is viewed against a cold, dark background, its spectrum consists of just bright emission lines.

17. When a chemical is heated in the Bunsen burner flame, the light produced is made of only specific wavelengths. Each chemical element has its unique series of wavelengths, which is like a fingerprint.

18. Absorption and Emission Spectra Iron in the Sun’s Atmosphere The upper (absorption) spectrum is a portion of the Sun’s spectrum from 425 to 430 nm. Numerous dark spectral lines are visible. The lower (emission) spectrum is a corresponding portion of the spectrum of vaporized iron. Several emission lines can be seen against the black background. The fact that the iron lines coincide with some of the solar absorption lines proves that there is some iron in the Sun’s atmosphere. It also raises the question as to how the lines arise in absorption (upper) and emission (lower)

19. A grating spectrograph separates light from a telescope into different colors by passing it through a diffraction grating, which has many tiny parallel grooves.

20. Diffraction Gratings in Familiar Objects This peacock feather contains numerous natural diffraction gratings. The role of the parallel lines etched in a human-made diffraction grating is played by parallel rods of the protein melanin in the feathers. CCDs and DVDs store information on closely spaced bumps located on a set of nearly parallel tracks. Light striking these tracks systematically reflects different colors in different directions—it behaves like a diffraction grating.

21. This image and graph show a/an: • Absorption Line Spectrum B. Emission Line Spectrum • C. Continuous Spectrum D. None of the above

22. This image and graph show a/an: • Absorption Line Spectrum B. Emission Line Spectrum • C. Continuous Spectrum D. None of the above

23. WHY these fingerprints? Physics at the atomic level: • Existence of electron (JJ Thompson 1897) • Atomic structure (Ernest Rutherford 1910) • Quantum mechanics (Niels Bohr 1913) • Quantum mechanics (Werner Heisenberg, Erwin Schrödinger, ca. 1927)

24. Rutherford Scattering Experiment (1910) Most helium nuclei (also called alpha particles) entering a thin foil scatter slightly as they pass through the medium, but some scatter backward, indicating that they have encountered very dense, compact objects. Such experiments were the first evidence that most matter is concentrated in what are now called atomic nuclei. Electrons must orbit these nuclei.

25. But … Electrons can’t orbit! • Orbit means they accelerate (recall def) • Which means they radiate away energy • And they spiral into the nucleus in a microsecond … • So, no chemical element can exist … • And neither can we … Hmm • NielsBohr proposed a rule that …

26. Certain special allowed orbits don’t radiate Simple mathematical rule with Planck’s constant “h”for these orbits. Later Quantum Mechanics (late 1920s) replaced the idea of an allowed orbit with an electron cloud. This works for all atoms, not just hydrogen.

27. Energy Level Diagram for Hydrogen An eV is an electron volt, a unit of energy. Energy of photon determines wavelength; next slide) When an electron moves from a lower energy to a higher energy level, a photon is absorbed. When an electron moves from a higher energy level to a lower energy level, a photon is emitted. The energy of the photon, and thus its wavelength, is the energy difference between the two energy levels.

28. Photon energy and wavelength λ = h c / E λ = wavelength (Greek “lambda”) h = Planck’s constant (.. Thermal radiation) c = speed of light E = energy of photon λ = 1240 / E … h and c are constants λ in nm (1 nm = 10-9 m), E in eV (electron volts, a unit of energy).

29. Balmer Lines in the Spectrum of a Star This portion of the spectrum of the star Vega shows eight Balmer lines, from Hα at 656.3 nm through Hθ at 388.9 nm.

30. Emission Spectra from Interstellar Gas Clouds Left: Stars in this interstellar gas cloud (NGC 2363 in the constellation Camelopardus, the Giraffe) emit absorption spectra. Electrons in the cloud’s hydrogen gas absorb and reemit the red light from these stars. NGC 2363 is located some 10 million ly away. (b) Part of the Rosette Nebula (NGC 2237), an interstellar gas cloud in the constellation Monoceros (the Unicorn). The green glow is generated by doubly ionized oxygen atoms (O III; oxygen atoms missing two electrons) in the cloud that emit 501-nm photons. The Rosette is 3000 ly away.

31. Light comes in “lumps” called: A. Protons B. Photons C. Phonons D. Pleurons

32. What relation exists between photon energy and wavelength: A. Longer wavelength means higher photon energy B. Longer wavelength means lower photon energy C. Wavelength has nothing to do with photon energy because wave and particle natures are independent aspects D. All photons have the same wavelength

33. Some more physics . . . • Doppler effect • Radioactive decay • Effect of dust • Proper motion

34. Recall that the wavelength of light, and therefore the wavelength of the photons that light contains, is shifted when the source is traveling toward or away from the observer, the Doppler Effect.

35. The Transformation of Uranium into Lead This figure shows the rate that 1 kg of uranium decays into lead, as described in the text. The half life of Uranium is 4.5 billion years. The sample contains 0.125 kg of uranium after 13.5 billion years (3 half lives).

36. Dust dims and reddens Red light suffers less attenuation, compared to blue. Sunsets are red, and sometimes the Sun isn’t blinding at sunset. Related fact: the sky is blue.

37. Proper Motion The proper motion of a star is its motion perpendicular to our line of sight across the celestial sphere. This is so small that it can only be measured for the closest stars. The radial velocity of a star is its motion along our line of sight either toward or away from us. Using the spectrum, we can measure this for nearly every object in space!

38. Proper Motion of Barnard’s Star These are two superimposed images of Barnard’s star, taken a year apart in 2000 and 2001, showing the proper motion of the star during that time. In addition to having the largest known proper motion (10.3˝ per year), Barnard’s star is one of the closest stars to Earth.

39. Summary of Key Ideas By studying the wavelengths of electromagnetic radiation emitted and absorbed by an astronomical object, astronomers can learn about the object’s: ● temperature ● chemical composition ● rotation rate, ● companion objects ● movement through space.

40. Blackbody Radiation • A blackbody is a hypothetical object that perfectly absorbs electromagnetic radiation at all wavelengths. The relative intensities of radiation that it emits at different wavelengths depend only on its temperature. Stars closely approximate blackbodies. • Wien’s law states that the peak wavelength of radiation emitted by a blackbody is inversely proportional to its temperature—the higher its temperature, the shorter the peak wavelength. The intensities of radiation emitted at various wavelengths by a blackbody at a given temperature are shown as a blackbody curve. • The Stefan-Boltzmann law shows that a hotter blackbody emits more radiation at every wavelength than does a cooler blackbody. Total Power = Area x σ T4

41. Discovering Spectra • Spectroscopy—the study of electromagnetic spectra— provides important information about the chemical composition of remote astronomical objects. • Kirchhoff’s three laws of spectral analysis describe the conditions under which absorption lines, emission lines, and a continuous spectrum can be observed. • Spectral lines serve as distinctive “fingerprints” that identify the chemical elements and compounds comprising a light source.

42. Atoms and Spectra • An atom consists of a small, dense nucleus (composed of protons and neutrons) surrounded by electrons. Atoms of different elements have different numbers of protons, while different isotopes have different numbers of neutrons. • Quantum mechanics describes the behavior of particles and shows that electrons can only be in certain allowed orbits around the nucleus. • The nuclei of some atoms are stable, while others (radioactive ones) spontaneously split into pieces. • The spectral lines of a particular element correspond to the various electron transitions between allowed orbits of that element with different energy levels of those atoms. When an electron shifts from one energy level to another, a photon of the appropriate energy (and hence a specific wavelength) is absorbed or emitted by the atom.

43. Atoms and Spectra • The spectrum of hydrogen at visible wavelengths consists of part of the Balmer series, which arises from electron transitions between the second energy level of the hydrogen atom and higher levels. • Every different element, isotope, and molecule has a different set of spectral lines. • When a neutral atom loses or gains one or more electrons, it is said to be charged. The atom loses an electron when the electron absorbs a sufficiently energetic photon, which rips it away from the nucleus. • The motion of an object toward or away from an observer causes the observer to see all of the colors from the object blueshifted or redshifted, respectively. This effect is generally called a Doppler shift. • The equation that describes the Doppler effect states that the size of a wavelength shift is proportional to the radial velocity between the light source and the observer.

44. Key Terms proton quantum mechanics radial velocity radioactive redshift spectral analysis spectrograph spectroscope Stefan-Boltzmann law strong nuclear force transition (of an electron) transverse velocity weak nuclear force Wien’s law emission line spectrum energy flux excited state ground state ion ionization isotope Kirchhoff’s laws luminosity molecule neutron nucleus (of an atom) periodic table Planck’s law proper motion absorption line absorption line spectrum atom atomic number blackbody blackbody curve blueshift continuous spectrum (continuum) diffraction grating Doppler shift electromagnetic force electron element emission line

45. WHAT DID YOU THINK? • Which is hotter, a “red-hot” or a “blue-hot” object? • Of all objects that glow visibly from heat generated or energy stored inside them, those that glow red are the coolest.

46. WHAT DID YOU THINK? • What color does the Sun emit most brightly? • The Sun emits all wavelengths of electromagnetic radiation. The colors it emits most intensely are in the blue-green part of the spectrum. Because the human eye is less sensitive to blue-green than to yellow, and Earth’s atmosphere scatters blue-green wavelengths more readily than longer wavelengths, we normally see the Sun as yellow.

47. WHAT DID YOU THINK? • How can we determine the age of space debris found on Earth? • We measure how much the long-lived radioactive elements, such as 238U, have decayed in the object. Carbon dating is only reliable for organic materials that formed within the past 100,000 years. It cannot be used for determining the age of rocks and minerals on Earth or from space. These substances were formed more than 4.5 billion years ago. Radioactive carbon in them has long since decayed to stable isotopes.