1. Atoms and Spectra

2. The Sun’s Visible-Light Spectrum

3. The Amazing Power of Starlight
By analyzing light received from a star, astronomers learn about the star’s…
Chemical composition
Surface temperature
Radius
Total energy output
Density
Velocity relative to Earth
Rotation period
Magnetic Fields

4. Continuous Spectrum
Spectrum of a common (incandescent) light bulb spans all visible wavelengths, without breaks

5. Kirchhoff’s Laws of Radiation
A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths & produce a continuous spectrum.

6. Emission Line Spectrum
Thin or low-density cloud of gas emits light only at specific wavelengths
Depends on its composition & temperature,
Producing a spectrum with bright emission lines.

7. Kirchhoff’s Laws of Radiation
2. Low-density gas excited will emit light only at specific wavelengths & produce emission spectrum.
Light excites electrons in atoms to higher energy states,
Which transition back to lower states, emitting light at specific frequencies.

8. Emission Line Spectrum
LASER
Light
Amplified
Stimulated
Emission
Radiation

9. Absorption Line Spectrum
A cloud of gas between us & continuous source can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum.

10. Kirchhoff’s Laws of Radiation
3. Light with continuous spectrum passes through a cool, low-density gas, produces absorption spectrum.
Light excites electrons in atoms to higher energy states,
The frequencies of light correspond to the transition energies absorbed from the continuous spectrum.

11. How does light tell us about matter?

12. What is matter? Matter is protons, neutrons, & electrons
Use this figure to define the nucleus; protons, neutrons, electrons; scale of atom and “electron cloud.”
Matter is protons, neutrons, & electrons
Most of mass in nucleus
Most of space in electron “cloud”

13. What is matter?
Use this figure to define the nucleus; protons, neutrons, electrons; scale of atom and “electron cloud.”

14. Different Kinds of Atoms
Different Kinds of Atoms
Type of atom depends on # of protons
Most abundant: Hydrogen (H)
1 proton, 1 electron
2nd: Helium (He)
2 protons, 2 neutrons, and 2 electrons

15. Atomic Terminology Atomic Number = # of protons in nucleus
Atomic Mass Number = # of protons + neutrons

16. Chemical Fingerprints
Each type of atom has a unique spectral fingerprint.
Why?

17. Chemical Fingerprints
Each type of atom has a unique spectral fingerprint.

18. Electron Orbits
Electron orbits restricted to very specific radii and energies.
Electron energies different for each individual element.
r3, E3
r2, E2
r1, E1

19. Electron Orbits
Electron orbits restricted to very specific radii and energies.
Electron energies different for each individual element.
Ephoton = E3 – E1
Ephoton = E4 – E1

20. Electron Orbits
Electron orbits restricted to very specific radii and energies.
Electron energies different for each individual element.
Wrong energy isn‘t absorbed!

21. Chemical Fingerprints
Each type of atom has unique electron energy levels.
Each transition corresponds to a unique photon energy & wavelength.
Energy levels of hydrogen

22. Chemical Fingerprints
Downward transitions produce a unique pattern of emission lines.

23. Chemical Fingerprints
Upward transitions produce a pattern of absorption lines … at the same wavelengths.

24. Chemical Fingerprints
Observing the fingerprints in a spectrum tells us which kinds of atoms are present.

25. Example: Solar Spectrum
You can use this slide as an example of real astronomical spectra made from a combination of the idealized types. Here we have the continuous (thermal) spectrum from the solar interior; dark absorption lines where the cooler solar atmosphere (photosphere) absorbs specific wavelengths of light.

26. Three basic types of spectra
Continuous Spectrum
Emission Line Spectrum
Absorption Line Spectrum
Spectra of astrophysical objects are usually combinations of these three basic types.

27. Thought Question Which letter(s) labels absorption lines?D E

28. Thought Question Which letter(s) labels absorption lines?D E

29. Thought Question
Which letter(s) labels the peak (greatest intensity) of infrared light? A B C D E

30. Thought Question
Which letter(s) labels the peak (greatest intensity) of infrared light? A B C D E

31. Thought Question Which letter(s) labels emission lines?D E

32. Thought Question Which letter(s) labels emission lines?D E

33. Color and Temperature Orion Stars appear in different colors:
Stars appear in different colors:
Blue (like Rigel),
Yellow (like our sun)
Red (like Betelgeuse).
Colors tell us about star’s temperature.
Orion
Betelgeuse
Rigel

34. Color and Temperature
Read about Annie Cannon!

35. Author of “Galileo’s Daughter”
The Women of Harvard
Dava Sobel
Author of “Galileo’s Daughter”

36. The Spectra of Stars
Inner, dense layers of a star produce a continuous (blackbody) spectrum.
Cooler surface layers absorb light at specific frequencies.
=> Spectra of stars are absorption spectra.

37. How does light tell us the temperatures of planets and stars?
TWO Laws of Radiation!
Hotter objects emit more light at all frequencies per unit area.
Hotter = much, Much, MUCH, MUCH brighter
Hotter objects emit photons with a higher average energy.
Hotter = BLUER

38. Remind students that the intensity is per area; larger objects can emit more total light even if they are cooler.

39. How does light tell us the temperatures of planets and stars?

40. How does light tell us about the motions of planets & stars?
DOPPLER effect
Motion AWAY “stretches” wavelengths
Sounds are lower; light is redder
Motion TOWARD “compresses” wavelengths
Sounds are higher; light is bluer

41. Measuring the Shift
Stationary

42. Measuring the Shift
Stationary
Moving Away

43. Measuring the Shift
Stationary
Moving Away
Away Faster

44. Measuring the Shift
Stationary
Moving Toward
Toward Faster

45. Doppler shift tells us ONLY about part of an object's motion toward or away from us.

46. Doppler Shift Example:
Earth’s orbital motion around the Sun causes a radial velocity towards (or away from) any star.