ASTRO 101

1. ASTRO 101 Principles of Astronomy

2. Instructor: Jerome A. Orosz (rhymes with “boris”)Contact: • Telephone: 594-7118 • E-mail: jorosz@mail.sdsu.edu • WWW: http://mintaka.sdsu.edu/faculty/orosz/web/ • Office: Physics 241, hours T TH 3:30-5:00

3. Text: “Discovering the Essential Universe, Fifth Edition”by Neil F. Comins

4. Course WWW Page http://mintaka.sdsu.edu/faculty/orosz/web/ast101_fall2013.html Note the underline: … ast101_fall2013.html … Also check out Nick Strobel’s Astronomy Notes: http://www.astronomynotes.com/

5. Fall 2013 Astronomy Help Room No appointment needed! Just drop by! Where: Room 215, physics-astronomy building (PA-215). When: All semester long, at the following days and times: • Monday: 12 – 2 PM; 5 – 6 PM • Tuesday: 12 – 2 PM; 5 – 6 PM • Wednesday: 12 – 2 PM; 5 – 6 PM • Thursday: 1 – 2 PM; 3 – 6 PM

6. Exam 1: • N=61 (4 missing) • Average = 59.4 • low = 27.5, high = 99 • A 90%--100% • A- 85%--89% • B+ 80%--84% • B 75%--79% • B- 70%--74% • C+ 65%--69% • C 60%--64% • C- 50%--59% • D 40%--49% • F 0%--39%

7. The Celestial Sphere • The celestial poles and the celestial equator are the same for everyone. • The zenith and the horizon depend on where you stand. http://www.astronomynotes.com/nakedeye/s4.htm

8. Venus in the Geocentric View • Venus is always close to the Sun on the sky, so its epicycle restricts its position. • In this view, Venus always appears as a crescent.

9. Venus in the Heliocentric View • In the heliocentric view, Venus orbits the Sun closer than the Earth does. • We on Earth can see a fully lit Venus when it is on the far side of its orbit.

10. Venus in the Heliocentric View • The correlation between the phases and the size is accounted for in the heliocentric view.

11. Homework/Announcements • Homework due Tuesday, October 8: Question 5, Chapter 4 (Describe four methods for discovering exoplanets)

12. Coming Up: • The 4 forces of Nature • Energy and the conservation of energy • The nature of light • Waves and bundles of energy • Different types of light • Telescopes and detectors • Spectra • Emission spectra • Absorption spectra

13. The spectrum • Definition and types: • Continuous • Discrete • The spectrum and its uses: • Temperature • Chemical composition • Velocity

14. The spectrum • A graph of the intensity of light vs. the color (e.g. the wavelength, frequency, or energy) is called a spectrum. • A spectrum is probably the single most useful diagnostic tool available in Astronomy.

15. The spectrum • A spectrum can tell us about the temperature and composition of an astronomical object. • There are two types of spectra of concern here: • Continuous spectra (the intensity varies smoothly from one wavelength to the next). • Line spectra (there are discrete jumps in the intensity from one wavelength to the next).

16. The spectrum • Continuous spectrum. • Discrete or line spectra. Images from Nick Strobel (http://www.astronomynotes.com)

17. Thermal Spectra • The most common type of continuous spectrum is a thermal spectrum. • Any dense body will emit a thermal spectrum of radiation when its temperature is above “absolute zero”: • The “color” depends only the temperature; • The total intensity depends on the temperature and the size of the body. • This type of radiation is often called “black body” radiation.

18. Thermal Spectra • The most common type of continuous spectrum is a thermal spectrum.

19. Black body radiation • Sample spectra from black bodies of different temperatures. Note that the area under the curves is largest for the hottest temperature. • There is always a well-defined peak, which crudely defines the “color”. The peak is at bluer wavelengths for hotter temperatures.

20. Important points • The luminosity (energy loss per unit time) of a black body is proportional the surface area times the temperature to the 4th power: • Hotter objects have higher intensities (for a given area), and larger objects have higher intensities.

21. Important points • The peak of the spectrum is inversely proportional to the temperature (hotter objects are bluer): • Hotter objects are bluer than cooler objects.

22. How light interacts with matter andthe line spectrum.

23. What Things Are Made Of • Atoms are the basic blocks of matter. • The number of protons determines the element. For example hydrogen has 1 proton, helium has 2 protons, uranium has 92 protons, etc. • In most cases, the number of electrons equals the number of protons.

24. How Light Interacts with Matter. • Atoms are the basic blocks of matter. • In the 1850s, it was discovered that an element, when burned, gave off a unique emission line spectrum.

25. How Light Interacts with Matter. • An electron will interact with a photon. • An electron that absorbs a photon will gain energy. • An electron that loses energy must emit a photon. • The total energy (electron plus photon) remains constant during this process.

26. How Light Interacts with Matter. • Electrons bound to atoms have discrete energies (i.e. not all energies are allowed). • Thus, only photons of certain energy can interact with the electrons in a given atom.

27. How Light Interacts with Matter. • Electrons bound to atoms have discrete energies (i.e. not all energies are allowed). • Thus, only photons of certain energy can interact with the electrons in a given atom. Image from Nick Strobel (http://www.astronomynotes.com)

28. How Light Interacts with Matter. • Electrons bound to atoms have discrete energies (i.e. not all energies are allowed). • Each element has its own unique pattern of energies.

29. How Light Interacts with Matter. • Electrons bound to atoms have discrete energies (i.e. not all energies are allowed). • Each element has its own unique pattern of energies, hence its own distinct line spectrum. Image from Nick Strobel (http://www.astronomynotes.com)

30. How Light Interacts with Matter. • An electron in free space can have any energy. • It can absorb a photon of any energy • It can lose any amount of energy (E) by emitting a photon with energy equal to E • An electron in an atom can only have very specific values of its energy E1, E2, E3, … EN) • The electron can absorb a photon with an energy equal to (E1-E2), (E1-E3), (E2-E3), … and jump to a higher level • The electron can lose an amount of energy equal to a change between levels (E1-E2), (E1-E3), (E2-E3), … and move down to a lower level

31. How Light Interacts with Matter. • An electron in an atom can only have very specific values of its energy E1, E2, E3, … EN) • The electron can absorb a photon with an energy equal to (E1-E2), (E1-E3), (E2-E3), … and jump to a higher level • The electron can lose an amount of energy equal to a change between levels (E1-E2), (E1-E3), (E2-E3), … and move down to a lower level • Since each element has its own unique sequence of energy levels (E1, E2, E3, … EN), the differences between the levels are also unique, giving rise to a unique line spectrum

32. Emission spectraandabsorption spectra.

33. Emission and Absorption • If you view a hot gas against a dark background, you see emission lines (wavelengths at which there is an abrupt spike in the brightness).

34. Emission and Absorption • If you view a continuous spectrum through cool gas, you see absorption lines (wavelengths where there is little light).

35. Emission and Absorption Image from Nick Strobel (http://www.astronomynotes.com)

36. The spectrum • View a hot, dense source, get a continuous spectrum. • View that hot source through cool gas, get an absorption spectrum. • View that gas against a dark background, get emission spectrum.

37. The spectrum • View a hot, dense source, get a continuou s spectrum. • View that hot source through cool gas, get an absorption spectrum. • View that gas against a dark background, get emission spectrum.

38. Tying things together: • The spectrum of a star is approximately a black body spectrum. • Hotter stars are bluer, cooler stars are redder. • For a given temperature, larger stars give off more energy than smaller stars.

39. In the constellation of Orion, the reddish star Betelgeuse is a relatively cool star. The blue star Rigel is relatively hot.

40. Tying things together: • The spectrum of a star is approximately a black body spectrum. • Hotter stars are bluer, cooler stars are redder. • For a given temperature, larger stars give off more energy than smaller stars. • However, a closer look reveals details in the spectra…

41. The Line Spectrum • Upon closer examination, the spectra of real stars show fine detail. • Dark regions where there is relatively little light are called lines.

42. The Line Spectrum • Today, we rarely photograph spectra, but rather plot the intensity vs the wavelength. • The “lines” where there is relatively little light show up as dips in the curves.

43. The Line Spectrum • Today, we rarely photograph spectra, but rather plot the intensity vs the wavelength. • The “lines” where there is relatively little light show up as dips in the curves. • These dips tell us about what elements are present in the star!

44. Atomic Fingerprints • Hydrogen has a specific line spectrum. • Each atom has its own specific line spectrum.

45. Atomic Fingerprints • These stars have absorption lines with the wavelengths corresponding to hydrogen!

46. Atomic Fingerprints • The Sun (and other stars) have absorption lines with the wavelengths corresponding to iron.

47. Atomic Fingerprints. • One can also look at the spectra of other objects besides stars, for example clouds of hot gas. • This cloud of gas looks red since its spectrum is a line spectrum from hydrogen gas.

48. The Doppler Shift: Measuring Motion • If a source of waves is not moving, then the waves are equally spaced in all directions.

49. The Doppler Shift: Measuring Motion • If a source of waves is moving, then the spacing of the wave crests depends on the direction relative to the direction of motion.

50. The Doppler Shift: Measuring Motion • Think of sound waves from a fast-moving car, train, plane, etc. • The sound has a higher pitch (higher frequency) when the car approaches. • The pitch is lower (lower frequency) as the car passes and moves further away.