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For Monday, Apr. 14

For Monday, Apr. 14. Reading: Section 13.4 Assignments: Mini-Project #4 (due today) Homework #5 (due Mon. Apr. 14) FREE PUBLIC LECTURE : tonight at 7:30 pm (GMCS 333) “ Let There Be Light: The History of the Universe From the First Billion Years to the Present ”

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For Monday, Apr. 14

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  1. For Monday, Apr. 14 Reading: Section 13.4 Assignments: Mini-Project #4 (due today) Homework #5 (due Mon. Apr. 14) FREE PUBLIC LECTURE: tonight at 7:30 pm (GMCS 333) “Let There Be Light: The History of the Universe From the First Billion Years to the Present” LUNAR ECLIPSE: night of APRIL 14 (Totality 12:06 -1:24 AM)

  2. Being a Good Astronomer: The Unknown • Try to use science we learn on Earth to understand things we can’t experiment on… • What is “normal” in astronomy? •  how do stars work? •  when is a star abnormal? • …use the Sun for comparison • Surprises are usually interesting… .

  3. Measuring Star Characteristics How different are other stars? • Distance (parallax) • Luminosity • Surface Temperature • Size • Mass • Chemical Composition • Speeds (radial and transverse velocities)

  4. How Far are Stars?  ONLY method for directly measuring distances in astronomy!

  5. Which of the stars in the picture below shows a measurable parallax? G E I A K C H F J B D

  6. For Wednesday, Apr. 16 Reading: Section 14.1, 14.2 Assignments: Homework #5 (due today) Homework #6 (due Mon. Apr. 21) Article #2 choice (due Mon. Apr. 21)

  7. Lunar Eclipse – tonight

  8. d 1 AU p Distance (d) Parallax: use geometry: • p: parallax angle • d: distance between Earth and star Typical distances between stars in Milky Way: about 1 “parsec” 1 parsec (pc) = 3.26 light-years = 206,265 AU !!!

  9. A Scale Model How far away is the nearest star? to Proxima Centauri --- near Balboa Park (almost 7 km)… 260,000x Earth-Sun distance 6800x Sun-Pluto distance Earth’s orbit (3 cm) Pluto’s orbit (1 meter) SPACE IS VERY EMPTY!!

  10. Thought Question: The brightest star in the sky (Sirius) has a parallax of about 0.4”. What is its distance in parsecs and in light-years? (Enter your answer in light-years, rounded to the nearest whole number.)

  11. Thought Question: On Earth, the parallax angle measured for the star Procyon is 0.29 arcseconds. If you were to measure Procyon’s parallax angle from Venus, what would the parallax angle be? (Note: Venus’ orbit is smaller than Earth’s orbit.) • More than 0.29 arcseconds • 0.29 arcseconds • Less than 0.29 arcseconds • Zero arcseconds (no parallax)

  12. Other Stars… Sirius (brightest star in sky) 23 Sun’s luminosity 9940 K temperature Proxima Centauri (nearest star) 0.0008 Sun’s luminosity 3000 K temperature

  13. Flux vs. Luminosity light collector • flux (F):energy reaching each square meter of collecting surface per time also called apparent brightness units: J / (m2·s)=W / m2 (what our eyes measure) SURFACE AREA OF A SPHERE • luminosity (L): total amount of energy released per time • units: Watt (W): 1 W = 1 J / s •  property of star: its “power”

  14. Flux vs. Luminosity • luminosity (L): rate of energy release • If L is constant, equal amounts of energy flow through each sphere each second… BUT: flux (F): rate of energy reaching each square meter of surface …energy spreads out over a larger area SURFACE AREA OF A SPHERE

  15. Thought Question: How much brighter does the Sun appear to us on Earth compared to what you would see standing on the dwarf planet Eris (67.7 AU from the Sun on average)? (Enter your answer rounded to the nearest whole number.)

  16. Brightness Earth’s orbit Sun

  17. Thought Question: Imagine you are comparing the brightness of two stars. Star A’s luminosity is 5 times higher than star B’s, and star A is 3 times farther away from you than star B. What is the ratio of the brightness of star A to the brightness of star B? (Enter the ratio as a two digit number: if the ratio is 2/3, enter “23”) Star A is 5/3 as bright as star B. Star A is 5/9 as bright as star B. Star A is 9/5 as bright as star B. Star A is 3/5 as bright as star B.

  18. Luminosity (L) How to calculate L for stars: • measure brightness (flux) at Earth • measure distance • use inverse-square law: centi-firefly?

  19. Surface Temperature O B • star colors change as temperature changes Compared to Sun:  hotter stars look blue-white  cooler stars look red  Sun is actually white A F G (Sun) K M

  20. Surface Temperature (T) How to measure: • overall color or most intense wavelength • spectral lines

  21. Spectral Types hottest coolest • pattern of absorption lines reveals star temperature •  reads like a barcode or a fingerprint

  22. Radius • R = 7  105 km • measured by knowing distance from Earth and its angular size Sun: . Jupiter 0.1 R Earth 0.01 R 0.5º 1 AU

  23. Approximate size of Sun: . Betelgeuse

  24. For Friday, Apr. 18 Reading: Sections 16.1, 16.2 and Math Tools 16.1 Assignments: Homework #6 (due Mon. Apr. 21) Article #2 choice (due Mon. Apr. 21)

  25. Spica (bright star in Virgo)

  26. Temperature, Size, Luminosity Two things can increase LUMINOSITY of a star: • REMEMBER THERMAL RADIATION! A HOTTER OBJECT RELEASES MORE LIGHT PER SECOND FROM EACH BIT OF SURFACE A LARGER AREA RELEASES MORE LIGHT PER SECOND: HOT COOL SAME AREA SAME TEMPERATURE SAME TEMPERATURE

  27. Star Sizes Stars release THERMAL RADIATION: • brightness of each piece of surface only depends on temperature average flux from star’s surface flux due to thermal radiation (Stefan-Boltzmann Law) We can calculate the size of the star!

  28. The HR Diagram …a “snapshot” of star properties Star properties change very slowly, so we can’t see them change…

  29. Thought Question In the graph below, which star (each represented by a dot) must have the smallest size? A B C Luminosity D F E G H I Temperature

  30. Thought Question: The stars Antares and Mimosa have about the same luminosity, but Mimosa is 8 times hotter than Antares. What is the ratio of the radii?

  31. Dwarfs and Giants • LUMINOSITY of a star depends on: • surface temperature • size If a star is LUMINOUS but COOL: If a star is HOT but LOW LUMINOSITY it must be small (small surface area)  WHITE DWARF it must be big (large surface area)  GIANT

  32. The HR Diagram …a “snapshot” of star properties Star properties change very slowly, so we can’t see them change… Luminosity Temperature

  33. Thought Question: If you took a star that was the same mass as the Sun and made it 10 times smaller (in diameter), how would its density compare to the Sun’s? VOLUME = LENGTH  WIDTH  HEIGHT

  34. Sirius B • temperature: 25000 K! • 1/40th Sun’s luminosity! • 1/100th the size of the Sun (Earth size!) • 106x as dense …like crushing an elephant into a teaspoon SIRIUS B (Hubble Space Telescope image)

  35. Radius (R) main sequence stars: . • How to calculate: • blackbodies:  brightness of thermal radiation at star’s surface: • average brightness released by surface: So: or

  36. Messier 9 - • A globular star cluster • About 300,000 stars • About 12 billion yrs old 25 light-yrs across

  37. Measuring Ages with Stars Outside the solar system, the objects that can be age-dated most accurately are stars… HIGH-MASS (SPICA) SUN VERY LOW-MASS (PROXIMA CENTAURI) .

  38. Main Sequence Stars highest mass Luminosity SPICA: 11x Sun’s mass, 20000x Sun’s luminosity lowest mass SUN Temperature PROXIMA CENTAURI 0.1x Sun’s mass, 0.0006x Sun’s luminosity .

  39. Rules for Stars To survive, stars must be in balance, or EQUILIBRIUM: • Energy flows are balanced: • stars are continuously losing energy by radiation •  stars MUST have an energy source OR ELSE their temperatures would drop rapidly • Forces are balanced: • gravity is always trying to crush a star •  another force MUST oppose gravity OR ELSE the star would collapse

  40. For Monday, Apr. 21 Reading: Sections 16.3, 16.4, 16.5 Assignments: Homework #6 (due Mon. Apr. 21) Article #2 Selection (due Mon. Apr. 21) Complete Online Survey for Extra Credit Optional Mt. Laguna Observatory Trip: week of Apr. 28 (weather permitting)

  41. Kepler 186f • first rocky extrasolar planet in habitable zone (liquid water may be present) • star is a cool (M type) • 4 other planets in system • 490 light-yrs away

  42. Pressure units: force per area (N / m2) COLLISIONS OF PARTICLES CREATE PRESSURE: DENSITY (number per volume): crowded particles  more collisions per sec.  more pressure TEMPERATURE: faster particles  more frequent, more violent collisions  more pressure Pressure animation

  43.  Pressure Gas must have enough pressure to support weight of everything above it Pressure at center must be largest because it supports the rest of the star…  … so gas becomes DENSE (150 g/cm3… 15 lead) and HOT (1.5  107K)!

  44. Thought Question: ClassAction question

  45. The Sun’s Lifetime SOURCE OF ENERGY ENERGY LOST FROM SUN (LUMINOSITY) FUEL • Sun’s luminosity: 4  1026 J/s • Sun’s age: about 4.6  109 yr • What source can provide energy for Sun for this long?

  46. Candy Sun If the Sun’s mass was all “Milky Way” candy bars (58.1 g, 280 Calories), how much energy could be released by burning the whole thing? (Enter the scientific notation exponent of the energy in J.) If the Sun was powered by these candy bars, how long could it maintain its current luminosity? (Enter the scientific notation exponent of the time in yr.)

  47. Hydrogen Fusion • STARS NEED: • hydrogen gas • high temperature for high-speed collisions between nuclei • After several reactions, 4 hydrogen nuclei fuse together into 1 helium nucleus

  48. Nuclear Energy and E=mc2 mass of a proton • Hydrogen nucleus: 1 proton: • Helium nucleus parts: 2 protons: 2 neutrons: • Actual helium nucleus mass: HELIUM ATOM HAS LESS MASS (0.8%) THAN ITS PARTS!!  “LOST MASS” IS CONVERTED TO ENERGY!

  49. Star Lifetime • Star’s stored nuclear energy comes from mass that will be converted (E=mc2):

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