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Properties of Stars

Properties of Stars.

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Properties of Stars

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  1. Properties of Stars

  2. “All men have the stars,” he answered, “but they are not the same things for different people. For some, who are travelers, the stars are guides. For others they are no more than little lights in the sky. For others, who are scholars, they are problems. For my businessman they were wealth. But all these stars are silent. You – you alone – will have the stars as no one else has them.” Antoine de Saint-Exupery (1900 – 1944) from The Little Prince

  3. WHAT DO YOU THINK? • How nearto us is the closest star other than the Sun? • What colorsare stars, and why do they have these colors? • How luminousis the Sun compared with other stars?

  4. WHAT DO YOU THINK? 4. Are brighter stars hotter than dimmer stars? 5. Compared to the Sun, what sizesare other stars? 6. Are most stars similar to the Sun, one star with planets, or in multiple-star groups?

  5. A Snapshot of the Heavens • How can welearn about the lives of stars, which last tens of millions to hundreds of billions of years? • we will never observe a particular star evolve from birth to death • so how can we study stellar evolution?

  6. How can we Study the Life Cycles of Stars? • Key: All stars were NOT born at the same time. • stars we see today are at different stagesin their lives • we observe only a brief moment in any one star’s life • by studying large numbers of stars, we get a “snapshot” of one moment in the history of the stellar community • we can draw conclusions just like we would with human census data…we do stellar demographics!

  7. A Snapshot of the Heavens • What two basic physical propertiesdo astronomers use to classify stars? • What does that classification tell us?

  8. Classification of Stars • Stars were originally classified based on: • their brightness • their location in the sky • This classification is still reflected in names of the brightest stars…those we can see with our eyes:

  9. Classification of Stars Order of brightness within a constellation Latin Genitive of the constellation  Orionis  Geminorum

  10. Classification of Stars • The old classification scheme told us little about a star’s true (physical) nature. • a star could be very bright because is was very close to us; not because it was truly bright • two stars in the same constellation might not be close to each other; one could be much farther away

  11. Classification of Stars • In 20th Century, astronomers developed a more appropriate classification system based on: • a star’s luminosity • a star’s surfacetemperature • These properties turn out to depend on: • a star’s massand • its stage in life • measuring these=> reconstruct stellar life cycles

  12. WHAT DO YOU THINK? • How near are stars?DISTANCE • What colors are stars?TEMPERATURE • How luminous are stars?LUMINOSITY • Are brighter stars hotter?TEMP vs. LUMIN. • What sizes are stars?SIZE • Single or Multiple?ORBITS MASS

  13. Step 1: Distance! From Parallax

  14. PARALLAX • Determines distance based on Earth’s Orbit around Sun. • SMALL angular shift! • Even closest stars (4.3 light years) show shift less than 1/4000th of a degree!

  15. PARALLAX • Examplehttp://www.solstation.com/stars/61cygni2.htm • Animationhttp://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/otherstars.htm • Current telescopes can measure angles as small as 1/400,000th of a degree (400+ light years)

  16. Step 2: TEMPERATURES! From Spectra!

  17. COLORSof stars lead toSurface Temperatures!

  18. Pickering’s Team, led by Williamina Fleming Annie Cannon

  19. Initial Spectral Type Classification System A B C D E F G H etc Based on “strengths”of Hydrogen Absorption lines

  20. Revised Spectral Type Classification System O BAFGKM OhBeAFineGirl/Guy, KissMe! 50,000K 3,000 K Temperature

  21. Higher Temperature => Bluer Higher Temperature => BRIGHTER

  22. Step 3: BRIGHTNESS! From distance and apparent magnitude

  23. How BRIGHT are stars? • Bright because they are nearby…or… • Bright because they are really, truly bright?

  24. Bright because they are really, truly bright? Bright because it is nearby…

  25. Apparent Magnitudes Ancient method for measuring stellar brightness from Greek astronomer Hipparchus (c. 190 – 120 B.C.) Magnitude Scale This scale runs backwards: The bigger the number, the fainter the star Brightest stars are #1, next brightest are #2, etc.

  26. Inverse Square Law for Light – how distance relates to brightness…

  27. Inverse Square Lawfor Light – how distance relates to brightness…

  28. So what can we know about stars? • We measure distances of nearby stars with PARALLAX • We deduce actual brightness from distance and the inverse-square law and… • We determine surface temperatures from spectra

  29. So where does knowing absolute brightness&spectra get us?

  30. Plot the two known quantities Surface temperature Vs. Luminosity (Absolute Brightness) against one another on a graph

  31. Plot all known stars! Our sun

  32. Plot all known stars! Betelgeuse

  33. Sun is much, much dimmer than most “bright” stars How does our Sun compare with other stars? Sun is brighter than most “nearby” stars

  34. The “HR” Diagram

  35. The “HR” Diagram 90% of all stars lie on the main sequence! WHY??

  36. Stellar Lives!

  37. BRIGHT HOT COOL FAINT

  38. Hypotheses to Explore • Stars are born like children, cool and small, and then heat up and grow brighter over time. Or… • Stars are like candles,hot and bright, and then cool off and get dimmer over time.

  39. Hypotheses to Explore • Stars are born like children… • Stars are like candles… • Stars are born with differenttemps and brightnesses, and change little over 90% of their lives.

  40. Hypotheses to Explore • IF…. Stars are born like children, cool and small, and then heat up and grow brighter over time. • THEN… new clusters of stars should all be hot O stars, and old clusters only M stars.

  41. Hypotheses to Explore • IF…Stars are like candles,hot and bright, and then cool off and get dimmer over time. • THEN… new clusters should show only M stars, and older ones should show O stars.

  42. Hypotheses to Explore • IF…. Stars are born like children, cool and small, and then heat up and grow brighter over time. • IF…Stars are like candles,hot and bright, and then cool off and get dimmer over time. • But “new” clusters like the Pleiades show all kinds of stars!

  43. The Pleiades

  44. Step 4: HR Diagram! From spectra and absolute brightness

  45. Diameter (Size) of Stars • Calculated from known values: • Luminosity • Temperature • Laws of Physics • Stefan-Boltzmann Law: Luminosity ~ Surface Area x T4 • PingPong balls, volleyballs, and Stars…

  46. Bigger HUGE TINY Smaller

  47. Determining Size • Larger stars can be less dense at edges • Less dense gas will change absorption lines

  48. Luminosity Classes of Stars • Based on Spectral Line Shapes& Density • Tied to physical SIZE • We’ll discover they are unstable…

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