Chapter 10 Measuring the Stars What are the properties of Stars? What are the patterns among Stars?

2. Chapter 10 Measuring the Stars What are the properties of Stars? What are the patterns among Stars?

3. From the Sun we’ve learned: • stars are far away • stars are bright • stars are hot • stars are massive • How FAR AWAY? (measure stellar DISTANCES) • How BRIGHT? (measure stellar LUMINOSITY) • How HOT? (measure stellar SPECTRAL TYPE) • How MASSIVE? (measure stellar MASS) • we already know how to determine the composition

5. Measuring Distances Easy way: use a ruler

6. Parallax An apparent shift in the relative positions of two objects when viewed from different perspectives

7. hyperphysics.phy-astr.gsu.edu/HBASE/Astro/para.html

8.  view 

9. uly view uly

10. tan p = B d d = 1.A.U. tan p p B

11. For distances to stars comparable to the Earth-Sun distance, parallax would be easily measured Parallax angle

12. For distances to stars comparable to the Earth-Sun distance, parallax would be easily measured tan p = B d d = 1.A.U. tan p 1 A.U. p ( Parallax angle 

13. For small angles tan p --> p d = B = 1AU tan p tan p d = 1 p But stars are at much greater distances from the solar system, and at great distances from one another June December The parallax of the nearest stars cannot be measured without a telescope Sincethe ancients were unable to observe stellar parallax, they would not accept the Sun as the center, even tough they considered the possibility.

14. Angular Measurements Angles are measured in degrees; Degrees are divided into 60 arcminutes; Arcminutes are divided into 60 arcseconds • Full circle = 360º • 1º = 60 (arcminutes) • 1 = 60 (arcseconds)

15. Measuring Distances d = 1 p parsec = parallax + arcsec sec is the distance to a star with a parallax of 1 second of arc.  = 1 1 arc sec 1 pc = 206,265 AU 1 pc = 3.2 ly

16. Example: Nearest star, a Centauri, has a parallax of p = 0.76 arc seconds d = 1 p d = 1 = 1.3 pc = 4.3 LY 0.76 This method does not work for stars farther away than 50 pc.

17. PARSEC is not a speed PARSEC is a DISTANCE

18. From the Sun we’ve learned: • stars are far away • stars are bright • stars are hot • stars are massive • How FAR AWAY? DISTANCE • How BRIGHT? LUMINOSITY • How HOT? SPECTRAL TYPE • How MASSIVE? MASS

19. Stellar brightness -- what is it? The brightness of an object depends on both distance and energy output

20. Amount of energy output a star radiates is called the Luminosity (L): theenergy per second Amount of starlight that reaches Earth is called the Apparent brightness (m)

21. Luminosity passing through each sphere is the same Divide luminosity by area to get brightness

22. The amount of light received (Brightness) from the source is proportional to its energy output (Luminosity (L) )and inversely proportional to the square of the distance (d): Luminosity Brightness = 4π (distance)2

23. Two Kinds of Brightness • Apparent Magnitude (m):How bright the object appears to us on Earth. • Absolute Magnitude (M):How bright a star actually is, its intrinsic brightness

24. Two Kinds of Brightness • Apparent Magnitude:How bright the object appears to us on Earth. m = 1 brightest m = 6 dimmest

25. As the Sun sets, some stars are visible. These are the first magnitude stars. Later, when twilight is over, more stars are visible. These are the second magnitude stars, and so on…

26. Two Kinds of Brightness • Apparent Magnitude:How bright the object appears to us on Earth. • Which would look brighter? Vega, m = 0.03 Antares, m = 1.06 • Which would look brighter? Sirius, m = -1.4 Venus, m = -4.4

27. Two Kinds of Brightness • Apparent Magnitude:How bright the object appears to us on Earth. • Which would look brighter? Vega, m = 0.03 Antares, m = 1.06 • Which would look brighter? Sirius, m = -1.4 Venus, m = -4.4

28. Two Kinds of Brightness • Absolute Magnitude:How bright a star actually is, its intrinsic brightness PROBLEM:stars are at different distances from Earth and so it’s hard to know which stars are ACTUALLY brighter versus which APPEAR bright The Absolute Magnitude that a star would have if it were at a distance of 10 pc.

29. Compare some stars: Absolute Apparent MSun = 4.8 mSun = -26 MSirius = 1.4 mSirius = -1.46 MBetelgeuse = -5.6 mBetelgeuse = 0.50 Which star looks brightest from Earth?Which star is brightest?

30. Star light, Star bright …. qualitatively quantitatively Magnitude : apparent and absolute

31. Most luminous stars: 106 LSun Least luminous stars: 10-4 LSun (LSun is luminosity of Sun)

32. From the Sun we’ve learned: • stars are far away • stars are bright • stars are hot • stars are massive • How FAR AWAY? DISTANCE • How BRIGHT? LUMINOSITY • How HOT?SPECTRAL TYPE • How MASSIVE? MASS

33. Temperature Color AND …

34. These BLACKBODY Radiation curves will provides a relation between TEMPERATURE and COLOR

35. Hipparchus developed the magnitude scale in ancient Greece. The brightest star: apparent magnitude = 1 The faintest star: apparent magnitude = 6

36. The “computors” at Harvard finished the classification process. (1890)

37. Spectra of Stars

38. Remember the Balmer Series? The “calculators” used H lines in spectra

39. Classification Scheme A B C D E . . . S Annie Jump Cannon (1863-1941)

40. Classification Schemebased on the strength of Balmer lines • Originally … • A strongest H lines • B less strong H lines • O Weakest H lines • 10,000 stars • publishes in 1890 A B C D E . . . S

41. A B C D E . . . S

42. Antonia Maury (1866-1952) Spectral classes might make more sense if arranged by temperature

43. temperature strength

44. Keep in mind that this classification is for the star’s SURFACE, only!!!!! Summary of Spectral Classes

45. Meghnad Saha offered the explanation (theory of thermal ionization of atoms) which was confirmed at Harvard by Cannon and Maury’s work. Cecelia Payne-Gaposchkin (1900-1979) First PhD in Astronomy from Harvard/Radcliffe Provided a convincing argument that stars are mostly made of hydrogen.

46. From the Sun we’ve learned: • stars are far away • stars are bright • stars are hot • stars are massive • How FAR AWAY? (measure stellar DISTANCES) • How BRIGHT? (measure stellar LUMINOSITY) • How HOT? (measure stellar SPECTRAL TYPE) • How MASSIVE? (measure stellar MASS)

47. Direct mass measurements are possible only for stars in binary star systems About half of all stars are in binary systems The orbit of a binary star system depends on strength of gravity

48. Binary Star system

49. Measure the mass of this?????

50. The binary star IW Tau is revealed through adaptive optics. The stars have a 0.3 arc second separation. The images were taken by Chas Beichman and Angelle Tanner of JPL.