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Stars

Stars. Luminous gaseous celestial body – spherical in shape held by its own gravity. How do we study stars?. Light!!. Stellar Radiation. H fusion occurs in star’s interior converting mass to E (mass deficit). T must be ~ 10 7 K, for nuclei to overcome Coulomb force & fuse.

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Stars

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  1. Stars Luminous gaseous celestial body – spherical in shape held by its own gravity

  2. How do we study stars? • Light!!

  3. Stellar Radiation • H fusion occurs in star’s interior converting mass to E (mass deficit). • T must be ~ 107 K, for nuclei to overcome Coulomb force & fuse. • Interior of the star is so hot it is plasma.

  4. BE of He higher than BE 4H. He - 4. H isotopes.

  5. Excess E is carried away by g photons & neutrinos n. Some E gets absorbed in star heats interior more & exerts outward pressure.

  6. Stellar Equilibrium- outward P from radiation balances gravity inward in stable stars.

  7. Stable Stars maintain sizeThe sun is stable

  8. Ex 1. The sun is losing mass at 4.26 x 109 kg/s. At what rate does the sun emit energy? • Assuming the mass is converted to E. • E = mc2. • (4.26 x 109 kg/s)(3 x 108m/s)2. • 3.83 x 10 26 J each second.

  9. Star Power Luminosity (L) = total power output of a star W or J/s. As we just calculated the sun converts mass to Energy Sun L = 3.9 x 1026 W.

  10. Luminosity (W) depends on: • Surface Area • Temperature • Which equation relates power to A & T?

  11. Stars often regarded as black body • L = sAT4. • L = s4pr2T4. • L – Watts J/s • A surface A m2 • T Kelvin • s = 5.67 x 10-8W/ m2 K4.

  12. ApparentBrightness (b): how bright stars appear. What we see from Earth depends on L & distance from Earth

  13. Def. Apparent brightness • radiation from star that is incident on the Earth per m2.

  14. Calculation of ApparentBrightness (b): L = luminosity in W d = distance to Earth m b = apparent brightness W/m2. Intensity

  15. Ex 2: The apparent brightness of a star is 6.4 x 108 W/m2. If its distance to Earth is 50 LY, find its luminosity.

  16. b4pd2 = L • (6.4 x 108 W/m2) (4p)(4.73 x 1017 m)2. • d = (9.46 x 10 15 m/LY)(50 LY) = 4.73 x 1017 m • 1.8 x 10 45 W

  17. Finding Star TemperatureRemember Black Bodies?

  18. Wein’s Displacement Law relates peak l & surface temp for black body. T in Kelvin l in meters Star’s spectra similar to black body.

  19. as T inc. • Tot intensity increase for all l. • Peak changes to shorter l higher f.

  20. Ex 3: A star has a surface temp of 17 000 K and L = 6.1 x 10 29 W. a. What is the peak l? b. Find its radius.

  21. Use Stephen Boltzmann to find R.

  22. Solar Spectrum • Some radiation l absorbed by outer layers. • Can identify elements in outer layers. • If H is present, H will absorb l = to dif between Bohr orbit levels. Form black lines.

  23. Motion & Speed of Stars • Doppler Effect/Red or Blue shift gives info. • Absorption lines shift toward longer or shorter l, depending on motion.

  24. Red Shift Spectrum – stars movingaway from us show dark line shift. • Find v, direction by shift of line spectra.

  25. Blue Shift – moving toward usAmount of Shift relates to speed of motion

  26. List 3 observations we can make using light to get information about stars.State what we can learn from each type of observation.

  27. Use Spectrum to find: • Chemical composition surface • (absorption spectrum) • Motion toward or away from Earth • Red/blue shift • Surface temp • Peak l (color)

  28. Ex 4: Our sun has T = 6000 K and L = 3.9 x 1026 W. If star Z has T = 4000 K, &L = 5.2 x 10 28 W would expect:It to be larger or smaller to our sun?Calculate its radius in terms of our sun’s radius. • Larger • 26 x Rsun.

  29. Early Star Classification • Spectral Class • Color Temperature Composition.

  30. Sun

  31. Stellar spectra • http://www.youtube.com/watch?v=jjmjEDYqbCk • From 4:48

  32. Star Types

  33. Types of Stars • Single – not bound to another. Sun. • Binary – 2 stars appear close. Most bound together by grav. • Cepheid – varies in brightness on regular cycle of days – changing size. • Red Giant – Old star. H burning is over. Low surface T. High L, lg area. • Supergiant – very heavy star fuses elements beyond carbon. • White dwarf – solar mass but planetary size no more fusion.

  34. Binary Stars – • Optical binary – appear together but not physically near each other.

  35. Visual binaries orbit together around center of mass. Can be distinguished visually. Mass can be determined from period of revolution & separation.

  36. Eclipsing Binary – Cannot see separate stars but 1 passes in front of the other so observed brightness varies with regular period.

  37. Animation of eclipsing binary • http://www.youtube.com/watch?v=zoekfYomfjI

  38. Why is there a larger dip in intensity for 1 position? Brighter/hotter star blocked bigger dip in light curve.

  39. Spectroscopic Binarytoo close to distinguish eclipse but can see doppler shift

  40. Red & Blue Shifted w/motion

  41. Binary Star Types4 min. • http://www.youtube.com/watch?v=1kFFwHkxBiI

  42. Star Classification

  43. Spectral Classes. • Stars characterized by temperature, absorption lines & color. OBAFGKM • Oh be a fine girl – kiss me. • Then subdivided in 10 smaller groups 0-9. • Sun – G2.

  44. H-R diagram graphs temp against luminosity – Not Linear • Be able to identify general regions of star types on the H-R diagram • 90% Stars on Main sequence.

  45. MS High Mass Cool, Super-Large H-R Diagram Fast Burners Cool, Large Small, Hot MS Low Mass Long Lives

  46. HR Diagramstart at 1:24 http://www.youtube.com/watch?v=yX0HWr9xQ6M

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