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Stars

Understand the enigmatic celestial bodies with gaseous spheres and stellar radiation, unveiling fusion processes, equilibrium, luminosity, temperature, and spectral analysis. Dive into the fascinating world of stars through the science of black body radiation, Doppler effect, and early star classification.

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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. StellarEquilibrium- outward P from radiation balances gravity inward in stable stars.

  7. Stable Stars maintain size.The 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 are regarded as black bodies • L = sAT4. • L = s4pr2T4. • L – Watts J/s. L = Power • 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. Apparent brightness • radiation from star that is incident on the Earth per m2. It’s like Intensity

  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. Absorption of l

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

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

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

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

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

  29. 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 it be:larger or smaller to our sun?Calculate its radius in terms of our sun’s radius. • Larger • 26 x Rsun.

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

  31. Sun

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

  33. Star Classification

  34. Spectral Classes. • Stars characterized by temperature, absorption lines & color. • Old Classification O – M.

  35. OBAFGKM • Oh be a fine girl – kiss me. • Then subdivided in 10 smaller groups 0-9. • Sun – G2.

  36. 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.

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

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

  39. Black body radiation 12 min • https://www.youtube.com/watch?v=TiOpUAI_9mk&autoplay=1&app=desktop

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