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Star Light, Star Bright

Star Light, Star Bright. Going from the Sun to other Stars. Giving a Star a Physical. Use Starlight! Distances- use stellar parallax Luminosity- same as sun (careful!) Temperature- same as sun Diameter- use Luminosity and Temperature Mass- save it for later. Distance. Stellar Parallax

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Star Light, Star Bright

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  1. Star Light, Star Bright Going from the Sun to other Stars

  2. Giving a Star a Physical • Use Starlight! • Distances- use stellar parallax • Luminosity- same as sun (careful!) • Temperature- same as sun • Diameter- use Luminosity and Temperature • Mass- save it for later

  3. Distance • Stellar Parallax • New unit- parsec (pc) • 1 pc= 206,265 AU= 3.1x1016m • “parallax in arc seconds” • Distance (pc)=1/parallax (arc sec) • Lightyear-distance light travels in one year 1pc=3.3ly

  4. Distance • Another Method- Standard Candles • Know the brightness a star should have • If it appears dimmer, it must be further away • Estimate distance based on dimness • Often used for extragalactic objects

  5. Parallax Earth 1 AU Star Parallax Angle Sun

  6. Sun’s Neighbors • Closest neighbor- Proxima Centauri • 1.3 pc away (4.3 ly) 300,000 times distance between Earth and Sun • About 30 stars within 4pc • Many are multiple star systems • We can measure parallax out to 100pc

  7. Diameter • Radius-Luminosity-Temperature relationship • Use color to find temperature • Use Stefan-Boltzmann Law to find Luminosity • L  R2  T4 • Star w/same T as Sun but Bigger L must be larger in size!

  8. Temperature • Remember that Blackbodies will appear different “colors” depending upon Temperature • Cool stars – Red • Hot stars – Blue

  9. Spectra of a Star • Indication of energy emitted at every wavelength of light • Tells us many things • Composition, temperature, luminosity, velocity, rotation speed are just some

  10. Spectral Classification • Detailed spectra of stars allow better classification • Lines that are present allow star to be “pigeonholed” • Orignal scheme was loosely based on color • 4 classes: White, Yellow, Red, Deep Red

  11. History of Spectral Types • Edward Pickering at Harvard • Hired “computers” • Williamina Fleming started with A • Strength of H lines only • She classified 10,000 stars • Pickering published her work in 1890

  12. History of Spectral Types • Annie Jump Cannon developed new scheme • Pared Fleming’s number of classes • Included subdivisons • Classified 400,000 stars in her lifetime • Her system is the standard used today

  13. Spectral Classification • Pickering (Harvard) assigned letters to original classes (A-M) • Annie Jump Cannon rearranged classes based on temperature (Payne’s system) • Non-alphabetical OBAFGKM (LT)(RNS)

  14. Families of Stars www.hubblesite.org

  15. Spectral Types O star • Ionized He, weak H lines • T>25,000 K • Electric Blue (peaks in UV) • Example: Stars in Orion’s Belt

  16. Spectral Types B star • Neutral He, moderate H lines • T=25,000 K-11,000K • Blue (peaks in UV) • Example: Rigel

  17. Spectral Types A star Very Strong H lines T=11,000-7,500K Peaks in Violet Example: Sirius, Vega

  18. Spectral Types F star Moderate H lines and Ionized Ca T=7,500-6,000K Blue Example: Polaris, Canopus

  19. Spectral Types G star Weak H lines and Strong Ionized Ca T=6,000-5,000K Yellow Example: Sun, Alpha Centauri

  20. Spectral Types K star Lines of neutral and singly ionized metal, some molecules T=5,000-3,500K Red Example: Arcturus, Aldeberan

  21. Spectral Types M star Strong Molecular Lines T=2,200-3,500K Red (Peaks in IR) Example: Betelgeuse, Proxima Centauri

  22. Spectral Types L star Strong Molecular Lines Includes Water !! T=1,300-2,200 Red (Peaks in IR) Likely a Brown Dwarf

  23. Spectral Types T star Strong Lines of Water and Methane Very Cool! T=900-1300K Red (Peaks in IR) Likely a Brown Dwarf

  24. Spectral Types RNS Special classes for “evolved” stars These stars are in old age Puffy atmospheres wash out some lines Others are easier to see

  25. Spectral Types • Further divisions 0-9 • Based on where temperature is in range • Lower the number- hotter the star • Sun is a G2 star, cooler than G1 hotter than G3

  26. Why different spectra? • Most stars have similar composition • Line strength is determined by number of excited electrons • What determines this? • Temperature differences!

  27. Combination of Tools • Spectral Class, Temperature, and Luminosity can be put together • Form a very useful tool • Hertzsprung-Russell (HR) diagram • Relates T, L, D , spectral class of any star! • Very important to Astronomers!

  28. HR Diagram • Demographic Chart • All stars are place on it based on two pieces information • Luminosity and Temperature (spectral class) • Can provide information about many things

  29. HR Diagram Hot, bright Cool, bright Luminosity Increasing Hot, dim Cool, dim Temperature Increasing

  30. HR Diagram Red Super Giants Red Giants Main Sequence Luminosity Increasing White Dwarfs Temperature Increasing

  31. Stellar Populations • HR diagram gives information about populations • Stars evolve and age • Star’s position on HR diagram =info about age • Not all stars in sky are same age! • Also info about fusion fuel

  32. Main Sequence • Most stars • Adult star • Majority of lifetime spent here • Hydrogen fusion • Stay in one location on diagram • Blue Supergiants to Red Dwarfs • Sun is on MS

  33. Red Giants • 10-1000x Radius of Sun (R) • 3000-6000K • Red Giants are older than MS stars of same mass • No Red Giants within 5pc of Sun • 1% of Solar Neighborhood • Stopped H-fusion

  34. White Dwarfs • Earth-sized (Tiny) • Very hot (>6000K) • Older than Red Giants • No H-fusion • 9% of Solar Neighborhood

  35. Luminosity Classes • Need more than Spectral Class • Example : Both Betelgeuse and Barnard’s Star are M type stars Betelgeuse is 100,000 times more Luminous!

  36. Luminosity Classes • Assign LC to distinguish types of stars of same Spectral Class • I Supergiants (Ia, Ib) • II Luminous Giants • III Regular Giants • IV Subgiants • V Main Sequence Stars

  37. Luminosity Classes • Betelgeuse is a M2Ia • Red, Supergiant • Barnard’s Star M5V • Red Dwarf, Main Sequence

  38. Distance Again • Find distance to ANY star • Measure energy received • Estimate luminosity from classification • Use inverse-square law to find distance • Spectroscopic Distance

  39. Stellar Masses • Can’t be found from just “size” • Two ways to determine • Binary Star system • Mass-Luminosity Relationship • Determines star’s location on MS and ultimately… It’s lifespan!

  40. Binary Star Masses • Two stars orbiting a common center • 3 types of Binary Stars • Visual Binary • Spectroscopic Binary • Eclipsing Binary

  41. Visual Binary • See two stars w/ eye or telescope • Example Alcor/Mizar in Big Dipper • Widely separated • Time of orbit can be observed directly • Brighter Star-Primary • Fainter Star-Secondary

  42. Spectroscopic Binary • Too closer together or too far away to see separate stars • Look for Doppler Shift in Spectral Lines • Moving toward us –Blue Shift • Moving away from us –Red Shift

  43. Spectroscopic Binary • Double-line SB • Two stars about same Luminosity • Two sets of lines observed • Each is Doppler Shifted • Single-line SB • One star is brighter than other • One set of lines observed • Doppler shifted also

  44. Spectroscopic Binary Animation http://csep10.phys.utk.edu/astr162/lect/binaries/spectroscopic.html

  45. Eclipsing Binaries • Rarest form • Orbital Plane is edge on • One star passes in front of other • Blocks light (eclipses!) • “Star” appears to vary dramatically in brightness Check it out! • Example: Algol, Sirius AB

  46. Finding Masses • Determine the period of orbit • Determine distance apart • Find the “balance point” of system • This is Center of Mass • Use this to determine total mass of system • Can’t find individual masses unless individual stars can be seen

  47. Single Star Masses • Binary techniques don’t work • Mass-Luminosity relationship • Larger Luminosity – Greater Mass • Luminosity  Mass4 • Example • A star 2x Mass of Sun (M) has a Luminosity 24 (16x) the Sun’s (L )

  48. IMPORTANT! • The Mass-Luminosity Relation applies to Main Sequence Stars only! • Red Giants and White Dwarfs must use approximations

  49. Mass-Luminosity Relation • Range of Masses on MS is not very large • 0.1M -100M • Smaller than this-don’t “turn on” • Larger than this –too unstable

  50. Mass-Luminosity Relation • Also, tells about lifetimes • Big stars have more fuel but… • They burn it much, much faster so… • They live much shorter lifetimes than smaller stars • 1M - 10 billion years • 10M-20 million years

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