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Star Properties

Star Properties. Apparent Magnitude. System of Hipparchus Group of brightest stars 1 m Stars about ½ as bright as 1 m 2 m Stars about ½ as bright as 2 m 3 m • • • Naked Eye Limit 6 m. Apparent Magnitude. 19th century photographers learn how eye responds to light (Pogson)

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Star Properties

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  1. Star Properties

  2. Apparent Magnitude • System of Hipparchus • Group of brightest stars 1m • Stars about ½ as bright as 1m 2m • Stars about ½ as bright as 2m 3m • • • • Naked Eye Limit 6m

  3. Apparent Magnitude • 19th century photographers learn how eye responds to light (Pogson) • Doubling the brightness is not perceived as a doubling by the eye • Eye response is logarithmic • Ratio of 100 in brightness corresponds to a Difference of five magnitudes • Dm of 5  100X in light • Dm of 1  2.512X in light

  4. Some Apparent Magnitudes • Sun -26.8 • Full Moon -12.6 • Venus at brightest -4.4 • Sirius -1.5 • Naked Eye Limit 6.0 • Faintest Objects +30.0 • Hubble

  5. Learning the Brightness • Is a star bright... • Because it really is a bright star? • Because it is close to the Earth? • Stellar brightness depends on • Luminosity • Distance

  6. June Sun January Measuring Distance • Stellar Parallax

  7. June Sun January Stellar Parallax Parallax 1 AU

  8. Measuring Parallax 1 arcsec 1 AU 1 parsec

  9. Stellar Parallax When p is measured in arcsec and d is measured in parsecs One parsec: 206,265 AU 3.26 light years

  10. Stellar Parallax • Nearest star to Sun (largest parallax) • a Cen p = 0.7 arcsec • Limit of accurate parallax •  200 pcs (angles of 0.005 arcsec) • Hipparcos satellite (120,000 stars measured to 0.001 arcsec)

  11. Absolute Magnitude • The magnitude a star would have at 10 parsecs from the Sun. • The apparent (m) and absolute (M) magnitudes of a star at 10 pcs are the same. • M, m, and d are related. Knowing two allows you to compute the third.

  12. Putting the Pieces into Place Ejnar Hertsprung 1911 Henry Norris Russell 1913

  13. Luminosity Classes I Supergiants II Bright Giants III Giants IV Subgiants V Dwarfs

  14. Luminosity Class implies Size • Consider the Sun and Capella The Sun G2V M=5 Capella G2III M=0

  15. Luminosity Class implies Size • Equal sized pieces of each star are equally bright • Capella is 100X brighter (5 magnitudes) • Capella must have 100X as much area • Surface area  radius2 • Capella must be 10X larger than Sun.

  16. Luminosity Class in the Spectrum A3 Supergiant A3 Giant A3 Dwarf

  17. Sun G2V Vega A1V Betelgeuse M1I

  18. Which of these stars is hottest? • Sun G2V • Vega A1V • Betelgeuse M1I • Can’t compare

  19. Which of these stars is brightest? • Sun G2V • Vega A1V • Betelgeuse M1I • Can’t compare

  20. Which of these stars is smallest? • Sun G2V • Vega A1V • Betelgeuse M1I • Can’t compare

  21. Which of these stars is most distant? • Sun G2V • Vega A1V • Betelgeuse M1I • Can’t compare

  22. Spectroscopic Parallax • Observe the spectrum and apparent magnitude of a star • Classify the spectrum Main Sequence • Plot it on the H-R Diagram • Read off the M • From m and M compute distance

  23. B V Color Index 12000 K * * 7000 K * *

  24. Color Index Star Temperature mB mV . 1 12000 K 2.0 2.4 2 7000 K 3.0 3.1 Color Index = mB - mV = B-V 1 B-V = 2.0 - 2.4 = -0.4 2 B-V = 3.0 - 3.1 = -0.1

  25. Spectroscopic Parallax • Can now get distances to any object whose spectrum can be measured. • Limit  5000 pcs

  26. Study Tools • Review 1 • Review 2

  27. The Advantage of Color Index • Measures temperature just like Spectral Type • Much easier to obtain • requires two measurements of brightness • spectral type requires getting the spectrum

  28. M mV Spectral Type B-V Color-Magnitude Diagrams Standard H-R Diagram Color-Magnitude Diagram

  29. Color-Magnitude Diagrams • Useful for star clusters • Can substitute mV for MV since you know all the stars are the same distance away. • Star Clusters • Open (galactic) • Globular

  30. Structure of the Milky Way

  31. Open Clusters • Irregular shape • Few tens to few hundred stars • In the plane of the galaxy • Young stars

  32. Open clusters M37 M16 M45

  33. Color-Magnitude DiagramM45

  34. Globular Clusters • Spherical in shape • Hundreds of thousands of stars • Halo distribution about galactic nucleus • Old stars

  35. Globular Clusters SFA Observatory M5 M3

  36. Color-Magnitude DiagramM3

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