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Properties of Stars

Properties of Stars. Learning goals:. Explain what is meant by the parallax of a star, how we measure it and use it to find the distance to a star. Define brightness (see text), apparent magnitude, absolute magnitude.

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Properties of Stars

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  1. Properties of Stars Learning goals: • Explain what is meant by the parallax of a star, how we measure it and use it to find the distance to a star. • Define brightness (see text), apparent magnitude, absolute magnitude. • Describe the methods used to determine the temperature, luminosity, and radius of a star.

  2. Questions: Which stars are the brightest? Which stars are putting out the most watts? (luminosity = energy per second) NEED TO KNOW: Distances The most fundamental and accurate (within a certain range) means of finding distances is measuring the parallaxes of stars.

  3. PARSEC: Parallax ARc SECond A star having a parallax of 1 arc second is 1 parsec away 1 parsec (pc) = 3.26 light years 1 kiloparsec (1 kpc) = 1000 pc; 1 megaparsec (1 Mpc) = 1,000,000 pc Baseline is 1 Astronomical Unit Small angle formula for distance in AU’s:

  4. Measured Parallax of Stars • Works accurately for stars within about 200 pc (Hipparchos satellite) • Biggest problem: measuring the miniscule shift of a star against more distant stars 6.7 22 667 2170 ly • Explain what is meant by the parallax of a star, how we measure it and use it to find the distance to a star.

  5. Using SIMBAD to find the parallaxes of the stars of Exercise 2 41 Cygni data (partial) Parallax = 4.24 ± 0.16 mas or 0.00424 ± 0.00016 arc seconds Distance = 1/parallax = 1/0.00424 = 236 pc or ~770 ly

  6. Apparent Magnitude • Every 5 magnitudes difference means 100 x difference in brightness • One magnitude difference is 2.512 times in brightness. (2.5125 = 100) • Define brightness, apparent magnitude, absolute magnitude

  7. Using SIMBAD to find necessary measured (observed) quantities 41 Cygni data (partial) V = apparent magnitude through “visual” filter Think of it as mv . IR UV

  8. Work on star table -- SIMBAD Search 6,900 4.02 4.24 F5 Iab 4,800 4.23 16.22 G9.5 III 26,000 5.94 0.36 B0 Ib 3,900 3.72 3.87 K4.5 Ib-II

  9. Solar data

  10. Absolute Magnitude • Absolute magnitude is the apparent magnitude a star would have if its distance = 10 parsecs. • Relates luminosities by “placing” stars on common scale. • Smaller the absolute magnitude number, the more luminous the star. 41 Cygni dpc = 236 parsecs mv = 4.016 What does the answer tell you? • Define apparent magnitude, absolute magnitude

  11. 52 Cygni

  12. Relationship between absolute magnitude and luminosity - bring in the Sun! 41 Cygni’s calculations

  13. 52 Cygni

  14. Depends on • Size (radius, R) • Temperature Luminosity 41 Cygni • Describe the methods used to determine temperature, luminosity, radius

  15. 52 Cygni

  16. Work on star table -- SIMBAD Search 6,900 4.02 4.24 F5 Iab 4,800 4.23 16.22 G9.5 III 26,000 5.94 0.36 B0 Ib 3,900 3.72 3.87 K4.5 Ib-II

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