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Understanding Properties of Stars: Parallax, Brightness, and Luminosity

Explore the concepts of parallax, brightness, and luminosity to understand how we measure distance, absolute magnitude, and brightness of stars. Learn about methods used to determine temperature, luminosity, and radius of stars.

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Understanding Properties of Stars: Parallax, Brightness, and Luminosity

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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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