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Lecture 12- Stars: Distances and Magnitudes

Lecture 12- Stars: Distances and Magnitudes. Smaller parallax > greater distance Largest parallax = 0.75 arcseconds 1 arcsecond = dime at 1.3 miles. Units of Stellar Distance. 1 parallax-second = 1 parsec = 3.08E+16m =206,265 au If 1 au were 1cm, Alpha Centauri at a distance of 1.5 miles

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Lecture 12- Stars: Distances and Magnitudes

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  1. Lecture 12- Stars: Distances and Magnitudes Smaller parallax > greater distance Largest parallax = 0.75 arcseconds 1 arcsecond = dime at 1.3 miles

  2. Units of Stellar Distance • 1 parallax-second = 1 parsec = 3.08E+16m =206,265 au • If 1 au were 1cm, Alpha Centauri at a distance of 1.5 miles • The Light Year = (2.9979E+08 m/sec)*(3.156E+07 sec) = 9.461E+15 m • 1 parsec = 3.26 ly

  3. Sources of Parallax Measurements • Pre-1990: ground-based telescopes, ~ 1000 stars measured • Hipparchus: 118,000 stars measured • See book for breakdown on precision Hipparcos web page: http://astro.estec.esa.nl/Hipparcos/table361.html http://astro.estec.esa.nl/Hipparcos/table365-new.html

  4. What distances do we have for the nearest stars? • Appendix 12 of book • Hipparcos table 361 • http://astro.estec.esa.nl/Hipparcos/table361.html What do these numbers mean? What do they tell us about the nature of stars?

  5. What are the distances to the stars in Cygnus?

  6. Distances to the Stars of Cygnus 990pc 3.5 pc 22 parsecs 17pc 125 pc 467 pc 52 pc

  7. What does this very basic information (distances and brightnesses of naked eye stars) tell us about the nature of stars? This is a real question. I want some answers.

  8. Brightnesses of Stars: The Magnitude System How to quantitatively describe the brightness of stars?

  9. Magnitudes, Apparent and Absolute • Apparent magnitude is the brightness of an object as it appears to you • System due to Hipparchos (2nd century BC) • Nowadays system made more precise • Magnitude changes are “logarithmic”, each magnitude means factor of 2.512 in brightness • Brightness in magnitudes in Table 16.1 • Apparent magnitudes of brightest stars in Appendix 13

  10. Absolute Magnitude: a measure of the intrinsic brilliance of a star • Pick a star (any star) • Imagine moving it to a distance of 10 parsecs • The apparent magnitude it would have is its absolute magnitude • The absolute magnitude is a distance-independent quantity • Look at Appendix 13 and think about the meaning of the absolute magnitudes

  11. Why such a big deal about absolute magnitudes? • The difference between the apparent magnitude (m) and the absolute magnitude (M) is a measure of the distance to an object

  12. If you know the absolute magnitude M of a star (or other astronomical object) and you measure its apparent magnitude m, you then know its distance. This difference (m-M) is called the distance modulus

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