Lecture 12- Stars: Distances and Magnitudes

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Presentation transcript:

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 The Light Year = (2.9979E+08 m/sec)*(3.156E+07 sec) = 9.461E+15 m 1 parsec = 3.26 ly

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

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?

What are the distances to the stars in Cygnus?

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

This is a real question. I want some answers. 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.

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

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

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

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 (m-M) Distance (pc) 10 1 15.8 2 25.1 5 100.0 1000 20 100,000

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