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Properties of Stars

Properties of Stars. http://stardate.org/radio/program/delta-lyrae. 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 arc second, parsec. Define brightness, apparent magnitude, absolute magnitude.

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Properties of Stars

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  1. Properties of Stars

  2. http://stardate.org/radio/program/delta-lyrae

  3. 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 arc second, parsec. • Define brightness, apparent magnitude, absolute magnitude. • Describe the methods used to determine the temperature, luminosity, and radius of a star.

  4. 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.

  5. You already know about the parallax effect: Demonstrating parallax Parallax of Stars • Explain what is meant by the parallax of a star, how we measure it and use it to find the distance to a star.

  6. Define arc second How many degrees in a circle? How many arc minutes in a degree? How many arc seconds in an arc minute? How many arc seconds in a degree? How many arc seconds in a circle? __?__ radians = 360 degrees 1 radian = 57.3 degrees How many arc seconds in 1 radian? 360, 60, 60, 3600; 1,296,000; 2 pi; 206,265 arc sec/rad

  7. 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: • Define arc second, parsec

  8. 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.

  9. Explain what is meant by the parallax of a star, how we measure it and use it to find the distance to a star.

  10. 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

  11. Inverse square law for light p. 494

  12. Brightness How the star looks to US HERE ON EARTH. times farther away  100 Watt 1000 Watt 1 Watts 10 times farther away  2 x farther away, 1/4 as bright 3 x farther away, 1/9 as bright • Define brightness, apparent magnitude, absolute magnitude

  13. 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

  14. When you see only “magnitude,” that means APPARENT magnitude. • The magnitude (m) of star A is 1, the magnitude (m) of star B is 6. How many times brighter is A than B? • a) 5 b) 10 c) 100 d) 1000 • m of star C is 12, m of star D is 2: How many times brighter is star D than star C? (Or, equally stated, how many times dimmer is star C than star D?) • a) 10 b) 24 c) 100 d) 10,000 • The Sun is the brightest star in the sky, with an apparent magnitude of about -26.5 Sirius is next in line, with an apparent magnitude of -1.5; how many times brighter is the Sun than Sirius? • a) 25 b) 28 c) 100,000 d) 10,000,000,000

  15. Using SIMBAD to find the apparent magnitudes of the stars of Exercise 2 41 Cygni data (partial) V = apparent magnitude through “visual” filter Think of it as mv . IR UV

  16. 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 brightness, apparent magnitude, absolute magnitude

  17. Define brightness, apparent and absolute magnitude

  18. Luminosity Classes Supergiant I Bright-Giant II Giant III Sub-Giant IV Main Sequence Star (dwarf) V We estimate the luminosity of a star by measuring how broad the absorption lines are in its spectrum. At a given temperature, the less luminous stars have atoms colliding a lot more than in the giant stars. • Describe the methods used to determine temperature, luminosity, radius

  19. High Luminosity Low Low High Temperature

  20. Using SIMBAD to find the parallaxes of the stars of Exercise 2 41 Cygni data (partial) F5 Iab

  21. The H-R Diagram

  22. Relationship between absolute magnitude and luminosity - bring in the Sun!

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

  24. The H-R Diagram

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