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Stars

Stars. … how I wonder what you are. Goals. Stars are Suns. Are they: Near? Far? Brighter? Dimmer? Hotter? Cooler? Heavier? Lighter? Larger? Smaller? What categories can we place them in?. Linear size = how big something really is Meters, inches, light years, feet

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Stars

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  1. Stars … how I wonder what you are.

  2. Goals • Stars are Suns. • Are they: • Near? Far? • Brighter? Dimmer? • Hotter? Cooler? • Heavier? Lighter? • Larger? Smaller? • What categories can we place them in?

  3. Linear size = how big something really is Meters, inches, light years, feet Angular size = how big something looks Degrees, arcminutes, arcseconds, milliarcseconds Circle = 360 degrees 1 degree = 60 arcmin 1 arcmin = 60 arcsec 1 arcsec = 1000 mas Angular Size

  4. Distance • One proof of a heliocentric Universe is stellar parallax. • Tycho Brahe saw no parallax. • Copernicus: stars too far away. • Nearest star: Proxima Centauri Parallax angle = 0.76 arcsec • Tycho’s precision = 1 arcmin

  5. What is the distance of an object with a parallax angle of 1 arcsec? Distance = 206,265 AU This distance is 1 parsec (pc) 1 pc = 206,265 AU = 3.3 ly 1 lightyear = distance light travels in one year. The Parsec

  6. Distances • Closest star: Proxima Centauri parallax = 0.76 arcsec Distance = 1.3 pc or 4.3 lightyears

  7. Brightness • How bright are they really? • What is due to distance? • What is due to luminosity? • Luminosity: • Total energy radiated every second.

  8. Magnitude Scale • The SMALLER the number the BRIGHTER the star! • Every difference of 1 magnitude = 2.5x brightness. • Every difference of 5 magnitudes is a 100x difference in brightness.

  9. Star light, star bright • Sirius is magnitude -1.5 Polaris is magnitude 2.5 • Is Sirius really more luminous than Polaris? • No, Sirius is just closer.

  10. Apparent and Absolute • Apparent Magnitude = brightness (magnitude) of a star as seen from Earth.  m • Depends on star’s total energy radiated (Luminosity) and distance • Absolute Magnitude = brightness (magnitude) of a star at a distance of 10 pc.  M • Only depends on a star’s luminosity

  11. example • Our Sun: • m = -26.8, • distance = 4.8 x 10-6 pc So: M = 4.8 • Polaris: • m = 2.5, • distance = 132 pc So: M = -3.1 • Polaris is 1500 times more luminous than the Sun!

  12. Hot Stellar Spectra Cool Stellar Temperatures How hot are stars? • In Lecture 4 we learned about thermal radiation and temperature. • Since different stars have different colors, different stars must be different temperatures.

  13. Spectral Classification

  14. Stellar Masses How massive are stars? • Kepler’s Laws – devised for the planets. • Apply to any object that orbits another object. • Kepler’s Third Law relates: • Period: “how long it takes to orbit something” • Semimajor axis: “how far you are away from that something” • Mass: “how much gravity is pulling you around in orbit” • Where M is the Total Mass. • Can calculate the mass of stars this way.

  15. Binary Stars • Most stars in the sky are in multiple systems. • Binaries, triplets, quadruplets, etc…. • Sirius • Alcor and Mizar • Tatooine • The Sun is in the minority by being single.

  16. 0.005 arcsec

  17. Stellar Masses How massive are stars? • Most stars have masses calculated this way. • Result: • The more massive the star, the more luminous it is. • The more massive the star, the hotter it is.

  18. 50 mas Stellar Radii How big are stars? • We see stars have different luminosities and different temperatures. • Stars have different sizes. • If you know: • Distance • Angular size • Learn real size.

  19. Betelgeuse • Angular size = 50 mas • Parallax = 7.6 mas = 0.0076 arcsec • Apparent mag = 0.6 • Distance = 1/parallax = 132 pc • True size = distance * angular size = 1400 Rsol • Absolute Mag = m – 5log(d/10pc) = -5 • Our sun M ~5, Betelgeuse = 10,000x luminosity

  20. Angular versus Linear Supergiants, Giants and Dwarfs

  21. H-R Diagram • Can order the stars we see by: • Temperature (or spectral type) • Luminosity (or absolute magnitude). • And see where other qualities fall: • Mass • Radius

  22. The Main Sequence • Stars characterized by what holds them up. • 90% held up by heat of Hydrogen fusion? 4H  He + Energy

  23. Main Sequence & Thermal Radiation • The Main Sequence makes sense! • Hotter stars are bluer – Wien’ Law • Hotter stars are brighter – Stefan’s Law

  24. Homework #8 • For Feb 12: • Read B16.6, 17.1 – 17.2, Ty10 • Do Ch16 Problems: 6, 9, 21

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