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Chapter 10 Measuring the Stars: Giants, Dwarfs, and the Main Sequences Measuring the Stars * The Distances to the Stars * ________________ - an object’s apparent shift against more distant background stars • • Analyze stars at opposite ends of our orbit • Parallax is actually half the shift we see * Angles are difficult to measure because they become so small * Astronomers use arcseconds instead of degrees because of the small angles * * One parsec(pc) = ____________________ * Our Nearest Neighbors * Proxima Centauri (Alpha Centauri system) • • 0.77" parallax • 4.3 light years away * Barnard’s Star • 2nd closest star to the Sun • 0.55" parallax • 6.0 light years away * Ground based telescopes can view stars with parallax values as low as 0.01” * Space telescopes can view stars with parallax values as low as 0.005” * Map of our 30 closest stars * Stellar Motion * Stars have transverse and radial motion • Transverse - perpendicular to line of sight • Radial - along our line of sight * ___________________ - annual movement of a star across the sky as seen from Earth • _____________________ has the largest known proper motion of any star – 10.3"/year – Most stars have proper motions less than 1”/year * Luminosity and Apparent Brightness * Apparent Magnitude • Magnitude scale - ranks naked eye stars in 6 categories as originally defined by Hipparchus • • • 1st order stars are 2.512 times brighter than 2nd order stars • 1st order stars are 100 times brighter than 6th order stars * • All stars are measured from a standard distance of 10 pc or 33 light years • * Stellar Temperatures 1 * * * Looking at a star’s color can reveal if it’s hot or cold • • Temperature examples of common stars Spectral Classifications * Looking at a stars spectra can give us it’s temperature * A star’s surface temperature is given a spectral class using specific letters (classes) • Spectral letters are subdivided in number • Stellar Sizes * Direct Measurements • Only a few stars are close enough to measure their sizes directly • Knowing a stars angular size and distance away, simple geometry can provide the radius * Indirect Measurements • – Luminosity is proportional to r2 X temp.4 – Re-arrange equation to get r = luminosity1/2 / T2 – Star Mira has temperature half of the Sun and a luminosity 400 times that of the Sun – Radius must be 80R – Mira is a Giant * * Larger stars are Supergiants * Small hot bright stars are called white dwarfs * * * * * Main sequence stars arranged from upper left to lower right • Main sequence stars are ordinary stars • Hotter, more massive blue stars (top left) to cooler, less massive red stars (lower right) • • Our Sun is considered an average star * Red giants occupy the top right * Some stars are supergiants and are also located near the top right (_______________) * White dwarfs are hot, faint, small stars near the bottom of the diagram • Some white dwarfs are Earth sized * Bright and near stars plotted on diagram Extending the Cosmic Distance Scale * Spectroscopic Parallax • Knowing luminosity and apparent brightness can help to determine distance – Example: car headlights • Spectral class helps us determine its luminosity if it lies on the main sequence • Apply the inverse square law to determine distance Stellar Mass * ___________________________________ by looking at the common center of mass 2 • * Looking at the 2 stars orbits can tell us where the center of mass is, and can tell us how massive one star is compared to the other Many stars are in binary systems • Visual binaries • Spectroscopic binaries - measures Doppler shifts • _______________________ – stars eclipse one another End of Chapter 10 3