Download Chapter 10 Measuring the Stars: Giants, Dwarfs, and the Main

Chapter 10
Measuring the Stars: Giants, Dwarfs, and the Main Sequences
Measuring the Stars
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The Distances to the Stars
* ________________ - an object’s apparent shift against more distant background stars
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• Analyze stars at opposite ends of our orbit
• Parallax is actually half the shift we see
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Angles are difficult to measure because they become so small
* Astronomers use arcseconds instead of degrees because of the small angles
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* One parsec(pc) = ____________________
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Our Nearest Neighbors
* Proxima Centauri (Alpha Centauri system)
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• 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
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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
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Luminosity and Apparent Brightness
* Apparent Magnitude • Magnitude scale - ranks naked eye stars in 6 categories as originally defined by
Hipparchus
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• 1st order stars are 2.512 times brighter than 2nd order stars
• 1st order stars are 100 times brighter than 6th order stars
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• All stars are measured from a standard distance of 10 pc or 33 light years
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Stellar Temperatures
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Looking at a star’s color can reveal if it’s hot or cold
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• 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
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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
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– 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
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* Larger stars are Supergiants
* Small hot bright stars are called white dwarfs
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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)
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• 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
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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
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