Convection and Radiation layers differ on how heat is transferred
PHYS 162
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Outer Atmosphere
• Can see during eclipses. Interactions of solar wind
with Earth’s magnetic field and atmosphere causes
Aurora Borealis
PHYS 162
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Aurora Borealis – Northern Lights
seen at high latitudes as magnetic fields are lower in the atmosphere.
rarely seen in DeKalb. Photos are from Alaska and Maine
PHYS 162
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Test 1 Guide for short answer questions
• Motion of Sun, stars, and planets through sky vs season
• Galileo’s astronomical observations
• Kepler’s Laws of planetary motion
• Newton’s Laws of motion (mostly F=ma)
• how light is produced (accelerated charge) plus discrete
vs. continuous
• nuclear reactions in the Sun : p-p cycle
• Layers in the Sun
• 4 forces with examples
PHYS 162
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The Nature of Stars
• Measure properties of Stars
Distance
Mass
Absolute Brightness
Surface Temperature
Radius
• Find that some are related
Large Mass  Large Brightness
• Gives model of stellar formation and life cycle
PHYS 162
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Distances to Stars
• Important as determines actual brightness but hard to
measure as stars are so far away
Closest Alpha Centauri
4.3 light years = 4 x 1013 km
(1 AU = distance Earth to Sun = 8 light minutes)
• Close stars use stellar parallax (heliocentric parallax or
triangulation  same meaning)
• Can “easily” measure distance using parallax to a few
100 LY. Need telescope: first observed in 1838. Study
close stars in detail. Other techniques (later) for distant
stars
PHYS 162
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Distances to Stars - Parallax
PHYS 162
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Shifting Star Positions
•
•
•
•
The orbit of the earth is used as the base.
Near stars appear to move more than far stars
distance = (base length)/angle
define: 1 parsec = 1/(angle of 1 second of arc) = 3.3 LY
site A
December
angle
Sun
site B
June
PHYS 162
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Stellar Parallax
• A photo of the stars will show the shift.
July
PHYS 162
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Nearest Stars
61 Cygni
first
parallax
in 1838
PHYS 162
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Nearest Stars
•The larger the angle (T.Par. =
trigonometric parallax) the
closer the star
• many stars come in groups like
the 2 stars in the Sirius “binary
cluster”  close together,
within same “solar system”
•Alpha Centauri and Procyon
are close binary systems.
Proxima Centauri is a red dwarf
which probably orbits Alpha
Centauri every 500,000 years
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Parallax Data
• In 1900 only 60 stars had parallax measurements
• 1997-2000 a European satellite Hipparcos released
parallax measurements for more than 2,300,000
stars up to 500 LY distance
• 118,000 stars measured with .001 arc-second
resolution and 0.2% error on light intensity
• OLD(1990): 100 stars with distance known to 5%.
“NEW” (2005): 7000 such stars
• ESA Gaia satellite: 2013 0.00001 arc-second
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Luminosity of Stars
• Luminosity=Absolute Brightness=how
much light/energy a star produces
• Scale relative to Sun. So
Lsirius = 23LS means Sirius radiates 23 times
more energy than the Sun
• Stars range from .0001xLS to 1,000,000xLS
Another scale: “magnitude” often used. A log scale to
the power of ~2.5. YOU DON’T NEED TO KNOW.
The lower the Mag the brighter the object
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Absolute vs Apparent Brightness
Absolute Brightness/Luminosity means total energy output
Apparent Brightness is what is seen by eye or in a
telescope and so depends on distance (1/Distance2)
PHYS 162
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