THE SUN AND STARS
The star we see by day
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The Sun, Our Star
• The Sun is an average star.
• From the Sun, we base our understanding of all
stars in the Universe.
• No solid surface.
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Vital Statistics
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Radius = 100 x Earth (696,000 km)
Mass = 300,000 x Earth (1.99 x 1030 kg)
Surface temp = 5,800 K
Core temp = 15,000,000 K
Luminosity = 4 x 1026 Watts
Solar “Day” =
– 24.9 Earth days (equator)
– 29.8 Earth days (poles)
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In The Core
• Density = 20 x
density of Iron
• Temperature =
15,000,000 K
• Hydrogen atoms
fuse together.
• Create Helium
atoms.
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Nuclear Fusion
• 4H  He
• The mass of 4 H nuclei (4 protons):
4 x (1.6726 x10-27 kg) = 6.690 x 10-27 kg
• The mass of He nuclei: = 6.643 x 10-27 kg
• Where does the extra 4.7 x 10-29 kg go?
• ENERGY!  E = mc2
• E = (4.7 x 10-29 kg ) x (3.0 x 108 m/s)2
• E = hc/l  l = 4.6 x 10-14 m (gamma rays)
• So: 4H  He + light!
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Solar Cross-Section
• Progressively smaller
convection cells carry the
energy towards surface.
• See tops of these cells as
granules.
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Ha Sun
Photo by Big Bear Solar Observatory
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Prominences
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Aurorae
• The solar wind
passes out
through the
Solar System.
• Consists of electrons, protons and other charged
particles stripped from the Sun’s surface.
• When charged particles and magnetic fields interact:
light!
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Solar Cycle
• Increase in solar wind activity
- Coronal Mass Ejections
• Increase in Auroral displays on Earth
• Increase in disruptions on and around Earth.
Courtesy of SOHO/LASCO/EIT consortium.
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Magnetic fields and Sunspots
• At kinks, disruption in convection cells.
• Sunspots form.
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11-year sunspot cycle.
Center – Umbra: 4500 K
Edge – Penumbra: 5500 K
Photosphere: 5800 K
Sunspots
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Magnetic fields and Sunspots
• Where
magnetic
fields “pop
out” of Sun,
form sunspots.
• Sunspots
come in pairs.
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Stars
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Spectral
Classifications
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Supergiants, Giants, and Dwarfs
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Stellar Radii
How big are stars?
• We see stars have different
luminosities and different
temperatures.
• Stars have different sizes.
• If you know:
50 mas
– Distance
– Angular size
• Learn real size.
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