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Astronomy Picture of the Day
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The Sun Core temperature - 15 million K Surface temperature - 6000 K 99.9% of all of the matter in the solar system Entirely gaseous Supplies the energy essential for life on Earth All normal matter was created inside a star similar to the sun!
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The Sun The Sun in X-rays over several years The Sun is a star: a shining ball of gas powered by nuclear fusion. Mass of Sun = 330,000 M Earth Radius of Sun = 109 R Earth Luminosity of Sun = 4 x 10 26 Watts (Equivalent to 100 billion 1-megaton explosions every second!) Why doesn't the sun blow itself apart?
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Hydrostatic Equilibrium Internal heat and pressure directed outward Gravity pulling mass inward Two forces balance When all hydrogen transformed to helium, Sun will begin to collapse
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Let's start with the core, where the Sun's energy is generated. The Interior Structure of the Sun Core - nuclear fusion Radiation zone - completely ionized => no photon absorption Convection zone - less ionization => photon absorption Photosphere - radiation, the part of the sun that we see How do we get this info?
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What Powers the Sun? Nuclear Fusion: An event where the nuclei of two atoms join together. Need high temperatures. Why? To overcome electric repulsion. Energy is produced. (A small amount of mass = a lot of energy) E = mc 2. Sum of mass and energy always conserved in reactions. Mass is just “frozen” energy.
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Why High Temperatures? To overcome electric repulsion High temp => fast atomic motion (Molecular Motion Demo) Nuclear forces - very short range (10 -15 m) 100 times EM force 10 39 times gravity (“Fusion” Demo)
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How does energy get from core to surface? core "radiation zone": photons slowly drift outwards "convection zone" "surface" or photosphere: photons escape into space. photon path
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Sunspots (evidence for 11 year cycle) Roughly Earth-sized Last ~2 months Usually in pairs Follow motion of sunspots => can determine rotation rate (Sunspot Demo)
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Magnetic Fields Rotating Earth generates a magnetic field - Like bar magnet aligned with Earth’s axis Influences paths of electrically charged particles
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Aurorae Charged particles from solar wind spiral towards poles Collide with and excite atmospheric molecules Emission spectra!
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The Sun's Rotation The Sun exhibits Differential Rotation just like the Jovian planets. ~25-36 days What is this? (animation) What might this mean for the magnetic field of the Sun?
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Sun’s Magnetic Field Rotating sun also generates a magnetic field Differential rotation => magnetic field distortion Loops extending beyond photosphere can form
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Sunspots They are darker because they are cooler (4500 K vs. 5800 K). Related to loops in the Sun's magnetic field. radiation from hot gas flowing along magnetic field loop of Sun.
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Solar Flares Violent ejection of large amount of particles from Sun surface. (animation)
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Above the photosphere, there is the chromosphere, transition zone, and... The Corona Best viewed during eclipses. Solar wind => Evaporation 1 million tons/s lost. But Sun has lost only 0.1% of its mass from solar wind in its 4.6 billion year lifetime.
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Apparent Brightness vs. Luminosity Luminosity – Intrinsic power output Apparent Brightness – Perceived intensity when viewed from some distance Does an object emitting light appear brighter or dimmer as it moves away from you?
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Apparent Brightness and Distance Calculate luminosity, measure apparent brightness => figure out distance!
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