03_Sun - GEL36 SOLAR SYSTEM Lecture 3 The Sun(Ch 8 in Seeds...

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1 GEL36 SOLAR SYSTEM Lecture 3: The Sun (Ch. 8 in Seeds – read selectively!) Goal of the Lecture: To provide a descriptive summary of the general characteristics of the sun, discuss how energy is generated within the sun, and evaluate the influence of ‘solar weather’ on our planet. Many new concepts and ideas are introduced in this lecture and will be returned to and more fully explained as the course progresses (e.g., interiors of stars and planetary bodies, heat transfer mechanisms, temperature vs. heat, solar wind) I: General Characteristics of the Sun 1) Where in the galaxy? - our sun is a typical star among 100 billion stars in the Milky Way. Located about 2/3’s of way from the center of the galaxy in an outer limb of the spiral arms. - solar system revolves around the center of the galaxy at a velocity of 250 km/sec (562,500 mph) and completes one orbit in about 226 million years. 2) Rotation, distance - sun spins on its axis in the same direction as the Earth, that is, west to east, counterclockwise if viewed from above the north pole . (This is what you would expect from the solar nebula hypothesis) This common motion of rotation, similar to the spin direction of the planets is called prograde motion . (The opposite, clockwise motion of Venus is known as retrograde motion .) The similar orbits of the planets around the sun is also known as a prograde motion. (we'll use these terms for the rest of the class) - 1 AU from Earth - sun’s axis inclined 7° from vertical, relative to the plane of the ecliptic (Earth’s axis is inclined 23.5°) - Sun has a differential rotation - rotates faster near its equator (~25 days for one rotation) than at its poles (~36 days for one rotation) (remember, Earth takes 24 hrs for one rotation) 3) Dimensions Our sun is an average size star . . . - diameter is 1.4 million km (~109 x Earth diameter) - density is 1.4 grams/cc (cc = cubic centimeter) (water is 1 g/cc and Earth is 5.5 g/cc)
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2 - mass is 2 x 10 30 kg (equivalent to mass of 1000 Jupiters or 333,400 Earths) - volume of the sun is large enough to hold about a million Earths - >99% of all mass in solar system is contained in the sun 4) Composition The sun, like any star, is just a hot ball of gases, held together by gravity. - mostly hydrogen ( H ~75% by mass) and helium ( He ~25% by mass) - minor amounts of heavier elements (0.1%), but they do occur with the same relative abundance as the same elements on Earth and in meteorites - suggests common derivation from same solar nebula. So, if most of the sun is composed of hydrogen and helium, and if >99% of the mass of the solar system is contained in the sun, then most of the solar system is composed of H and He. II: Interior of the Sun
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03_Sun - GEL36 SOLAR SYSTEM Lecture 3 The Sun(Ch 8 in Seeds...

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