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Reminders! Website: http://starsarestellar.blogspot.com/ Lectures 1-7 are available for download as study aids. Reading: You should have Chapters 1-7 read, and Chapters 8 by the end of Tuesday. Homework: Homework #2 is due this Wednesday at the BEGINNING of class. Discussion: Next week, we will be reviewing for the midterm. Midterm!!! The midterm is Thursday, June 11th in class. Don’t be late! The test will cover Chapters 1-8. It will consist of 40 to 50 questions and you will have until 12 o’clock to complete it. The test is true-false and multiple choice. Make sure you have a SCAN-TRON 882 form and a #2 pencil. I will NOT have these available in class. Bring your student ID. The test is closed book, closed notes, and no calculators. There will be a sheet of equations available. A practice midterm is available. Take the practice test for 1.5 hours, and see how you do. EXTRA CREDIT!!! Get 1 extra percentage point for each mistake you find on the practice test! Homework Issues Practice your proportionalities! L ∝ T4 then T ∝ L1/4 If L goes up by a factor of 2, then T goes up by a factor of 21/4 ~ 1.2 Unit changes! Include units! 100 cm or 1 m or 10-3 km is all right. If your answer is simply 100, then you won’t get full credit! Jovian Planets Today’s Lecture: • Jovian Planets (chapter 7, pages 152-183) Jupiter and its moons Saturn and Titan Uranus Neptune The Solar System Top view of the solar system • All the planets orbit the Sun in the same direction. • Most of the planets rotate in the same direction too. Sun • And so does the Sun. • Orbits are all nearly in the same plane. Side view is a thin disk Origin of the Solar System “Nebular Hypothesis” (Kant, Laplace, 18th century) • Before the Sun was born, there was a gravitationally contracting, slowly spinning cloud of dust. • As it contracts due to gravity, is spins up to conserve angular momentum (figure skater effect) • Sun forms in the center, and the outer parts make a disk. • Planets form out of the disk. • Extra gas is blown away by the Sun’s winds. Terrestrial Planets • The inner planets: Mercury, Venus, Earth, and Mars • All have a smaller radius than the Earth • Not very massive (less than Earth), but dense (>1 g/cm3, which is the density of water) • Close to the Sun and few moons • Rocky outer parts and iron cores. What do you weigh on the moon? MMoon = MEarth/80, RMoon = REarth/3.7, F = ma The ratio of surface accelerations for the two bodies is • Weight = F = ma, so you would weight 1/6 on the Moon of what you weigh on Earth. • Note that mass and weight are different. Mass is intrinsic, weight is a force that depends where that object is Jovian Planets • The outer planets: Jupiter, Saturn, Uranus, Neptune • All have a much larger radii than the Earth • All are much more massive than the Earth, but they’re much less dense (0.7-1.6 g/cm3, where 1 g/cm3 is the density of water) • Far from the Sun and MANY moons • Mostly liquid, but have rock/iron cores • All are rotating very quickly (Jupiter has a radius 11 times the Earth, but a rotation time of 10 hours!) Jupiter: King of the Planets • Radius = 11 RE = 0.1 Rsun • Mass = 320 ME = 10-3 Msun • Composed mostly of H and He (like the Sun) • Colorful bands parallel to the equator. Lots of swirls and spots, indicating storms. • Oblate (squashed) shape because of rapid rotation (10 hours rotation period!) • Famous “Great red spot” is about 2-3 times the Earth’s diameter! >300 year old storm. • Visited many times, Voyager 1,2 (1979) Galileo (1996,1997) Jupiter’s Moons • Four Galilean moons: easily seen even through amateur telescopes as little points of light. • Io: Erupting volcanoes! Geologically youngest surface (the “pizza” moon). Interior molten due to tidal forces during its elliptical orbit around Jupiter. • Europa: Smooth, narrow, dark stripes and few craters. It looks like fractured soft ice--is there liquid water (or even life) below? • Ganymede, Callisto: old, hard ice with many craters • Over 50 more moons, and new smaller moons discovered each year. Many are captured asteroids. • Very thin ring at a distance of 1.8 time the radius Saturn: Magnificent Rings • Density of 0.7 g/cm3. It could float on an ocean of water! • Mostly H, He, with an Earth-like core. The structure is similar to Jupiter, with atmospheric bands. • Surrounded by magnificent rings! Ice chunks and icy rocks. • Rings are made of material that failed to form a moon (or a torn-up moon or comet) due to the tidal forces of Saturn (it is inside the “Roche limit”) • Rings in equatorial plane, but inclined 27° with respect to the orbital plane. They appear “edge-on” twice per 29-year orbital period (end of 2008) Saturn (continued) • Only 4-5 rings are generally visible from Earth. Dark “Cassini’s division” is between the 2 main rings. • Voyager (1980) found more than 100,000 ringlets about 20 m thick • The largest moon Titan: which is a very interesting object. • Titan has a thick N2 atmosphere like the Earth! Hazy atmosphere full of methane gas. • We have not seen methane or ethane oceans, but the surface appears covered by ice blocks. • Cassini spacecraft is currently orbiting Saturn 2004-now • Cassini spacecraft sent the Huygens probe to Titan to investigate Titan’s surface Uranus: The Sideways Planet • Porbit = 84 years • Not known to ancients; discovered by William Herchel (1781) • H, He mostly; ammonia and methane clouds • Very cold: 60 K • Axis of rotation tipped 98°! (Almost in the orbital plane.) Extreme seasons, in terms of the Sun’s visibility Sun Uranus: Occultation of a star by the planet and its unexpected rings Star’s brightness vs time (expected) Uranus b time (observed) Uranus b time Rings around Uranus • 10 very narrow rings discovered in 1977, when Uranus passed in front of a bright star. • Some of the rings are very thin <10 km wide. “Shepherd moons”? ring • Voyager 2 (1986) later found the “shepherd moons” around some rings, as predicted. They keep the rings narrow (gravitational interactions). Neptune: Last of the gas giants • About the same size as Uranus • H, He; methane atmosphere • Neptune discovered in 1846 (J. Galle) as a result of analysis of Uranus’ orbit (Leverriere, Adams). • Neptune perturbs Uranus • 1613: Galileo saw it, but didn’t recognize it as a planet! Neptune: end of an odyssey • Voyager 2: August 1989, got within 5000 km • Saw complete (but clumpy) rings and an atmosphere much more dynamic than Uranus. Winds up to 1100 km/hr! • “Great Dark Spot” - size of Earth; wispy clouds of methane skirt around it. (The spot later disappeared.) • Triton: large moon with thin methane atmosphere. Fascinating, varied terrain, and active ice volcanoes! • Great way to end Voyager’s 12 year odyssey!