Reminders!Website: http://starsarestellar.blogspot.com/ Lectures 1-7 are available for download as study aids.Reading: You should have Chapters 1-7 read, andChapters 8 by the end of Tuesday.Homework: Homework #2 is due this Wednesday at theBEGINNING of class.Discussion: Next week, we will be reviewing for themidterm.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 12o’clock to complete it.The test is true-false and multiple choice. Make sure you have aSCAN-TRON 882 form and a #2 pencil. I will NOT have theseavailable 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 1.5hours, and see how you do.EXTRA CREDIT!!! Get 1 extra percentage point for eachmistake you find on the practice test!Homework IssuesPractice your proportionalities!L ∝ T4 then T ∝ L1/4If L goes up by a factor of 2, then T goes up by a factorof 21/4 ~ 1.2Unit changes!Include units!100 cm or 1 m or 10-3 km is all right. If your answer issimply 100, then you won’t get full credit!Jovian PlanetsToday’s Lecture:• Jovian Planets (chapter 7, pages 152-183)Jupiter and its moonsSaturn and TitanUranusNeptuneThe Solar SystemSunTop view of thesolar system• All the planets orbitthe Sun in the samedirection.• Most of the planetsrotate in the samedirection too.• And so does the Sun.• Orbits are all nearly inthe same plane.Side view isa thin diskOrigin of the Solar System“Nebular Hypothesis” (Kant, Laplace, 18th century)• Before the Sun was born, there was a gravitationallycontracting, slowly spinning cloud of dust.• As it contracts due to gravity, is spins up to conserveangular momentum (figure skater effect)• Sun forms in the center, and the outer parts make adisk.• 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 thanthe Earth• Not very massive (less thanEarth), but dense (>1 g/cm3,which is the density of water)• Close to the Sun and fewmoons• Rocky outer parts and ironcores.What do you weigh on the moon?MMoon = MEarth/80, RMoon = REarth/3.7, F = maThe ratio of surface accelerations for the two bodies is• Weight = F = ma, so you would weight 1/6 on the Moon ofwhat you weigh on Earth.• Note that mass and weight are different. Mass is intrinsic,weight is a force that depends where that object isJovian 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’remuch less dense (0.7-1.6 g/cm3, where 1 g/cm3 is thedensity 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 timesthe 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 andspots, indicating storms.• Oblate (squashed) shape because of rapid rotation (10hours rotation period!)• Famous “Great red spot” is about 2-3 times the Earth’ sdiameter! >300 year old storm.• Visited many times, Voyager 1,2 (1979) Galileo(1996,1997)Jupiter’s Moons• Four Galilean moons: easily seen even through amateurtelescopes as little points of light.• Io: Erupting volcanoes! Geologically youngest surface(the “pizza” moon). Interior molten due to tidal forcesduring its elliptical orbit around Jupiter.• Europa: Smooth, narrow, dark stripes and few craters. Itlooks like fractured soft ice--is there liquid water (or evenlife) below?• Ganymede, Callisto: old, hard ice with many craters• Over 50 more moons, and new smaller moonsdiscovered each year. Many are captured asteroids.• Very thin ring at a distance of 1.8 time the radiusSaturn: 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 issimilar to Jupiter, with atmospheric bands.• Surrounded by magnificent rings! Ice chunks and icyrocks.• Rings are made of material that failed to form a moon (ora 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 tothe orbital plane. They appear “edge-on” twice per 29-yearorbital 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 about20 m thick• The largest moon Titan: which is a very interesting object.• Titan has a thick N2 atmosphere like the Earth! Hazyatmosphere full of methane gas.• We have not seen methane or ethane oceans, but thesurface appears covered by ice blocks.• Cassini spacecraft is currently orbiting Saturn 2004-now• Cassini spacecraft sent the Huygens probe to Titan toinvestigate Titan’s surfaceUranus: The Sideways Planet• Porbit = 84 years• Not known to ancients;discovered by WilliamHerchel (1781)• H, He mostly; ammoniaand methane clouds• Very cold: 60 K• Axis of rotation tipped98°! (Almost in theorbital plane.) Extremeseasons, in terms of theSun’s visibilitySunUranus: Occultation of a star by theplanet and its unexpected ringsUranusUranustimebStar’s brightness vs time(expected)timeb(observed)Rings around Uranus• 10 very narrow rings discovered in 1977, when Uranuspassed in front of a bright star.• Some of the rings are very thin <10 km wide. “Shepherdmoons”?• Voyager 2 (1986) later found the “shepherd moons”around some rings, as predicted. They keep the ringsnarrow (gravitational interactions).ringNeptune: Last of the gas giants• About the same size asUranus• H, He; methaneatmosphere• Neptune discovered in1846 (J. Galle) as a resultof analysis of Uranus’orbit (Leverriere, Adams).• Neptune perturbsUranus• 1613: Galileo saw it, butdidn’t recognize it as aplanet!Neptune: end of an odyssey• Voyager 2: August 1989, got within 5000 km• Saw complete (but clumpy) rings and anatmosphere much more dynamic than Uranus.Winds up to 1100 km/hr!• “Great Dark Spot” - size of
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