Nebular Hypothesis, Condensation Sequence, Undifferentiated Material, 30 September 2013!About this course: !1.We are now beginning the solar system, which is the main part of this A.course. We started with history, and then went into the means by which astronomers understand things. !This part of the class starts with objects that are nearest to the earth and B.continues on to those farthest away from the earth, an order which has been in use since the 80s (though the information has changed). !The solar system system will be taught based on how all of the objects in it C.are related to each other rather than how they are different as the textbook does. The data is the same, but it points in a different direction.!In astronomy as well as many other sciences, our conception of the natural D.world is akin to a group of people's collective knowledge of an elephant they have each encountered in a dark room. Everyone has figured out snippets of the natural world (in the analogy, some have felt a tail, leg, or ear), but no one understands how the grand design of nature works (no one has seen the whole elephant). !Advances in the way humans understand nature happen when someone a.sees a relationship between two different snippets. !Almost all celestial objects are far from one another, and this distance E.means that astronomy deals in many different scales. !Our world is a small part of the solar system which is a small part of our a.galaxy, which is a tiny part of the universe. !These are all also different in scale in terms of size. !1.Space is so large that an ordinary image cannot show you both the scale b.and the relative size of planets. !If you can see an image of a celestial object, all you know about the c.space between it and the earth is that the space is transparent. Space is not completely a vacuum, but you cannot learn about it by looking this way.!Nebular Hypothesis: Take a Nebula, Condense and Stir!2. We used to think that objects in space (stars, planets, and moons) were A.different kinds of celestial objects, but in actuality they are cousins whose recipe is "Take a Nebula, Condense and Stir." The processes that make each object what it is depend on how much stuff it has, and how hot it got. !These process can be represented by the graph, which will be used many a.times in this unit and the following units. ! (see next page)!1.A nebula (pl. nebulae) is a cloud of gas and dust in space. This cloud is B.stirred somehow, condenses, and then spins in a direction. Collisions between the particles of the rotating nebula cause it to flatten out into a disk shape. Continuing collisions cause the particles to clump into planets. !Contents of a Nebula: !a.Low Melting Point "Volatile"!1.Gases: hydrogen, helium!A.Ices: water, carbon dioxide, methane, ammonia. !B.Tars: hydrocarbons. !C.High Melting Point "Refractory"!2.Dust: silicates, iron, nickel!A.Small exception if a star is near star death, in which case the nebulae 3.The mass in kilograms is in logarithmic form. (Or, how many 0s follow the number) (Found under "Photos" of this website.)Unlike masses, core temperatures are uncertain and difficult to derive. The numbers on this graph may change a little. !The highest temperatures of an object determines the processes the object underwent.is entirely hot and all the cold things are melted. !Astronomers are also now finding more ices like carbon monoxide in 4.smaller quantities.!A Nebula stars out large, and shrinks by condensing (becoming more b.concentrated/dense). !As the cloud shrinks, it usually starts to spin and flatten into a disk. !c.If you start with a roughly spherical nebula, the particles and 1.molecules in the nebula will hit each other at all sorts of odd angles.!Over time, the random motion will damp out. !2.There is almost always some direction which has left over momentum 3.that doesn't damp out, and this becomes the direction of the cloud's rotation. !A particle on the equator of this rotation will continue normally.!4.The particles that are at the ends of the sphere orbit the center of 5.gravity, so they have to cross the particle dense equator. !There are many collisions at the equator when these particles cross, 6.and over time the nebula takes on a disk shape. !Some collisions are gentle and slow. !A.Some collisions are violent, and may splatter particles out into B.space. !These particles usually get mopped up by other clumps of a.particles and planets. !Particles getting mopped up by planets happens today, and on b.earth. (ex. Chelyabinsk meteor in Russia last year.)!All the colliding particles clump together to form molecules, then grains, d.then planetismals (pieces of planets), then protoplanets, and later planets.!There is no official boundary between any of these stages. !1.This process is called accretion. !2.Condensation Sequence: As the planets form, their position in relation to the 3.sun, and therefore their temperature, determines their material composition. Though they start as masses of smashed together ingredients from the same nebula (undifferentiated material), heat affects what materials a planet retains.!The only materials that will condense in extreme heat are metals and rock, A.so planets nearest the sun are small and rocky. (ex. Mercury)!Farther away from the sun, a planet can condense not only rock and metal, B.but also ices. (ex. Mars)!Even farther from the sun, a planet can condense rock, metal, ice, and gases. C.This distance and beyond is where gas giants like Saturn can
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