Transcript oct8
Formation of the Solar System Simulation Terrestrial & Jovian planets Discussion Given the composition of the solar nebula, why do you think all the terrestrial planets have smaller masses than the Jovian planets? 98% hydrogen and helium 1.4% hydrogen compounds – CH4, NH3, H2O 0.4% silicate rocks 0.2% metals Discussion Can the Earth hold hydrogen and helium gas in its atmosphere? How do you know? Discussion Do you think any of the other terrestrial planets hold hydrogen and helium gas? Discussion Why do you think the cores of all the Jovian planets have a mass about 10 times the mass of the Earth? Jovian Planets Once at protoplanet reaches a mass of about 10 times that of the Earth, it can capture large amounts of gas directly from the solar nebular, becoming a Jovian planet. Discussion Why do you think Uranus and Neptune didn’t get as big as Jupiter and Saturn? What about Pluto and the other TNO’s Just the proto-cores of would-be Jovian planets that never got massive enough to hold H and He. Doppler method for extra solar planet detection Discussion What planet characteristics (mass and distance from the star) will be easiest to find with the Doppler method? Explain your reasoning. Extra Solar planets Many extra-solar planets are Jupiter-like planets which lie very close to their star. NASA’s Kepler mission indicates that hot Jupiter’s are not very common. Kepler results Planetary Migration Most likely these hot Jupiters formed beyond the frost-line, but due to close encounters with other protoplanets lost orbital speed and spiraled in toward the star. The Sun Discussion Why does the Sun shine? Discussion How do you know the Sun is hot? Discussion How do we know the temperature of the Sun? Discussion Why is there less solar intensity at sea level than there is at the top of Earth’s atmosphere? Discussion Where do you think that energy goes? Discussion Why isn’t the Sun a perfect blackbody? Solar Data Radius: Mass: Composition: Mean density: Luminosity: 109 Earth radii 333,000 Earth masses 74% hydrogen 25% helium 1.41 g/cm3 3.86 1026 Watts Discussion How do we know the mass of the Sun? The Sun as a big cosmic light bulb Suppose every human being on Earth turned on 1000, 100-watt light bulbs. With about 6 billion people this would only be 6 1014 watts. We would need 670 billion more Earth’s doing the same thing to equal the energy output of the Sun. Cooling Ember theory Anaxagoras (500 – 428 B.C.E.) believed the Sun was a very hot, glowing rock about the size of Greece. Discussion If the Sun were cooling down over time, how could we tell? Thermal equilibrium The Sun is not measurably heating up or cooling down. No cooling ember At the rate that the Sun is emitting energy, the Sun must have been much hotter just a few hundred years earlier, making life on Earth impossible. The Sun must have an energy source; a way of generating its own heat. Discussion Given the composition of the Sun, why is it unlikely that it could be heated by the burning of wood or coal? Kelvin-Helmoltz contraction As things contract gravitationally, they become hotter. Discussion Why do you think gravitational contraction leads to a temperature increase? Discussion If the Sun is getting its energy from KelvinHelmoltz contraction, how could you prove this? Do you think this is an easy thing to do? Explain. Hydrostatic Equilibrium The Sun is not measurably expanding or contracting The age of the Sun Sedimentary rocks on Earth which were deposited in liquid water are 3.8 billion years old. Rocks containing fossils are 3.5 billion years old. The Sun must have been shining for at least this long. What energy source can keep the Sun hot for 3.8 billion years? Burning coal: Sun would last 10,000 years Kelvin-Helmholtz contraction: if the Sun’s heat were generated from contraction of the Sun’s mass, it would shine for only 25 million years. E = m c2 Energy equals the mass times the speed of light squared. Matter is a form of frozen energy. The Sun is huge! A little bit of matter can be turned into a large amount of energy. If the Sun’s mass could be converted to energy it could shine for hundreds of billions of years. The Sun needs to convert 4.3 million tons of matter to energy every second. The Sun’s Mass is Converted to Energy 4 hydrogen atoms have a mass of 6.693 10-27 kg (four protons) 1 helium atom has a mass of 6.645 10-27 kg (two protons and two neutrons) Thus, 0.048 10-27 kg are converted to energy. Thermonuclear Fusion The Sun fuses 4 hydrogen atoms together to produce 1 helium atom releasing energy. In the Sun about 600 million tons of hydrogen is converted to helium per second. How does it work? We need a new form of matter called antimatter. Antimatter is made up of antiparticles which have the same mass as ordinary particles but opposite charge. Matter and antimatter will annihilate each other if they come in contact producing energy. Proton-Proton chain Helium nuclei can be built up one proton at a time in what we call the proton-proton chain. Normally, two protons will repel each other with the electrostatic force, but if they are smashed together with enough force they can stay together via the strong nuclear force. Changing protons into neutrons is a very slow process, at the Sun’s temperature, it takes billions of years to convert two protons into a deuterium nucleus. Neutrinos Neutrinos () are particles that only interact with matter via the weak nuclear force (the force responsible for radioactive decay). To stop a typical neutrino emitted from the Sun would require 1 light-year (5 trillion miles) of lead. How do we know thermonuclear fusion is taking place in the Sun? “We do not argue with the critic who urges that stars are not hot enough for this process; we tell him to go and find a hotter place.” Eddington (1926) We can test the theory that the Sun is powered by thermonuclear fusion by: 1. Modeling the solar interior 2. Direct observations of solar neutrinos Discussion Which acrobat would you rather be and why? Discussion What does this mean for the pressure on the gas as you descend into the interior of the Sun? Pressure increases toward the center of the Sun To maintain equilibrium, the pressure below each layer of the Sun must be greater than the pressure above that layer. Discussion What happens if you squash a gas? Density increases toward center of the Sun The Sun is gaseous. If you apply pressure to a gas is compresses, i.e. it’s density goes up. Temperature increases toward the center of the Sun As the pressure goes up toward the center of the Sun, the temperature also increases. Discussion According to the previous graphs, where is fusion taking place in the Sun? Explain. Fusion only takes place in the Sun’s core In the inner 1/4 of the Sun’s radius can fusion take place. Even at 15 million K, it takes on average 14 billion years at a rate of 100 million collisions per second to fuse two protons to produce a deuterium atom. Discussion Fusion keeps the Sun hot, but fusion requires the Sun to be hot. How did the Sun ever get hot enough to start fusion? Discussion What would happen if the Sun started to contract? What happens to the density, temperature, pressure, rate of fusion etc? Discussion What would happen if the Sun started to expand? What happens to the density, temperature, pressure, rate of fusion etc? Negative feedback The Sun is stabilized by this negative feedback. Contraction/higher core temperatures, increased fusion rates, expansion and cooling. Expansion/core cooling, decreased fusion rates, contraction. Discussion What happens if all fusion in the Sun ceases?