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Unformatted text preview: August 30, 2010 Reading assignment, Cosmic Catastrophes, Chapter 6 plus Section 5.1, Section 1.2.4 and Section 2.3 for background Last lecture posted as pdf on class web site Astronomy in the News? NASA consulted on the psychology of isolation on behalf of the trapped Chilean miners. Pic of the Day – some early water on Mars might not have been acidic. Note Mars is “bigger” than the Moon when viewed from close up. White Dwarfs (Section 5.1) White Dwarf – dense core left behind by low mass stars (less than 8 solar masses) after red giant and planetary nebular phase. Essentially every white dwarf formed since beginning of the Galaxy is still here 10-100 billion of them (~ 100 billion stars total), but a few white dwarfs have blown up. Most are dim, undiscovered, see only those nearby, none naked eye Sirius, brightest star in the sky, has a white dwarf companion. Can’t see the white dwarf with the naked eye, too small, dim, but Sirius is easy if you look for it at the right time. Find Sirius for the extra credit sky watch project. Discussion Point: White dwarfs have about the same mass as the Sun and about the same radius as the Earth. How does the gravity of a white dwarf compare to the Sun and the Earth, and why? What do we know about white dwarfs? Mass ~ Sun Most are single, 0.6 M (solar masses) Some in binary systems, higher mass Size ~ Earth Density =
mass volume ~1% radius of Sun →
106 grams c. c. ~ tons cubic centimeter OR MORE! HUGE GRAVITY! X X
X Gravity the same here Gravity here much stronger Same mass, smaller size, gravity on surface is larger because you are closer to the center. Gravity on surface acts as if all mass beneath were concentrated at a point in the center -- Newton/Calculus Huge gravity compresses a white dwarf -requires special pressure to support it (Section 1.2.4, Section 2.3) Normal pressure -- thermal pressure Motion of hot particles -- Pressure depends on Temperature Quantum Pressure -- Quantum Theory Uncertainty Principle -- Can’t specify position of any particle exactly. If you squeeze and “locate” a particle more precisely, its energy gets more uncertain, and larger on average. Exclusion Principle -- No two identical particles (electrons, protons, neutrons) can occupy same, place with same energy, but they can if one has more “uncertainty” energy. Pressure depends only on density, not on temperature Figure 1.4 Demonstration thermal pressure, quantum pressure - need volunteers. Discussion point: How does the different form of the pressure, thermal or quantum, affect the behavior of stars? What happens if the star puts in excess nuclear energy? What happens if the star loses excess energy to space? Same C mass in all three cases One Minute Exam: Where is gravity strongest? A. A B B. C. Insufﬁcient information Quantum Pressure -- just depends on squeezing particles, electrons for white dwarf, to very high density -- depends on density only -- does not depend on temperature Important Implication: Normal Radiate energy, pressure tries to drop, star contracts and gets hotter (and higher pressure) White Dwarf Radiate energy, temperature does not matter, pressure, size, remain constant, star gets cooler Opposite behavior Normal Star Regulated White Dwarf Unregulated put in energy, star expands, cools put in energy, hotter, more nuclear burning -- explosion! Figure 1.3 A normal star can and will radiate away thermal energy and hence structural energy. A brick cannot radiate its structural energy, A white dwarf cannot radiate away its quantum energy. Behavior of white dwarf, Quantum Pressure, worked out by S. Chandrasekhar in the 1930’s Limit to mass the Quantum Pressure of electrons can support Chandrasekhar limit ~ 1.4 M density ~ billion grams/cc ~ 1000 tons/cubic centimeter Maximum mass of white dwarf. If more mass is added, the white dwarf must collapse or explode! One Minute Exam If nuclear reactions start burning in an ordinary star like the Sun, what happens to the temperature? The temperature goes up The temperature remains constant The temperature goes down Insufﬁcient information to answer the question One Minute Exam If nuclear reactions start burning in a white dwarf, what happens to the temperature? The temperature goes up The temperature remains constant The temperature goes down Insufﬁcient information to answer the question SUPERNOVAE
Catastrophic explosions that end the lives of stars, Provide the heavy elements on which planets and life as we know it depends, Energize the interstellar gas to form new stars, Produce exotic compact objects, neutron stars and black holes, Provide yardsticks to measure the history and fate of the Universe. Reading: Chapter 6 Supernovae Also § 2.1, 2.2, 2.4 & 2.5 for background
Issues to look for in background: Why is it necessary for a thermonuclear fuel to get hot to burn - charge repulsion § 2.1 & 2.2 Core Collapse § 2.4 & 2.5 One type of supernova is powered by the collapse of the core of a massive star to produce a neutron star, or perhaps a black hole The mechanism of the explosion is still a mystery. The other type of supernovae (Type Ia) is thought to come from a white dwarf that grows to an explosive condition in a binary system. Chandra X-ray Observatory image Of Tycho’s supernova of 1572 These explode completely, like a stick of dynamite, and leave no compact object (neutron star or black hole) behind. Chapter 6 Supernovae
Historical Supernovae - in our Milky Way Galaxy observed with naked eye over 2000 years especially by Chinese (preserved records), but also Japanese, Koreans, Arabs, Native Americans, ﬁnally Europeans. SN 386 SN 1006 SN 1054 SN 1181 SN 1572 SN 1604 ~1680 SN 1987A Vela earliest record brightest Crab Nebula (Radio Source 3C58) Tycho Kepler Cas A nearby galaxy 10,000 years ago NS, jet? No NS NS, jets NS, jets No NS No NS NS? jets NS? jets NS, jets January 27, 2010 Reading assignment, Cosmic Catastrophes, Chapter 6 plus Section 5.1, Section 1.2.4, Section 2.3 for background Also § 2.1, 2.2, 2.4 & 2.5 for background Astronomy in the News? See if President Obama says anything about science, NASA in the State of the Union Address. What is the future of the US human space ﬂight program, and the NASA science program? Pic of the Day - Saturn’s moons Titan and Tethys from the Cassini spacecraft orbiting Saturn. ...
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- Fall '10