Final Exam Review: Jeopardy Questions by Dan Perley with (very) slight modifications by Nicholas McConnell

Transcription

1 Final Exam Review: Jeopardy Questions by Dan Perley with (very) slight modifications by Nicholas McConnell General Physics If two objects have the same mass, their center of gravity is located at this position. -> Halfway between them. If a visible light source is receding fast enough from an observer, its energy may be shifted into this part of the spectrum. -> Infrared (or radio) --- Or, just to redder wavelengths in the visible if it's not moving so fast. In general, this is the approximate maximum size of an object which is observed to vary in brightness on a timescale of about one year. -> One light-year --- See CS-266 on page 59 of your Course Reader. This is the ratio of the speed of the following two photons, as measured from Earth, as they travel through space: an ultraviolet photon emitted by the Sun, and an radio photon emitted by a spaceship traveling away from us at half the speed of light. -> One --- All photons move at the same speed in vacuum, period! The force of gravity and the intensity of a light source both fall off according to this mathematical law. -> Inverse square (law)

2 Solar System Uranus and Jupiter are examples of this kind of planet. -> Gas giant. (or Jovian planet) This celestial object is believed to have been created by a collision between Earth and a protoplanet. -> The Moon The presence of immense quantities of this gas causes the surface of Venus to be hotter than even the daytime side of Mercury. -> Carbon dioxide This region of icy, orbiting debris includes among its largest members the bodies Sedna, Quaoar, and Pluto. -> Kuiper Belt This is the largest primarily rocky object known in the Solar System. -> Earth --- An amusing factoid, in my opinion. The cores of the giant planets are probably mostly rocky and substantially larger than Earth, though.

3 Galaxies The Milky Way is this type of galaxy. -> Spiral In at least some cases, these very large galaxies are thought to be formed by mergers of smaller galaxies. -> Elliptical Star formation occurs in irregular and peculiar galaxies, as well as in this portion of a spiral galaxy. -> Spiral arms. This is the best-accepted explanation for why the rotation curve of a galaxy doesn't look at all like the rotation curve for the solar system, even out to very large distances. -> Dark matter Careful observations of stars in the two satellite galaxies of the Milky Way ruled out the possibility that a large fraction of the mass in the Universe is in the form of a class of objects grouped under this acronym... much to Governor Schwarzenneger's disappointment. -> MACHOs. --- MACHOs, MAssive Compact Halo Objects, were once thought to be a good candidate for dark matter. However, studies of gravitational lensing of stars in the Magellenic clouds have largely ruled them out.

4 Sun / Stars The specific nuclear fusion reaction that powers the Sun. -> Hydrogen to helium. How does the lifetime of a 2-solar-mass star compare to the Sun s lifetime (no math)? -> It is shorter. --- The lifetime of a star is inversely proportional to the cube of its mass. A larger star lives for a shorter time than a smaller star. (My apologies for expressing this item in question, rather than answer, format.) A star with a temperature of 0 Kelvins and a luminosity 50 times that of the Sun is a member of this evolutionary class. -> Red giant. What property is summarized in the mnemonic O Be A Fine Guy/Girl, Kiss Me Lovingly? Spectral Type ---Temperature decreases from O down through L. If the Sun were replaced by a star with ten times the Sun's surface temperature but of the same physical size, this is the distance from the Sun in AU that a planet would have to be to receive the same amount of energy as Earth does. -> --- Since the star is the same size as the Sun, the luminosity is simply proportional to the fourth power of the temperature: (L/Lsun) = (T/Tsun)^4 = 10,000. But the energy received by an object of constant area falls of as the inverse square of the distance, so to receive the same amount of energy as Earth, the planet would have to move the square root of 10,000 (=) times further away... more than twice the distance of the orbit of Pluto. (Note: No such stars actually exist - even the hottest (non-whitedwarf) stars are only about 6-7 times the Sun's temperature, and these are all somewhat larger than the Sun. This is just an exercise.)

5 Supernovae / Stellar remnants / black holes A type-ii supernova leaves this super-dense object in its wake. -> Neutron star (or black hole) These cosmic lighthouses have strong magnetic fields and spin rapidly. -> Pulsars Both types of supernovae emit the majority of their energy in this form. -> Neutrinos --- Visible light comprises only a small fraction of the energy emitted by any supernova. These two explosive phenomena are both thought to be triggered by matter from a nearby star accreting onto a white dwarf. -> Novae, and Type I-a supernovae. --- Type II supernovae are caused by core-collapse and explosion of a supergiant star. This is the ultimate fate of a binary system containing two neutron stars, each with a mass of about 3 solar masses. -> Coalescence (due to GR) and formation of a black hole. --- General relativity says that a close binary system will slowly decay, causing the objects to eventually merge. The upper limit of a neutron star is around 3 solar masses, so the resulting object (which would have a mass of about 6 solar masses) would be a black hole and not another neutron star.

6 Black Holes, Quasars and Cosmology This mysterious entity comprises about 70% of the current energy content of the universe. -> Dark energy A universe with a matter density precisely equal to the critical density is described by this kind of geometry. -> Flat (Euclidean). Quasars were first discovered by observations in this region of the electromagnetic spectrum. -> Radio The *only* supermassive black hole whose mass is known from measurements of the positions of stars in orbit around it is located in this galaxy. -> Milky Way --- Supermassive black holes in other galaxies were found by Doppler studies of gas orbiting the center of the galaxy. The age of the universe is determined by the inverse of this parameter - *if* the universe contains no matter or antigravity. -> The Hubble "constant". --- A matter-dominated universe would be younger than this. An antigravity-dominated universe would be older than this. (The real universe is a combination of matter and antigravity, with antigravity the dominant component at the present, although matter was the dominant component in the distant past. Interestingly, the two effects of the two components almost completely cancel each other out - so in fact the inverse of the Hubble constant is actually within 2% of the actual age of 13.8 billion years... even though the universe is definitely NOT empty.)

7 The Big Bang The full name of the "afterglow" of the big bang, which was discovered in > Cosmic (Microwave) Background Radiation. (CBR) This is the heaviest element created by the Big Bang itself. -> Lithium (no penalty for saying helium) --- A large amount of hydrogen and helium were created, along with a trace amount of Lithium-7. NO heavier elements were created in the Big Bang; they were all formed in stars and supernovae. In the theory we primarily discussed in class, the energy for inflation is thought to be related by a symmetry breaking between these two things. -> Forces (strong nuclear and electroweak, *or* gravity and grandunified) --- This is just one hypothesis, but is the one Alex focused on in lecture. The two main problems with the original bang theory were that the universe is very nearly flat, and this. -> The homogeneity problem: universe (the CBR in particular) is too uniform on large scales. "The Big Bang happened at a specific point in the universe currently occupied by this galaxy." -> NO ANSWER. The Big Bang did not happen at a "specific point". Everybody who *doesn't* answer this question (or says "no answer") gets 0 points!!!

8 Final Jeopardy: The Origin of the Elements An element, and the process or object which ultimately created it. Possible Answers: Hydrogen - Created in the Big Bang. Helium - Created in the Big Bang, somewhat later than hydrogen. A relatively small amount was created in normal (and giant) stars, and in supernovae. Lithium - Created in the Big Bang, somewhat later than hydrogen. A small amount was created by cosmic rays (not covered in this class). Beryllium, Boron - Created by cosmic rays interacting with other elements (not covered in this class). Carbon, Nitrogen, Oxygen - Created in red giant stars, and to some extent in supernovae. Fluorine through Iron - Created in supergiant stars and in supernovae (in various proportions depending on the particular element). Elements heavier than Iron - Created ONLY in supernovae.