Physics, Light, Radiation, Spectra ---------------------------------- If two objects have the same mass, their center of gravity is located at this position. -> Halfway between them. The force of gravity and the intensity of a light source both fall off according to this mathematical law. -> Inverse square (law) 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. 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 element helium was first discovered using observations of this spectral feature in the light coming from the Sun. -> Absorption lines Solar System ------------ The seventh planet from the Sun. -> Uranus. 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, Eris, 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, but are basically inseparable from the gaseous exterior of those planets. 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 --- Not all ellipticals are necessarily formed this way, but at least some are formed by collisions of spirals. 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 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. AGN and Quasars --------------- Quasars were first discovered by observations in this region of the electromagnetic spectrum. -> Radio The *only* supermassive black whole 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 rotating gas. 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-255 on page 58 of your Course Reader. This is the ultimate energy source for quasars and AGN. -> Gravity --- The gravitational potential energy of objects far away from the object is turned into kinetic energy as it falls in, into thermal energy as it contacts other infalling matter, and finally to radiation due to blackbody emission (and other processes). The efficiency of this process for is about 10%, much better than nuclear fusion (which is only about 1% efficient.) Sun / Stars ----------- The specific reaction that powers the Sun. -> Nuclear fusion of hydrogen into helium. If a star's spectrum peaks in the infrared, it belongs to this spectral class. -> M (K also accepted.) A star with a temperature of 3000 Kelvins and a luminosity 50 times that of the Sun is a member of this evolutionary class. -> Red giant. --- Supergiants (in general) have luminosities of at least 1,000 times that of the Sun. If two equal-mass stars collided early in a star's lifetime to create a new star with twice the amount of hydrogen available for fusion as either possessed individually, how (qualitatively) would the lifetime of the star be affected? -> It would be shortened. --- 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. 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. -> 100 --- 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 (=100) times further away... more than twice the distance of the orbit of Pluto. (Note: No such stars actually exist – even the hottest (non-white-dwarf) 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.) Supernovae / Stellar Remnants / Black Holes ------------------------------------------- A type-II supernova leaves this super-dense object in its wake. -> Neutron star, or black hole. These objects were briefly mistaken for extraterrestrial intelligence shortly after their discovery. -> 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. Cosmology --------- A universe with a matter density precisely equal to the critical density is described by this kind of geometry. -> Flat (Euclidean). This mysterious entity comprises about 70% of the current energy content of the universe. -> Dark energy --- The cosmological constant is one possibility for what might comprise this dark energy. The current rate of change of the expansion rate of the universe. -> Accelerating (or increasing) If the universe was isotropic, static and stars had existed forever, this would be the minimum surface temperature on Earth. -> 3000-10000 Kelvins (about the surface temperature of a star) The inverse of the Hubble constant is a measurement of this property – if the universe contains no matter or antigravity. -> The age of the universe. --- 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.) The Big Bang ------------ The full name of the "afterglow" of the big bang, which was discovered in 1965. -> 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. Essentially no heavier elements were created in the Big Bang; they were all formed in stars and supernovae. The energy for inflation is thought to have been provided by a symmetry breaking between these two things. -> Forces (strong nuclear and electroweak, *or* gravity and grand-unified) --- This is the theory we discussed in lecture, but there are other theories, too. 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". --- This question exists because you will probably run across this fallacy sometime in the future, and should know confidently how to deal with it! Comprehensive: The Origin of the Elements ----------------------------------------- An element, and the event or object responsible for its formation. ->Possible Answers: Hydrogen – Created (early) 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. 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. The process that created most of the helium in the universe. -> Nucleosynthesis after the big bang. All free neutrons that had not decayed combined with protons to form deuterium, and then helium. Most of the helium in the universe came from this process: the amount synthesized in stars is significant but significantly less. (However, as noted above, all elements past #5 come from stars and supernovae. Only the lightest elements are primordial.)