PREPARATION FOR FINAL EXAM QUIZ ASTB23. SAMPLE QUESTIONS - *SOME* ARE ANSWERED ASTB23H3F for eam time & location see syllabus. 8 pages of hand written (not printed or photocopied) of *own* notes are allowed (i.e., up to 4 sheets of paper). Calculator is required; phones & other electronics are not allowed. Exam booklets and candidate forms will be provided. Please bring an ID. _______________________________________________________________________________ Remember that it's not enough to say a statement is false. You need to circle at least one wrong word in the statement, and of course circle "N" in Y/N column. _______________________________________________________________________________ This set of questions is almost all devoted to the post-midterm part of the course. Material from the first half of the course is in the preparatory questions supplied for the midterm (see the course web page http://planets.utsc.utoronto.ca/~pawel/ASTB23). The symbol ^ is exponentiation, like 2^3 = 8; also 1e13 = 1 * 10^13 etc. Ignore any misspellings or grammar issues in questions. Your job is one of a scientific editor, not a copy editor. ________________________________________________________________________________ [Y] There is currently no specific name to call something larger than the Universe, which is defined as the observable part of that something bigger (observable because of the finite speed of light) [ ] Stars form when the Jeans criterion indicates the gravitational potential is larger than kinetic energy, either thermal energy of molecules, or turbulent streaming motions of gas [Y] At large redshift, z > 1000, the universe was fully ionized; after recombination, radiation (which we now see as the Cosmic Microwave Background Radiation, CMBR for short, but had wavelengths smaller than 1 micrometer at this redshift) decoupled from matter in an expanding, cooling universe, because neutral atoms have little tendency to absorb or emit radiation, compared with ions and free electrons in plasma. [N] Mass to light ratio M/L = 1 (here, M and L are given in solar units) would indicate that the gaalxy is dominated by dark matter. Solar neighborhood's value is close to 5. [Y] Mass of a brown dwarf is in the range 5-80 Jupiter masses, that is 0.005 to 0.08 solar masses [ ] Stars with mass higher than the sun have partly convective interiors, stars less massive have convective envelopes. [ ] Neutron stars can have density ~1e15 g/cm^3 [N] Radius of a star on the main sequence is approximately proportional to its mass. Therefore, a neutron star of mass 1.3 solar masses has somewhat larger radius than the sun [ ] Millisecond pulsars had rapidly rotating progenitors (parent stars) [ ] Superluminal motion means motion faster than the speed of light. It is only an illusion, caused by a small viewing angle of a jest of material moving toward us. [N] Total amount of matter flowing onto a solar-mass protostar through a protostellar disk in 10^6 yr or disk's existence is of order 10^{-1} solar mass. [ ] Supermassive black holes probably exist in the centers of most galaxies. [Y] Quasars are an extreme example of AGNs (Active Galactic Nuclei), fed by a large amount of gas from the merging protogalactic clouds. [Y] Gravitational lenses can produce an odd number of images of a distant quasar, or distort its image into arcs. [Y] AGN unification scheme assumes that all types of AGNs including Seyfert galaxies, blazars and quasars, are really the same type of objects seen from a different viewing angle [N] Galactic year is the time for the sun to circle the center of our Galaxy once. It is longer than 200 Gyr [Y] The sun is near Orion's arm of teh Milky Way [ ] Milky Way and Galaxy mean the same thing, but Galaxy and galaxy not necessarily the same object [N] Gravitational lens can decrease the total brightness of a background object. [Y] Accretional luminosity powers quasars. Their luminosity can approach the luminosity of a whole galaxy. [ ] Superluminal motions of knots in a jet are caused by the fact that the size of the emitting region expressed in units of time (of flight of photons) is smaller than the observed variability time scale. [ ] Mergers of galaxies in a cluster of galaxies are much more frequent than physical collisions of stars in a galaxy: a typical galaxy has a fairly large chance to encounter another galaxy but a typical star has nearly zero chance of striking another star, at least in time interval equal to Hubble time. [N] Velocity curve of our Galaxy shows that the sun's linear speed is in the range 420+-22 km/s [ ] Impulse approximation used to compute the relaxation time in stellar systems is a first order perturbation theory: we evaluate the perturbation on an unperturbed, straight-line orbit [ ] William Herschel built the largest telescope of the 18th century (with 126 cm diameter mirror). He discovered the spiral structure of many nebulae, including galaxy NGC 7331 that is discussed in the textbook. [N] W. Parsons (a.k.a. Lord Rosse) built the largest telescope of the 19th century (183 cm diameter mirror) and discoverd the the first spiral structure of a nebula: galaxy NGC 7331 that is discussed in the textbook. [ ] We can call the stellar disk of a spiral galaxy dynamically cold, because the epicyclic (non-Keplerian) motions are small compared with the mean circular flow. In contrast, a spherical globular cluster, galactic halo or an elliptical galaxy would be an example of kinematically hot system, because the random velocity components of stars at any place in the galaxy would be of order of the circular velocity. [ ] Turnoff point on an H-R diagram of a stellar cluster shows the age of the cluster: it moves up the main sequence (that is, toward larger luminosity) as the Universe evolves [N] The thickness of the so-called thick disk of the Milky Way is of order 130 kpc [Y] Horizonal branch stars are helium-burning stars. They undergo pulsations. [ ] Accelerating expansion of the universe was measured using core-collapse supernovae (or SN II) as standard candles [Y] One of the main reasons why we think supermassive black holes occupy the very centers of many galaxies, is that velocity of objects within a small radius (inside the central parsec) often rise to ~1000 km/s. [ ] Most of the energy is emitted by galaxies in the X-ray and ultraviolet spectral range [Y] The most massive star known, eta Carinae A, is in a 5-yr period binary with another massive star eta Carinae B. They emit about 5 and 1 million solar luminosities, accordingly [N] Most of the energy emitted by galaxies is in the ultraviolet and X-ray parts of spectrum. [ ] Dark matter consists of unknown particles moving at relativistic speeds [N] Most microwave background radiation photons have wavelengths between 0.1 and 0.13 micrometers. [Y] Part of antenna noise in old TV sets is from CMBR (cosmic microwave background radiation) [N] CMBR was discovered in 1999 in a baloon experiment launched from Antarctica [Y] The SLING amplification mechanism of spiral modes in disk galaxies relies on there being no inner Lindblad Resonance in the galaxy. [ ] In a quasi-stationary spiral pattern theory of Lin and Shu, spiral arms are density waves. [Y] Population II objects are older and move on more elliptic orbits than pop I objects [ ] Molecular clouds belong to population II [ ] Young OB-type stars belong to population I [Y] The main spiral pattern resides inside the corotational radius, hence the galactic material overtakes the spiral pattern in its motion around the center of a galaxy [ ] Dark lanes (dusty, molecular interstellar medium) are seen on the concave side of the spiral arm, whereas the OB associations of young massive stars are found on the convex side of the spiral arm. (Lets call the side of a tea cup where there's tee the "concave" side, the other side being "convex") [ ] The theory of spiral pattern says that normally the spiral pattern is leading rather than trailing type, and the number of arms is m=2 [ ] Redshift z is defined as the ratio of observed and emitted radiation's wavelengths [ ] You observe a spiral galaxy almost edge-on. The brighter side of it (the one with a less unobscured view of its star formation sites) is most probably approaching us and thus somewhat redshifted [N] Elements C, N, and O, as well as iron, are mostly produced in the quiescent burning stage on main sequence. [Y] More than 90 percent of energy of a supernova is emitted as neutrinos [ ] More than 90 percent of energy of sun's radiation is emitted as neutrinos [Y] One of the explanations for Champagne Supernova (twice as bright as expected and typical of other type Ia supernovae) is that it resulted from a merger of two similar, massive white dwarfs, each close to having Chandrasekhar mass [Y] Galactic disks are optically thin to the 21 cm line of hydrogen: unlike the visible light, radio waves penetrate the dust clouds easily [ ] The reason radio waves penetrate dust clouds easily is that the electromagn. radiation with wavelength much shorter than the dust grain size has weak interaction with the grain. [ ] Crab nebula contains a pulsar remnant (spinning many times per second) [Y] Stars less massive than 8 solar masses do not leave black holes as remnants of evolution [ ] Lenticular galaxies have so little gas that it is hardly observable as a disk + spiral arms [Y] Pulsars were discovered in 1967 and initially designated LGM-1 (Little Green Men-1), because no known objects were small enough to be able to produce short enough bursts of radiation [ ] Jocelyn Bell was a graduate student when she discovered pulsers. She received Nobel Prize in Physics for the discovery [N] 21 cm emission line is due to recombination of hydrogen [N] Neutronization is a process of converting neutrons and protons into electrons and neutrinos [Y] Starquakes on neutron stars occur because of a decrease in spin rate, as a result of radiation from a pulsar eventually depleting its rotational energy reservoir [N] Classification of elliptic galaxies ends at type E9 [Y] The relationship V^2 = G M(R) /R (where M(R) is the mass inside radius R) for the circular velocity in disk galaxy is only approximate, because they are not spherical, in which case the formula would be exact. In practice, the deviation from a true value of V at any radius is < 20% [ ] Black holes were first put forward as a hypothesis by John Michell and Pierre-Simon Laplace, who noticed that surface gravity and escape speed from stellar surface grow with the siaze na mass of a star, eventually allowing massive enough stars to confine radiation [N] Herschel concluded that the sun is far from the center of the Galaxy [ ] E. Hubble proved observationally that all the nebulae were gas clouds in the Milky Way [ ] The rotation curves of disk galaxies show that the dark halos are much more extended than their visible disks [ ] In the so-called singular isothermal sphere, where density drops with the square of the distance, mass M(r) grows with radius without bound as r --> infinity [Y] Little dark matter is needed to understand the rotation in the inner parts of galaxies. But within the visible/radio radii of galaxies, the fraction of dark to light matter grows to 90% (in case of dwarf galaxies). [ ] About 1/3 of spiral galaxies are grand design spirals with m=2 arms. [N] Both leading and trailing arms are frequently observed in spiral galaxies. [ ] The observed spiral patterns cannot be made of objects fixed inside the arms, as the spirals would quickly wrap up toghtly and finally disappear due to the differential rotation of the disk. [ ] One of the highlights of 19th century astronomy were observations of spiral structure of some nebulae, most famously the precise drawings of M51 and its companion, by Lord Rosse (William Parsons) of England. [N] James Lick was put in charge of using the largest telescope in the world (in his times) on Mt. Hamilton near San Francisco. He discovered that galaxies show redshift. [Y] If, by magic, all the stars were to start living shorter lives (say, by a factor of 2), then the spiral arms we see would become much narrower, as they are outlined by the short-lived massive stars. [Y] Lin-Shu hypothesis of a quasi-stationary spiral structure postulates a single and constant rotation speed of the whole pattern. [Y] Dispersion relation is, in general, an expression binding wave number k and frequency omega of a wave. In galactic dynamics, we often use a WKB version, valid for short radial wavelengths (or large wavenumbers k). [ ] Stability criterion (Safronov-Toomre criterion) can be derived from the WKB dispersion relation for waves in a galactic disk [ ] Self-gravity and the rotation of the disks participate in establishing the spiral wave patterns in disk galaxies. The waves we see (short leading waves) are therefore not exact analogs of sound waves. [Y] Galaxies are observed to be close to the gravitational instability condition, but always a safe factor of 1.5 or 2 away from the instability. [N] Safronov-Toomre number Q is a non-dimensional measure of stability against gravitational breakup of a disk [ ] Toomre number Q is proportional to a product of surface density and soundspeed (or velocity dispersion) in the disk. [Y] Lindblad resonance occurs wherever the frequency with which stars in the disk encounter spiral waves equals the natural frequency of radial epicyclic oscillations (called kappa). [ ] Neutron stars' equation of state has adiabatic index close to 2 [ ] Corotational resonance occurs wherever the spiral pattern rotates at twice the angular speed of the disk material, if the number of arms is 2 [N] Electrons belong to the group of particles called barions [Y] Jupiter's interior is under quantum pressure of degenracy, just like a white dwarf's interior [N] Pauli's principle states that two or more spin-1/2 particles can be in the same quantum state [ ] In our universe, the furthest objects at distances larger than 20 billion light years can be observed with the Hubble Space Telescope [ ] One of the most famous galaxy clusters is in the constellation of Hercules, onother one in the constellation of Vega [Y] Some galaxies, like certain models of our own, have no inner Lindblad resonance. They may produce the closed loop of waves (standing pattern) by reflection of trailing waves off the center of the galaxy, as in the SWING amplification mechanism. [N] Einstein Cross is a collection of four stars resembling a cross [ ] Galaxies which do have inner Lindblad resonances may have an amplification mechanism called WASER, in which waves are only present outside the ILR. [N] A very small fraction of disk galaxies have bars [Y] We can use Virial Theorem to estimate one of the three quantities if two other are measured: mass, mean speed of constituents, and core radius of a stellar system, if in equilibrium [ ] One way to achieve equilibrium is through 2-body relaxation (orbit modification during mutual encounters, not necessarily physical collisions) [N] Dark matter may consist of relativistic elementary particles which have not yet been discovered, and move at close to the speed of light [y] When M31 and the Milky Way will merge, the merger remnant will not be a spiral galaxy, but much more like an elliptical galaxy. [ ] Galaxy mergers may decrease the number of spirals and increase the number of elliptical galaxies [ ] Heber Curis, the participant in the Great Debate in 1920, became convinced based on his observations at Lick observatory that spiral nebulae are distant galaxies, much beyond the limits of our Galaxy. [ ] Harlow Shapley worked at Mt. Wilson observatory in southern California, and found that globular clusters form a system centered not on the sun but a faraway Galaxy Center. [Y] H. Shapley confused two types of variable stars he observed in the nebulae: RR Lyrae and Cepheids. He obtained too large a distance to the nebulae. He also cited incorrect observations of Van Maanen who claimed to have detected the rotation of spiral nebulae. [Y] M87 is the central, giant elliptical of the Virgo cluster of galaxies. [N] M87 has a one-sided jet. It's a mystery why the jet is one-sided [N] There is an empirical correlation between the bulge mass M_b and the supermassive black hole mass M_BH in galaxy centers: the black hole mass decreases as the inverse of M_b. [ ] Hubble law states that recessional speed of a galaxy is proportional to its distance. This established the universal expansion [ ] Hubble law states that recessional speed of a galaxy is proportional to the redshift z [ ] Henrietta Swan-Leavitt first noticed the luminosity-period relationship for variable stars, turning stars like delat Cephei and its kin into 'standard candles'. Hubble found Cepheids in M31 which allowed the first distance measurement of Andromeda galaxy. [N] Tully-Fisher relationship states that the maximum circular velocity in a disk galaxy scales with the fourth power of its luminosity [ ] Faber-Jackson relationship relates the luminosity of an elliptical galaxy to the (approximately) fourth power of its observed velocity dispersion. [Y] If sigma is the velocity dispersion of an isothermal stellar system, and T its temperature, then sigma^2 = kT/m, where k= Boltzmann constant, and m = mass of each constituent object (e.g., star). [ ] Isothermal spherical system has velocity dispersion sigma and the circular speed V independent of radius (we call it flat rotation curve). [ ] Plummer sphere has an asymptotically decreasing V(R) speed [N] Microwave backgroud radiation is slightly anisotropic (like a dipole) because our Galaxy moves with respect to it at a relativistic speed. [ ] Supernovae type Ia can be used as very bright and accurate standard candles for the purpose of distance measurements. This is how the accelerated expansion was discovered. [Y] Up to about a quarter of its rest mass (m c^2) is released before the mass m of matter is swallowed by a black hole. [Y] Scharzschild radius is a radius around a non-rotating black hole, from which nothing (including light) can escape. [N] Hawking radiation from small black holes makes them disappear at a slower rate than their more massive cousins [ ] In Hubble diagram, ellipticals can be of types E0 to E7 [Y] Toomre criterion predicts gravitational instability of disks against collapse and the fragmentation of galactic disks if the dispersion of stellar velocities in the disk is sufficiently small for a fixed surface density of the disk, or conversely if the surface density is large enough for a given dispersion of velocities. [Y] About 1/2 of galaxies have a rotating central bar [N] Andromeda galaxy has 36 elliptic galaxy companions [ ] Relaxation time of a galaxy due to star-star encounters is 1e12 yr, and thus much larger than the Hubble time, which is equal one billion years. [Y] Typical density law of stellar density in stellar clusters and galaxies has a nearly constant central part (known as flat density core). [ ] One of the 4 criteria for spiral galaxy classification into several subtypes is based the pitch angle of a spiral in a disk. Sc type has the lowest values. [N] Sagittarius A* is a member of the Local Group, i.e. a satellite of the Milky Way [ ] Andromeda galaxy (M31) will collide with our Galaxy in about 4-5 Gyr. [ ] The discovery of the expansion of the universe was made in the beginning of the 20th century by the use of Cepheid variable stars [ ] The discovery of the accelerating expansion of the universe was made in the beginning of the 20th century by the use of Cepheid variable stars [ ] All globular clusters are suspected of living longer than 20 relaxation times, thus having undergone the gravitational core collapse and formation of a massive black hole in the center. [Y] Dwarf galaxies tend to contain a larger percentage of dark matter than regular, bright ones. [ ] Spherically symetric formula for the gravity force is not strictly applicable to flat systems like galaxy disks, but provides a reasonable approximation to circular velocity values anyway [ ] Whenever we see only one jet, we may suspect a special viewing geometry of a galactic nucleus, hiding the opposite jet, which would appear heavily redshifted and less luminous to us to the extent that it is actually unobservable. [Y] BOOMERANG first flew above Antarctica for 10 days in 1998, observing the anisotropy of the background microwave radiation. WMAP later extended its results. The results of both agree with the hypothesis that the space-time (vacuum) of the universe on large scales has flat, or Euclidean geometry [N] Photons and all other massless particles do bend space-time around them, therefore general relativity theory of Einstein does not describe well their trajectories. [ ] The dispersion of velocities is the r.m.s. deviation of velocities from their average. [Y] Velocity dispersion of population I stars is smaller than the corresponding quantity among population II stars. [] Galaxies have built their bulges first, and disks (if any) later. [Y] Lyman alpha forest is a spectroscopic signature in the spectrum of a distant quasar, of many almost totally transparent gaseous protogalactic clouds in the early universe. Such object do not exist any longer in the modern (nearby) universe. [N] Hubble's constant is equal about 13.8 Mpc/(km/s). [ ] If a galaxy has an exactly flat rotation curve, then its epicyclic frequency (kappa) is (square root of 2)*Omega (Omega being the angular speed of matter), at each radius [ ] The Galactic year's length (orbital period) does not change with radius, at least in the flat rotation curve galaxies. [Y] Some globular clusters may have undergone the gravitational core collapse of their cores. However, few massive black holes were found in them. This is explained by the input of energy from close stellar binary systems into core energetics. [Y] Constant rotation curve systems obey V^2 = GM(R)/R [ ] Galaxies which do not show constant rotation curves, do not obey V^2 = GM(R)/R [ ] Linearly rising rotation curve V(R) signifies the solid body rotation, i.e. a constant orbital period. [N] One of Newton's theorems states that the gravitational potential of a spherical shell is zero inside the shell, and non-zero outside. [ ] In order to use the formula V(esc)^2 = 2 |Phi|, one needs to assume that the gravitational potential is well-normalized to zero at infinity [Y] The thick disk of our Galaxy was once much thinner. The processes which increased its thickness include the stirring by massive molecular clouds and interactions with spiral arms, as well as mergers with dwarf galaxies. [ ] The first indication that Milky Way's rotation speed does not go to zero as the distance w.r.t. the center established in the constellation of Sagittarius, was obtained by Dutch astronomer Jan Oort in the 1930s. [ ] Two theorists, Peebles and Ostriker, noticed that if dark matter formed halos around each galaxy, then their otherwise strange orbital stability (or normal matter) would be justified. [N] Virgo cluster is a cluster of about 1500 galaxies at a distance of 16 kpc [ ] Arcs seen on the sky around massive objects are a result of them acting as gravitational lenses [ ] The Cosmic Microwave Background Radiation (CMBR) is about 14 billion years old [Y] The Universe is about 14 billion years old [ ] Variations in CMBR from one point on the sky to another, after accounting for the simple effect of sun's motion in space and nonuniform emission from dust clouds in our own galaxy, are very small, only ~1e-5 in units of the full solar flux. [Y] The first observational evidence for flatness of the spacetime was obtained by a baloon experiment Boomerang in 1998-1999 [N] 3/4 of the universe (by energy or mass) is the mysterous dark matter, and most of the remaining 1/4 consists of some kinds of undiscovered, weakly interacting elementary particles. [ ] Relaxation time in stellar systems is the time after 5pm