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The April Brooks Observatory sessions The one remaining session is this Thursday, April 24. You may also attend regular Friday public observing at Brooks, weather permitting, this Friday, April 25. Bring a Or Friday, May 2. In blue ticket. that case, report due 12 noon May 7. Your report will be due at the final exam, Wednesday, April 30. Not required if you have already been to Brooks this semester & written a report. As before, take elevator to 5th floor of this building, walk up to 6th floor. Bring your blue ticket with your name and my name (Nancy Morrison) written on it. Extra blue tickets are available. The last available planetarium shows are: Fridays, April 25 and May 2, 7:30 PM Saturdays, April 26 and May 2, 1:00 PM. Report due as noted above. Homework 6 is graded and available for pickup in back Planetarium and observing reports are also checked and available for pickup. This Wednesday Course evaluation questionnaire, 15 minutes at beginning of class About the final exam Wednesday, 30 April 2008, 7:30–9:30 PM, MH 1005 Comprehensive Between 75 and 100 multiple-choice questions similar to tests Counts 37.5% of grade (75 points out of 200) Study suggestions • Old tests: make sure you understand missed questions • Homework, especially frequently missed questions • Look for connections across segments of course Frequently missed questions, Homework 6 1. Which of these stars has the hottest core? (a) red giant (b) red main-sequence star (c) blue main-sequence star Red giants are on their way to fusing helium into carbon, which is possible because the core has shrunk and heated up. 2. What would you be most likely to find if you returned to the solar system ten billion years from now? (a) white dwarf (b) neutron star (c) black hole The Sun will turn into a white dwarf, not a black hole or a neutron star. 3. What would happen if the Sun suddenly became a black hole without changing its mass? (a) The black hole would quickly suck in the Earth. (b) Earth would gradually spiral into the black hole. (c) Earth’s orbit would not change. The Earth’s orbit depends only on the mass of the object at the center of the solar system. This question does not state that the Sun will become a black hole. It is only asking what would happen if, somehow, a black hole were substituted for the Sun. 5. Where in the Galaxy would you least expect to find an ionization nebula? (a) halo (b) disk (c) spiral arm Ionization nebulae are found near hot, young stars, which are concentrated in spiral arms. The spiral arms are within the disk. So they are least likely to be found in the halo. Importance and implications of Hubble’s Law • The galaxies move away from each other—not just from our galaxy. • Distances between galaxies enlarge; space expands; the universe is expanding. • Actually, clusters of galaxies do not expand and, of course, galaxies themselves do not expand. Therefore, it is more accurate to say that the clusters of galaxies move apart. 427 • Estimate distances to very distant galaxies – Measure the radial velocity of a galaxy from its spectrum, then divide the radial velocity by the Hubble Constant to calculate the galaxy’s distance. – In this way, estimate distance to a galaxy even when you know nothing about it other than its radial velocity. 428 Active galaxies and quasars An active galaxy is a galaxy that has an unusually bright point source of light at its center. Often, a jet of material is being expelled from the center of the galaxy. The bright, point light source cannot be just stars. • It is too bright and too small. • Its spectrum is unlike the spectrum of starlight. 429 The small size of the central light source • The fact that this light source is a point is not the issue; even a quite large object would appear as a point of light at the great distance of a typical galaxy. • The changes that occur in the brightness of the source put a limit on how big the source can be. 430 • The most rapid change possible would be for the whole object to brighten simultaneously and instantaneously. • But an external observer will not see an instantaneous change. • Instead, light from the edge of the source nearest the observer reaches him/her first. 431 • The observer will see a gradual brightening. • The time required for the brightening will be (in our case) equal to the time needed for light to cross half of the source. • Again: this reasoning sets an upper limit on the size of the source. 432 • The brightness of an active galactic nucleus can vary in the course of times as short as 1 week. Conclude: the source of the energy is at most 1 light week across. • But it emits the energy equivalent of millions of stars! • Obviously, a highly efficient process for energy conversion is at work. 433 Currently favored model for the energy source at the center of an active galaxy • Hidden inside is a massive black hole—perhaps 1 million Suns. or much more! • Surrounding it is an accretion disk of material spiralling into the hole. • The accretion disk radiates the energy that we observe. • Stars and interstellar material from the surrounding galaxy supply material to the accretion disk. 434 Quasars “Quasar” stands for “quasi-stellar radio source.” • Discovered as bright, powerful sources of radio waves • In visible-light images, they look like stars (points of light). • But spectra are not starlike: emission-line spectra with very large redshifts. In recent years, many quasars have been discovered from the large redshift alone. In fact, most are not radio sources. Therefore, the more neutral term, “quasi-stellar object (QSO)” is sometimes used instead of “quasar.” 435 Similarities to active galactic nuclei • The spectra are very similar. • The luminosities are similar, but quasars are generally more luminous. • Similar variations in brightness • Many quasars are located in the centers of galaxies. These similarities suggest that quasars are the same type of object as active galactic nuclei, just farther away. So the same model is applied to both. 436 Properties of the universe The universe: everything about which we can get information through light (1) Expansion of space • Hubble Relation: clusters of galaxies move apart with speeds proportional to their mutual separations • We imagine space itself as a stretchable fabric with clusters of galaxies pasted onto it. • Mutual recession of clusters of galaxies is caused by expansion of space itself. Hence, “expansion of the universe” 437 Imagine you are living on the surface of a balloon (Fig. 15.18). Light waves are “stretched” as they travel through the expanding fabric (Fig. 15.19). The farther & longer they travel, the more they are “stretched:” an explanation for Hubble’s Law of redshifts. (2) Large-scale structure Clusters of galaxies are the largest gravitationally bound structures. They are grouped into superclusters. Superclusters line up in filaments, which are separated by empty spaces or voids. 438 (3) Looking back in time • We see galaxies as they were when the light left them. • The farther away a galaxy is, the farther back in time we are looking. • For example: in the Hubble Ultra Deep Field, very faint, distant galaxies are seen as they were fairly soon after they formed. 439 (4) Age of the universe • Current estimates – Based on observed radial velocity of any galaxy and its estimated distance; assumes this rate has been constant over time – About 14 billion years. Uncertainty 2 billion years. • Compare with age of oldest known objects: globular star clusters, about 12 to 15 billion years. But has the expansion rate always been the same as it is now? The mutual gravitational pull of all the galaxies in the universe could be slowing it down. 440 (5) Average density of the universe Estimate the average density in a large volume: reckon up the total mass in the volume, divide by the volume 441 To determine the mass of a galaxy • Study Doppler shifts in outer disk to learn orbital speeds of stars (rotation of galaxies) • Study galaxies in clusters to learn their motion in relation to each other Learn: in most galaxies, this method indicates more mass than the detected stars in the galaxy can account for. This fact argues that much of the matter in galaxies is dark. 442 The critical density • The average density that would cause the cosmic expansion to slow down continually but never quite stop • There are good theoretical reasons for thinking that the average density of the universe is equal to the critical density. 443 (6) Acceleration A new standard candle for great distances: white dwarf supernovae • A different kind of stellar explosion • In a binary system, a white dwarf gathers material from its companion until its mass exceeds the maximum for a white dwarf. • Then it explodes, producing a great burst of light but leaving no collapsed remnant. 444 • Because the mass and composition of the exploding star are always the same, the luminosity of the explosion is uniform. • These explosions have been calibrated in nearby galaxies. • Allow distance estimates to greater distances than ever before. • The farthest ones are closer than their redshifts would indicate. This means that the universe expanded more slowly in the past than it does now: the expansion is accelerating. 445 No one knows for sure what is causing the acceleration, but we give it a name anyway: dark energy. Dark energy is a property of space itself, even in vacuum. If we assume the dark energy has certain properties, then it has the equivalent of enough mass to bring the average density of the universe up to the critical value. (Remember E = mc2!) 446 (7) The cosmic background radiation • Definition & properties – An infrared & microwave “glow” from empty sky – Thermal radiation at about 3o Kelvin – Almost perfectly uniform in brightness • Interpretation – This radiation fills the universe as if we were inside a room with glowing walls. – In the past, the waves were less “stretched.” 447 – Observers then would have measured the temperature of the “walls of the room” to be higher than we do now. – The farther back in time, the hotter and denser the universe – Suggests that the universe originated in an extremely hot, dense state: a primordial cosmic fireball – The brightness of the background is very slightly nonuniform, so the primordial fireball was too. – Denser regions in the fireball eventually gave rise to superclusters in the universe today, through gravitational contraction. 448 – Theory predicts the angular size of those denser, brighter regions today, depending on the geometry of space. Geometry • In flat space (like a table top), the angular size of an object decreases in proportion to its distance. 449 • But in curved space (like the surface of a sphere), the angular size of an object still decreases with increasing distance from an observer, but not as much. • So if the universe were curved, the brighter spots in the cosmic background would be larger (in angular size) than they actually are. • A spacecraft called WMAP has found the angular sizes of the bright spots to match the prediction of the theory of a flat universe. 450 (8) Flatness of space The geometry of empty space is determined by the average density, and vice versa. If space is flat, rather than curved like the surface of a sphere, the density equals the critical value. The consequence is that the universe will never stop expanding, but the expansion will continually slow down. But it’s remarkable that the density is just equal to the critical density, since it could have been much larger or much smaller. This is a very special situation. 451