Astronomy Final Exam Chapters 13-16 13.1 White Dwarfs? What is a white dwarf? A white dwarf is the core left over from a lowmass star, sup ported against the crush of gravity by electron degeneracy pressure. A white dwarf typically has the mass of the Sun compressed into a size no larger than Earth. No white dwarf can have a mass greater than 1.4MSIf you want to learn more check out isabella germinario
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un. ? What can happen to a white dwarf in a close binary system? A white dwarf in a close binary system can acquire hydrogen from its compan ion through an accretion disk that swirls toward the white dwarf’s surface. As hydrogen builds up on the white dwarf’s surface, it may begin nuclear fusion and cause a nova that, for a few weeks, may shine as brightly as 100,000 Suns. In extreme cases, accretion may continue until the white dwarf’s mass exceeds the white dwarf limit of 1.4MSun, at which point it will explode as a white dwarf supernova. 13.2 Neutron Stars? What is a neutron star? A neutron star is the ball of neutrons created by the collapse of the iron core in a massive star supernova. It resembles a giant atomic nucleus 10 kilometers in radius and with more mass than the Sun. ? How were neutron stars discovered? Neutron stars spin rapidly when they are born, and their strong magnetic fields can direct beams of radiation that sweep through space as the neutron star spins. We see such neutron stars as pulsars, and these pulsars provided the first direct evidence for the exis tence of neutron stars. ? What can happen to a neutron star in a close binary system? Neutron stars in close binary systems can accrete hydrogen from their companions, forming dense, hot accretion disks. The hot gas emits strongly in X rays, so we see these systems as Xray binaries. In some of these systems, frequent bursts of helium fusion occur on the neutron star’s surface, causing Xray bursts. 13.3 Black Holes: Gravity’s Ultimate Victory ? What is a black hole? A black hole is a place where gravity has crushed matter into oblivion, creating a hole in the universe from which nothing can ever escape, not even light. The event horizon marks the boundary between our observable universe and the inside of the black hole; the black hole’s Schwarzschild radius is the size of the event horizon. ? What would it be like to visit a black hole?You could orbit a black hole just like you could any other ob ject of the same mass. However, you’d see strange effects for an object falling toward the black hole: Time would seem to run slowly for the object, and its light would be increasingly red shifted as it approached the black hole. The object would never quite reach the event horizon, but it would soon disappear from view as its light became so redshifted that no instrument could detect it. ? Do black holes really exist?No known force can stop the collapse of a stellar corpse with a mass above the neutron star limit of 2 to 3 solar masses, and theoretical studies of supernovae suggest that such objects should sometimes form. Observational evidence supports this idea: Some Xray binaries include compact objects far too massive to be neutron stars, making it likely that they are black holes. 13.4 The Origin of GammaRay Bursts ? What causes gammaray bursts? No one knows exactly how gamma ray bursts are produced, but the energy required to make one suggests that gammaray bursts occur when certain kinds of black holes are formed. At time least some gammaray bursts appear to come from unusually powerful super nova explosions that may create black holes. Others may come from the creation of black holes through mergers of neutron stars in close binary systems. 14.1 The Milky Way Revealed ? What does our galaxy look like?The Milky Way Galaxy consists of a thin disk about 100,000 lightyears in diam eter with a central bulge and a spherical region called the halo that surrounds the entire disk. The disk contains most of the gas and dust of the interstellar medium, while the halo contains only a small amount of hot gas and virtually no cold gas. ? How do stars orbit in our galaxy?Stars in the disk all orbit the galactic center in about the same plane andin the same direction. Halo and bulge stars also orbit the center of the galaxy, but their orbits are randomly inclined to the disk of the galaxy. Orbital motions of stars allow us to determine the distribution of mass in our galaxy. 14.2 How is gas recycled in our galaxy? Stars are born from the gravitational col lapse of gas clumps in molecular clouds. Massive stars explode as supernovae when they die, creating hot bubbles in the inter stellar medium that contain the new ele ments made by these stars. Eventually, this gas cools and mixes with the interstellar medium, turning into atomic hydrogen gas and then cooling further, producing star forming molecular clouds. We call this process the star–gas–star cycle. ? Where do stars tend to form in our galaxy?Active starforming regions, marked by the presence of hot, massive stars and ionization nebulae, are found mostly in spiral arms. In each arm, a spiral density wave causes gas clouds to crash into each other and to form clusters of new stars. 14.3 The History of the Milky Way? What do halo stars tell us about our galaxy’s history? The halo generally contains only old, lowmass stars that havea much smaller proportion of heavy elements than stars in the disk. Halo stars therefore must have formed early in the galaxy’s history, before the gas settled into a disk. visual skills check ? How did our galaxy form?Halo stars probably formed in several different protogalactic clouds of hydrogen and helium gas. Gravity pulled those clouds together to form a single larger one. The collapse of this cloud continued until it formed a spinning disk around the galactic center. Stars have formed continuously in the disk since that time, but stars no longer form in the halo. 14.4 The Mysterious Galactic Center ? What lies in the center of our galaxy? 406 Part V Galaxies and Beyond Use the following questions to check your understanding of some of the many types of visual information used in astronomy. Answers are provided in Appendix J. For additional practice, try the Chapter 14 Visual Quiz at www.masteringastronomy.com. Visible light from stars Infrared light from stars Radio emission from molecules Xray emission from hot gas 1600 AU 0.2" Motions of stars near the center of our galaxy suggest that it contains a black hole about 4 million times as massive as the Sun. The black hole appears to be poweringa bright source of radio emission knownas Sgr A*. 15.1 Islands of Stars? What are the three major types of galaxies? (1) Spiral galaxies have prominent disks and spiral arms. (2) Elliptical galaxies are rounder and redder than spiral galaxies and contain less cool gas and dust. (3) Irregular galaxies are neither disklike nor rounded in appearance. Radar Hyades Pleiades MainSequence Fitting Distant Standards Hubble’s Law v = H0 d Our measurements of galaxy distances depend on a chain of methods. The chain begins with radar ranging in our own solar system and parallax measure ments of distances to nearby 438 Part V Galaxies and Beyond the big picture Putting Chapter 15 into Perspective The universe is filled with galaxies that come in a variety of shapes and sizes. In order to learn the histories of these galaxies, we must also consider how the universe itself has evolved through time. Much of our current understanding of the structure and evolution of the universe is based on measurements of distances to faraway gal axies. These measurements rely on a chain of techniques in which each link in the chain builds upon the links that come before it. It has been less than a century since Hubble first proved that the Milky Way is only one of billions of galaxies in the universe. This discovery, and his subsequent discovery of the universe’s expansion, provided the foundation upon which modern cosmology has been built. Measurements of the rate of expansion tell us that the uni verse began about 14 billion years ago. Although we do not yet know the complete story of galaxy evolution, we are rapidly learning more. Galaxies probably all began as protogalactic clouds, but they do not always evolve peacefully. Some galaxies collide with neighbors, often with dramatic results. The tremendous energy outputs of quasars and other active galactic nuclei, including those of radio galaxies, are probably powered by gas accreting onto supermassive black holes. The centers of many presentday galaxies must still contain the supermassive black holes that once enabled them to shine as quasars. ? How are galaxies grouped together?Spiral galaxies tend to collect in groups that contain up to several dozen galaxies. Elliptical galaxies are more common in clusters of galaxies, which contain hundreds to thousands of galaxies, all bound together by gravity. 15.2 Distances of Galaxies? How do we measure the distances to galaxies Our measurements of galaxy distances depend on a chain of methods. The chain begins with radar ranging in our own solar system and parallax measure ments of distances to nearby stars, then relies on standard candles to measure greater distances. What is Hubble’s law?Hubble’s law tells us that moredistant ? Why do galaxies differ?Differences between presentday galaxies probably arise both from conditions in their protogalactic clouds and from colli sions with other galaxies. Slowly rotating or highdensity protogalactic clouds may form elliptical rather than spiral galaxies. Ellipticals may also form through the merger of spiral galaxies. 15.4 Quasars and Other Active Galactic Nuclei ? What are quasars?Some galaxies have unusually bright centers known as active galactic nuclei. A quasar is a particularly bright active galac tic nucleus. Quasars are generally found at great distances from us, telling us that they were more common early in the history of the universe. ? What is the power source for quasars and other active galactic nuclei? Supermassive black holes are thoughtto be the power sources for active galactic nuclei. As matter falls into a supermassive black hole through an accretion disk, its gravitational potential energy is transformed with enormous efficiency into thermal en ergy and then into light. ? Do supermassive black holes really exist? Observations of orbiting stars and gas clouds in the nuclei of galaxies suggest that all galaxies may harbor supermassive black holes at their centers. The masses we measure for those black holes are closely related to the properties of the galaxy around them, suggesting that the growth of these black holes is closely tied to the process of galaxy evolution. distance galaxies are moving away faster:v ? H0 ? d, where H0 is Hubble’s constant. It allows us to determine a galaxy’s distance from the speed at which it is moving away from us, which we can measure from its Doppler shift. ? How do distance measurements tell us the age of the universe? Combining distance measurements with velocity measure ments tells us Hubble’s constant, and the inverse of Hubble’s constant tells us how long it would have taken the universe to reach its present size if the expansion rate had never changed. Based on Hubble’s constant and estimates of how it has changed with time, we now estimate the age of the universe at about 14 billion years, which restricts our view of the universe to objects with lookback times smaller than that age. 15.3 Galaxy Evolution ? How do we observe the life histories of galaxies? Today’s telescopes enable us to ob serve galaxies of many different ages because they are powerful enough to detect light from objects with lookback times almost as large as the age of the universe. We can therefore assemble “family albums” of galaxies at different distances and lookback times. ? How did galaxies form?The most successful models of galaxy formation assume that galaxies formed as gravity pulled together regions of the universe that were ever so slightly denser than their surroundings. Gas collected in protogalactic clouds, and stars began to form as the gas cooled. 16.1 Unseen Influences in the Cosmos ? What do we mean by dark matter and dark energy? Dark matter and dark energy have never been directly observed, but each has been proposed to exist because it seems the simplest way to explain a set of observed motions in the universe. Dark matter is the name given to the unseen mass whose gravity governs the observed motions of stars and gas clouds. Dark energy is the name given to whatever may be causing the expansion of the universe to accelerate. 16.2 Evidence for Dark Matter ? What is the evidence for dark matter in galaxies? 300 200 100 0 0 10 20 30 40 50 60 70 80 90 100 distance from center (thousands of lightyears) The orbital velocities of stars and gas clouds in galaxies do not change much with distance from the center of the galaxy. Applying Newton’s laws of gravitation and motion to these orbits leads to the conclusion that the total mass of a galaxy is far larger than the mass of its stars. Because no detectable visible light is coming from this matter, we call it dark matter. ? What is the evidence for dark matter in clusters of galaxies? We have three different ways of measuring the amount of dark matter in clusters of gal axies: from galaxy orbits, from the tempera ture of the hot gas in clusters, and from the gravitational lensing predicted by Einstein. All of these methods agree, indicating that the total mass of a cluster is about 50 times the mass of its stars, implying huge amounts of dark matter. ? Does dark matter really exist?We infer that dark matter exists from its gravitational influence on the matter we can see, leaving two possibilities: Either dark matter exists or there is something wrong with our understanding of gravity. We cannot rule out the latter possibility, but we have good reason to be confident that our current understanding of gravity is correct and dark matter is real. ? What might dark matter be made of? There does not appear to be enough ordinary matter in the form of stars, planets, and gas clouds to account for all the dark matter. Therefore, scientists suspect that most of it is made of exotic particles known as WIMPs that have yet to be directly detected. 16.3 Structure Formation? What is the role of dark matter in galaxy formation? Because most of a galaxy’s mass is in the form of dark matter, the gravity of that dark matter is probably what formed proto galactic clouds and galaxies from slight density enhancements in the early universe. ? What are the largest structures in the universe? Galaxies appear to be distributed in gigan tic chains and sheets that surround great voids. These giant structures trace their origin directly back to regions of slightly enhanced density early in time. 16.4 The Universe’s Fate ? Will the universe continue expanding forever? Even before we consider the possibility of dark energy, the evidence points to eternal expan sion. The critical density is the average matter density the universe would need for the strength of gravity to eventually halt the expansion. The overall matter density of the universe appears to be only about 25% of the critical density.