Astro 3 final studyguide
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Astro 3 final studyguide

Course: ASTR 3, Winter 2008

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Astro 3 Final Study Guide...Summaries Exam Multiple choice Exam: Thursday, March 20, 2008 3:00 PM - 6:00 PM MS 4000A 3/9/2008 2:51:00 PM E Chapter Review: Introduction Early observers grouped the stars visible to the naked eye into patterns called constellation which they imagined were attached to a vast celestial sphere centered on the Earth. Constellations have no physical significance, but are still used to...

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3 Astro Final Study Guide...Summaries Exam Multiple choice Exam: Thursday, March 20, 2008 3:00 PM - 6:00 PM MS 4000A 3/9/2008 2:51:00 PM E Chapter Review: Introduction Early observers grouped the stars visible to the naked eye into patterns called constellation which they imagined were attached to a vast celestial sphere centered on the Earth. Constellations have no physical significance, but are still used to label regions in the sky. Celestial coordinates are a more precise way of specifying a star's location on the celestial sphere. The nightly motion of the stars across the sky is a result of the Earth's rotation on it's axis. Because of the Earth's revolution on it's axis around the Sun, we see different stars at night at different times of the year. The Sun's apparent yearly path around the celestial sphere (or the plane of Earth's orbit around the Sun) is called the ecliptic. We experience seasons because Earth's rotation axis in inclined to the ecliptic plane. At the summer solstice, the Sun is highest in the sky and the length of the day is greatest. At the winter solstice, the Sun is lowest and the day is shortest. Because of precession, the orientation of the Earth's axis change slowly over the course of thousands of years. As the Moon orbits Earth, it keeps the same face permanently turned towards our planet. We see lunar phases as the fraction of the Moon's sunlit face visible varies to us. A lunar eclipse occurs when the Moon enters Earth's shadow. A solar eclipse occurs when the Moon passes between Earth and the Sun. An eclipse may be total if the body in question is completely obscured, or partial is only a portion of the surface is affected. If the Moon happens to be too far from the Earth for its disk to completely hide the Sun, an annular eclipse occurs. Because the Moon's orbit around Earth is slightly inclined to the ecliptic, solar and lunar eclipses are relatively rare events. Astronomers use triangulation to measure the distance to planets and stars, forming the foundation of the cosmic distance scale, the family of distance-measurement techniques used to chart the universe. Parallax is the apparent motion of a foreground object relative to a distant background as the observer's position changes. The larger the baseline--the distance between the two observation points--the greater the parallax. The scientific method is a methodical approach employed by scientists to explore the universe around us in an objective manner. A theory is a framework of ideas and assumptions used to explain some set of observations and make predictions about the real world. These predictions in turn are amenable to further observational testing. In this way the theory expands, and science advances. Chapter 1 Review: The Copernican Revolution Geocentric models of the universe, such as the Ptolemaic model, have the Sun, the Moon, and all the other planers orbiting Earth. Copernicus's heliocentric view of the solar system holds that the Earth, like all other planets, orbit the Sun. This model naturally explains the retrograde motion and the observed brightness variations of the planets. The widespread realization that the solar system is Sun-centered and not Earth-centered is known as the Copernican Revolution. Galileo Galilei was the first experimental scientist. His telescopic observations of the Sun, the Moon, and the planets provided experimental evidence against the heliocentric theory and supporting Copernicus's heliocentric model. Johannes Kepler constructed a set of three simple laws describing the motions of the planets, explaining the mass of observational data accumulated by Tycho Brahe. 1. 2. 3. Kepler's three laws of planetary motion state that: planetary orbits are ellipses having one focus a planet moves faster as it's orbit takes it closer to the Sun the semi-major axis of the orbit is related in a simple way to the planet's orbit period. The average distance from Earth to the Sun is one astronomical unit, today precisely determined by bouncing radar signals off the planet Venus. To change a body's velocity, a force must be applied. The rate of change of velocity, called the Gravity attracts the planets to acceleration, is equal to the applied force divided by the body's mass. the Sun. Every object having any mass exerts a gravitational force on all other objects, and the strength of this force decreases with distance according to an inverse-square law. Chapter 2-Light and Matter Electromagnetic radiation travels through space in the form of a wave. Any electrically charged object is surrounded by an electric field that determines the force it exerts on other charged objects. When a charged particle moves, information about that motion is transmitted throughout the universe by the particles's changing electric and magnetic fields. information travels at the speed of light in the form of an electromagnetic wave. The electromagnetic spectrum consists of (in order of increasing frequency) radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. The opacity of the Earth's atmosphere--the extent to which it absorbs radiation--varies greatly with the wavelength of the radiation. Only radio waves, some The infrared wavelengths, and visible light can penetrate the atmosphere and reach the ground. The temperature of an object is a measure of the speed with which its constituent particles move. The intensity of radiation emitted by an object has a characteristic distribution called a blackbody curve, that depends only on the temperature of the object. Wien's law tells us that the wavelength at which the object's radiation peaks is inversely proportional to its temperature. power of the temperature. Many hot objects emit a continuous spectrum of radiation, containing light of all wavelengths. A hot gas may instead produce an emission spectrum, consisting only of a few well-defined emission lines of specific frequencies or colors. Passing a continuous beam of radiation through cool gas will produce absorption lines at precisely the same frequencies as would be present in the gas's emission spectrum. Atoms are made up of negatively charged electrons orbiting a positively charged nucleus consisting of positively charged protons and electrically neutral neutrons. The number of protons in the nucleus determines the type of element the atom Stefan's law states that the total amount of energy radiated is proportional to the fourth 2 represents. In the Bohr model of the hydrogen atom, the atom has a minimum-energy ground state, representing it's "normal" condition. When the electron has a higher-than-normal energy, the atom is in an excited state. For any given atom, only certain, well-defined energies are possible. In the modern view, the electron is envisaged as being spread out in a "cloud" around the nucleus but still having a sharply defined energy. When an electron moves from one energy state to another in an atom, the difference in the energy between the states is emitted or absorbed in the form of "packets" of electromagnetic radiation--photons. Because the energy levels have definite energies, the photons also have definite energies that are characteristic of the type of atoms involved. The energy of the photon determines the frequency, and hence the color, of the light emitted or absorbed. Out perception of the wavelength of a beam of light can be altered by source's velocity relative to us. This motion-induced change in the observed frequency of a wave is called the Doppler effect. Any net motion of the source away from the observer causes a redshift--a shift to lower frequencies--in the received beam. Motion toward the observer causes a blueshift. The extent of the shift is directly proportional to the source's radial velocity relative to the observer. Chapter 3: Telescopes A telescope is a device designed to collect as much light as possible from some distance source and deliver it to a detector for detailed study. Reflecting telescopes are preferred by astronomers because large mirros are lighter and much easier to construct than large lenses, and they also suffer from fewer optical defects. Instruments may be placed inside the telescope at the prime focus, or a secondary mirror may be used to reflect light to an external detector. Most modern telescopes use charge-coupled devices to collect and store data in digital form for later analysis. The light gathering power of a telescope depends on its collecting area, which is proportional to the square of the mirror diameter. To study the faintest source of radiation, astronomers must use large telescopes. Large telescopes also suffer least from the effects of diffraction and hence can achieve better angular resolution once the blurring effects of Earth's atmosphere are overcome. The amount of diffraction is proportional to the wavelength of the radiation under study and inversely proportional to the size of the mirror. The resolution of most ground-based optical telescopes is limited by seeing--the blurring effect of Earth's turbulent atmosphere, which smears the pointlike images of a star into a seeing disk a few arc seconds in diameter. Astronomers can improve a telescopes optics, in which a telescope's environment and focus are carefully monitored and controlled, and adaptive optics, in which the blurring effects of atmospheric turbulence are corrected for in real time. Radio telescopes are conceptually similar in construction to optical reflecting telescopes, although they are generally much larger than optical instruments, in part because so little radio energy reaches Earth from space. Their main disadvantage is that diffraction on long-wavelength radio waves limits their resolution. Their principal advantage is that they allow astronomers to explore a new part of the electromagnetic spectrum and of the universe--many astronomical radio emitters are completely undetectable in visible light. In addition, tadio observations are largely unaffected by Earth's atmosphere, weather, and the location of the Sun. 3 To increase the effective area of a telescope, and hence improve its resolution several instruments may be combined into an interferometer, in which the interference pattern of radiation received by two or more detectors is used to reconstruct a high-precision map of the source. Using interferometry, radio telescopes can produce images much sharper than those from the best optical equipment. Infrared and ultraviolet telescopes are similar in basic design to optical systems. telescopes study the X-ray and gamma-ray regions of the electromagnetic spectrum. High-energy X-ray telescopes can form images of their field of view, although the mirror design is more complex than fore lower-energy instruments. Famma-ray telescopes simply point in a certain direction and count photons received. Infrared studies in some parts of the infrared range can be carried out using large ground-based systems. However, because of atmospheric opacity, most infrared, and all ultraviolet, X-ray, and gamma- ray observations, must be made from space. Different physical processes can produce very different kids of electromagnetic radiation, and the image of a given object in long-wavelength, low-energy waves may bear little resemblance to its appearance in high-energy X- or gamma-rays. Observations at wavelengths spanning the electromagnetic spectrum are essential to a complete understanding of astronomical events. Chapter 4: The Solar System The planets that make up our solar system all orbit the Sun counterclockwise, as viewed from above Earth's North Pole, on roughly circular orbits that lie close to the ecliptic plane. The orbit of Mercury is the most eccentric and also has the greatest orbital inclination. The spacing between planetary orbits increases as we move outward from the Sun. The inner terrestrial planets--Mercury, Venus, Earth, and Mars--are all of comparable density and generally rocky, whereas all the outer jovian planets--Jupiter, Saturn, Uranus, and Neptune--have much lower densities and are made up mostly of gaseous or liquid hydrogen and helium. Most asteroids orbit in a broad band called the asteroid belt, lying between the orbits of Mars and Jupiter. The Trojan asteroids share Jupiter's orbit, remaining 60 ahead or behind the planet as it moves around the Sun. A few Earth-crossing asteroids have orbits that intersect Earth's orbit and will probably collide with our planet one day. The largest asteroids are a few hundred kilometers across. Most are much smaller. Asteroids in the inner part of the asteroid belt are predominantly rocky in composition; asteroids orbiting farther out contain fractions of water, ice, and organic material. Comets are fragments of icy and rocky material that normally orbit far from the Sun. We see a comet by the sunlight reflected from the dust and vapor released if its orbit happens to bring it near the Sun. Stretching behind the kilometer-size nucleus f a comet is a long tail, formed by the interaction between the cometary material and the solar wind. Most comets are thought to reside in the Oort cloud, a vast reservoir of cometary material tens of thousands of astronomical units across, completely surrounding 4 the Sun. Short-period comets, having orbital periods of less than about 200 years, originate the in the Kuiper belt, a broad band of material beyond the orbit of Neptune. A meteroid is a piece of rocky interplanetary debris smaller than 100 m across. Any meteroid, asteroid, or comet fragment that enters Earth's atmosphere produces a meteor, a bright streak of light across the sky. meteorite. If part of the body causing the meteor reaches the ground, the remnant is called a Each time a comet rounds the Sun, some cometary material becomes dislodged, forming a Larger meteoroids Most meteorites meteoroid swarm--a group of small micrometeoroids that travel in the comet's orbit. are pieces of material chipped off asteroids following collisions in the asteroid belt. are between 4.4 and 4.6 billion years old. The organization of the solar system, and the properties of planets, asteroids, and comets listed above, point to formation as a one-time event that occurred 4.6 billion years ago. An ideal theory of the solar system must provide strong reasons for the observed characteristics of our planetary system yet at the same time be flexible enough to allow for deviations. As the solar nebula contracted under it's own gravity, it began to spin faster, eventually forming a disk. Protoplanets formed in the disk and became planets, while the central protosun eventually evolved Particles of interstellar dust helped to cool the solar nebula and acted as condensation Small clumos of matter grew by accretion, into the Sun. nuclei, allowing the planet-building process to begin. gradually sticking together and growing into moon-size planetesimals whose gravitational fields were strong enough to accelerate accretion. objects remained. Planets in the outer solar system became so large that they could capture the hydrogen and helium gas in the solar nebula, forming the jovian worlds. remaining gas in the solar nebula. inner regions of the solar nebula. also form. When the Sun became a star, its strong winds blew away any As planetesimals collided and merged, only a few planet-sized The terrestrial planets are rocky because they formed in the hot Farther out, the nebula was cooler, so water and ammonia ice could cloud by the gravitational fields of Many leftover planetesimals were ejected into the Oort the outer planets, leaving the Kuiper belt behind. The asteroid belt is a collection of planetesimals that never formed a planet, probably because of Jupiter's gravitational influence. More than 150 extrasolar planets are now known. All were discovered by observing their parent star wobble back and forth as the planet orbits; one has since been observed passing in front of the srat, reducing the star's brightness slightly. Most systems found so far contain a single, massive planet None look much like our solar comparable to Jupiter, on an orbit taking it close to the central star. system, but this may be because current observational techniques are most sensitive only to the sorts of systems seen so far. planetary systems. It is currently nor known whether our own solar system is unusual or normal among Chapter 5: Earth and its Moon 5 The six main regions of Earth are (from inside to outside) a central metallic core, which is surrounded by a thick rocky mantle and topped with a thin crust. Above the surface is the atmosphere, composed primarily of nitrogen and oxygen. Surface winds and weather in the lower atmosphere or troposphere are cause by convection, whereby heat moves from one place to another by rising or sinking streams of air. Higher still lies the magnetosphere, where charged particles from the Sun are trapped by Earth's magnetic field. The Moon has a partially differentiated interior consisting of a semi-solid core, a mantle, and a thick crust. It has no atmosphere, because it's gravity is too weak to retain one, and no magnetosphere. The daily tides in Earth's oceans are caused by the gravitational effects of the Moon and the Sun, which raise tidal bulges in the oceans. Their size depends on the orientation of the Sun and the Moon relative to Earth. A differential gravitational force is called a tidal force, even when no oceans or even planets are involved. The tidal interaction between Earth and the Moon is causing the Earth's spin to slow and is responsible for the Moon's synchronous orbit, in which the same side of the Moon always faces our planet. At high altitudes, in the ionosphere, the atmosphere is kept ionized by the absorption of high-energy radiation and particles from the Sun. Between the ionosphere and the troposphere lies the ozone layer, where incoming solar ultraviolet radiation is absorbed. Both these layers help protect us from dangerous radiation from space. The greenhouse effect is the absorption and trapping of infrared radiation emitted by Earth's surface by atmospheric gases (primarily carbon dioxide and water vapor). It makes our planet's surface some 40 K warmer than would otherwise be the case. We study Earth's interior by observing how seismic waves produced by earthquakes travel through the mantle. Earth's iron core consists of a solid inner core surrounded by a liquid outer core. The process by which dense material sinks to the center of the planet while lighter material rises to the surface is called differentiation. The differentiation of Earth implies that our planet must have been at least partially molten in the past, because of bombardment by material from interplanetary space and heat released by radioactivity in Earth's interior. Earth's surface is made up of enormous slabs, or plates. The slow movement of these plates across the surface is known as continental drift, or plate tectonics. Earthquakes, volcanism, and mountain building are associated with plate boundaries, where plates may collide, move apart, or rub against one another. The "fit" of the continents to one another and the ages of rocks near oceanic ridges argue in favor of this theory. The motion of the plates is thought to be driven by convection in Earth's mantle. On the Moon, the crust is too thick and the mantle too cool for plate tectonics to occur. The main surface features on the Moon are the dark maria, and the lighter-colored highlands. Craters of all sizes, caused by impacting meteoroids, are found everywhere on the lunar surface. The high-lands are older than the maria and are much more heavily cratered. Meteoric impacts are the main source of erosion on the lunar surface. There is no volcanic activity on the Moon's cooling mantle shortly after extensive lava flows formed more than 3 billion years ago. Charged particles from the solar wind are trapped by Earth's magnetic field lines to form the Van Allen belts. When particles from the Van Allen belts hit the Earth's atmosphere, they heat and ionize the atoms there, causing the atoms to glow in an aurora. Planetary magnetic fields are produced by the motion of rapidly rotating, electrically conducting fluid (such as molten iron) in a planet's core, which accounts for the absence of a lunar magnetic field. 6 The most likely explanation for the formation of the Moon is that the newly formed Earth was struck by a Mars-sized object. The core of the impacting body remained behind as part of the core of our planet, and debris splattered into space formed the Moon. Chapter 9: The Sun The Sun is a star, a glowing ball of gas held together by it's own gravity and powered by nuclear fusion at its center. The photosphere is the region at the Sun's surface from which virtually all the visible light is emitted. The main interior regions of the Sun are the core, where nuclear reactions generate energy; the radiation zone, where the energy travels outward in the form of electromagnetic radiation; and the convection zone, where the Sun's matter is in constant convective motion. The amount of solar energy reaching a one square meter area at the top o Earth's atmosphere each second is a quantity known as the solar constant. The Sun's luminosity is the total amount of energy radiated from the solar surface per second. Above the photosphere lies the chromosphere, the Sun's lower atmosphere. In the transition zone above the chromosphere, the temperature increases from a few thousand to around a million Kelvin. transition zone is the Sun's thin, hot upper atmosphere, the solar corona. solar radii, the gas in the corona begins to flow outward as the solar wind. Much of our knowledge of the solar interior comes from mathematical models. the observed properties of the Sun is the standard solar model. The model that best fits Above the At a distance of about 15 Helioseismology--the study of vibrations of the solar surface caused by pressure waves in the interior--provides further insight into the Sun's structure. The effect of the solar convection zone can be seen on the surface in the form of Lower levels in the convection zone also leave their mark in the granulation of the photosphere. photosphere in the form of larger transient pattern called supergranulation. Sunspots are Earth-sized regions on the solar surface that are a little cooler than the surrounding photosphere. They are regions of intense magnetism. Both the numbers and locations of sunspots vary in The overall direction of a roughly 11-year sunspot cycle as the Sun's magnetic field rises and falls. the field reverses from one sunspot cycle to the next. The 22-year cycle that results when the Solar activity tends to be direction of the field is taken into account is called the solar cycle. concentrated in active regions associated with sunspot groups; this activity plays an important role in heating the corona and driving the solar wind, which flows along open field lines from low-density regions of the corona called coronal holes. The Sun generates energy by converting hydrogen to helium in its core by the process of nuclear fusion. When four protons are converted to a helium nucleus in the proton-proton chain, some mass is lost. The law of conservation of mass and energy requires that this mass appear as energy, eventually resulting in the light we see. 7 Neutrinos are nearly massless particles that are produced in the proton-proton chain and escape from the Sun. Despite their elusiveness, it is possible to detect a small fraction of the neutrinos streaming Observations over several decades led to the solar neutrino problem--substantially few They accepted explanation, supported by recent from the Sun. neutrinos are observed than are predicted by theory. observational evidence, is that neutrino oscillations convert some neutrinos to other (undetected) particles en route from the Sun to Earth. Chapter 10: Measuring the Stars The distances to the nearest stars can be measured by stellar parallax. A star with a parallax of 1 arc second is 1 parsec-- about 3.3 light-years--away from the Sun. A star's proper motion, which is its true motion across the sky, is a measure of the star's velocity perpendicular to our line of sight. The apparent brightness of a star is the rate at which energy from the star reaches a detector. Apparent brightness falls off as the inverse square of the distance. Optical astronomers use the magnitude scale to express and compare stellar brightness. The greater the magnitude, the fainter the star; a difference of five magnitudes corresponds to a facto of 100 in brightness. Apparent magnitude is a measure of apparent brightness. The absolute magnitude of a star is the apparent magnitude it would have if placed at a standard distance of 10 pc from the viewer. It is a measure of the star's luminosity. Astronomers often measure the temperatures of stars by measuring their brightness through two or more optical filters, then fitting a blackbody curve to the results. Spectroscopic observations of stars provide an accurate means of determining both stellar temperatures and stellar composition. Astronomers classify stars according to the absorption lines in their spectra. The standard stellar spectral classes, in order of decreasing temperature are O B A F G K M. Only a few stars are large enough and close enough to Earth that their radii can be measured directly. The sizes of most stars are estimated indirectly through the radius-luminosity-temperature relationship. Stars are categorized as dwarfs comparable in size to, or smaller than, the Sun; giants up to 100 times larger than the Sun, and supergiants more than 100 times larger than the Sun. In addition to "normals" stars such as the Sun, two other important classes of star are red giants, which are cool, and luminous, and white dwarfs, which are small, hot, and faint. A plot of stellar luminosity versus stellar spectral class (or temperature) is called an H-R diagram. About 90 percent of all stars plotted on an H-R diagram lie on the main sequence, which stretches from hot, bright blue supergiants and blue giants, through intermediate stars such as the Sun, to cool, faint red dwarfs. Most main-sequence stars are red-dwarfs; blue giants are quite rare. About 9 percent of stars lie in the white-dwarf region, and the remaining 1 percent lie in the red-giant region. If a star is known to lie on the main sequence, measurement of its spectral type allows its luminosity to be estimated and it's distance to be measured. This method of distance determination, which is valid A star's for stars up to several thousand parsecs from Earth, is called spectroscopic parallax. 8 luminosity class allows astronomers to distinguish main-sequence stars from giants and supergiants of the same spectral type. Most stars are not isolated in space but instead orbit other stars in binary-star systems. In a visual binary, both stars can be seen and their orbits charted. In a spectroscopic binary, the stars cannot be resolved, but their orbital motion can be detected spectroscopically. In an eclipsing binary, the orbits are orientated in such a way that one star periodically passes in front of the other as seen from Earth and dims the light we receive. Studies of binary stars often allow stellar masses to be measured. The mass of a star determines its size, temperature, and brightness. Hot blue giants are much more massive than the Sun; cool red dwarfs are much less massive. High-mass stars burn their fuel rapidly and have much shorter lifetimes than the Sun. Low-mass stars consume their fuel slowly and remain on the main sequence for trillions of years. Chaper 11: The Interstellar Medium The interstellar medium occupies the space between stars. It is made up of cold (less than 100 K) gas, mostly atomic or molecular hydrogen and helium, and dust grains. Interstellar dust is very effective at blocking our view of distant stars, even though the density of our interstellar medium is very low. The spatial distribution of interstellar matter is very patchy. The dust preferentially absorbs short-wavelength radiation, leading to reddening of light passing though interstellar clouds. Emission nebulae are extended clouds of hot, glowing interstellar matter. Those associated with star formation are caused by hot O- and B-type stars heating and ionizing their surroundings. They are often crossed by dark dust lanes-- part of the larger molecular clouds from which they formed. Dark dust clouds are cold, irregularly shaped regions on the interstellar medium that diminish or completely obscure the light from background stars. The interstellar medium also contains many cold, dark, molecular clouds. Dust within these clouds probably both protects the molecules and acts as a catalyst to help them form. Molecular clouds are likely sites of future star formation. Often, several molecular clouds are found close to one another, forming a molecular cloud complex millions of times more massive than the Sun. Astronomers can study dark interstellar clouds by observing their effects on the light from more distant stars. Another way to observe these regions of interstellar space is through spectral analysis of the 21- centimeter line produced whenever the electron in a hydrogen atom reverses its spin, changing its energy very slightly in the process. Astronomers usually study these clouds through observations of other molecules that are less common than hydrogen but much easier to detect. Stars form when an interstellar cloud collapses under its own gravity and breaks up into smaller pieces. The evolution of the contracting cloud can be represented as an evolutionary track on the Hertzsprung-Russell diagram. A cold interstellar cloud (stage 1) containing a few thousand solar masses of gas can fragment into tens or hundreds of smaller clumps of matter, from which stars eventually form. As a collapsing prestellar fragment heats up and becomes denser (stages 2 and 3), it eventually becomes a protostar (stage 4)--a warm very luminous object that emits radiation mainly in the infrared portion of the electromagnetic spectrum. The protostar contracts as it radiates its internal energy into space (stage 5). 9 Eventually, the central temperature becomes high enough for hydrogen fusion to being (stage 6), and the protostar becomes a star (stage 7). Dark dust clouds and globules are examples of stage-1 clouds and stage-2 collapsing fragments. Examples of stage-3 objects have been observed in some star-forming regions, such as Orion. Stars in the T-Tauri phase are examples of stage-4/5 protostars. Low-mass stage-6 stars on their final approach to the main sequence are found in star-forming regions. Stars of different masses go through similar formation stages, but end up at different locations along the main sequence, high-mass stars at the top, low-mass stars at the bottom. The zero-age main sequence is the main-sequence predicted by stellar evolutionary theory. It agrees very well with observed main sequences. The most massive stars have the shortest formation times and the shortest main-sequence lifetimes. At the other extreme, some low-mass fragments never reach the point of nuclear ignition. Objects not massive enough to fuse hydrogen to helium are called brown dwarfs. They may be very common in the universe. A single collapsing and fragmenting cloud can give rise to hundreds or thousands of stars--a star cluster. Open clusters, which are loose, irregular clusters that typically contain from a few tens to a few thousands of stars, are found mostly in the Milky Way plane. They typically contain many bright blue stars, indicating that they formed relatively recently. Globular clusters are roughly spherical and may contain millions of stars. They include no main-sequence stars more massive than the Sun, indicating that they formed long ago. Infrared observations have revealed young star clusters within several emission nebulae. Eventually, star clusters break up into individual stars, although the process may take billions of years to complete. Chapter 12: Stellar Evolution Stars spend most of their lives on the main sequence in the core-hydrogen burning (stage 7) phase of stellar evolution. Stars leave the main sequence when the hydrogen in their cores is exhausted. With no internal energy source, the star's helium core is unable to support itself against its own gravity and begins to shrink. The star at this stage is in the hydrogen shell-burning phase, with nonburning helium at the center surrounded by a layer of burning hydrogen. The energy released by the contracting helium core heats the hydrogen-burning shell, greatly increasing the nuclear reaction rates there. As a result, the star becomes much brighter, while the envelope expands and cools. A solar-mass star moves off the main sequence on the H-R diagram first along the subgiant branch (stage 8), then almost vertically up the red giant branch (stage 9). As the helium core contracts, it heats up. Eventually, helium begins to fuse into carbon. In a star like the Sun, helium burning begins explosively, in the helium flash. The flash expands the core and reduces the star's luminosity, sending it onto the horizontal branch (stage 10) of the H-R diagram. The star now has a core of burning helium surrounded by a shell of burning hydrogen. An inner core of nonburning carbon forms, shrinks and heats the overlying burning layers, and the star once again becomes a red giant, even more luminous than before (stage 11). The core of a solar-mass star never become hot enough to fuse carbon. Such a star continues to brighten and expand until its envelope is ejected into space , 10 forming a planetary nebula (stage 12). At that point the core becomes visible as a hot, faint, and extremely dense white dwarf (stage 13). The white dwarf cools and fades, eventually becoming a cold black dwarf (stage 14). A nova is a star that suddenly increases greatly in brightness, then slowly fades back to its normal appearance over a period of months. It is a result of a white dwarf in a binary system drawing hydrogen-rich material from its companion. The gas spirals inward in an accretion disk and builds up on the white dwarf's surface, eventually becoming hot and dense enough for the hydrogen to burn explosively, temporarily causes a large increase in the dwarf's luminosity. Stars more massive than about 8 solar masses form heavier and heavier elements in their cores, at a more and more rapid pace. As they evolve into red supergiants their cores form a layered structure consisting of burning shells of successively heavier elements. The process stops at iron, whose nuclei can neither be fused together nor split to produce energy. As a star's iron core grows in mass, it eventually becomes unable to support itself against gravity and begins to collapse. At the high temperatures produced during the collapse, iron nuclei are broken down into protons and neutrons. The protons combine with electrons to form more neutrons. Eventually, when the core becomes so dense that the neutrons are effectively brought into physical contact with one another, their resistance to further squeezing stops the collapse and the core rebounds, sending a violent shock wave throughout the rest of the star. The star explodes in a core-collapse supernova. Type I supernovae are hydrogen poor and have a light curve similar in shape to that of a nova. Type II supernovae are hydrogen rich and have a characteristic bump in the light curve a few months after maximum. A Type II supernovae is a core-collapse supernova. A Type I supernovae is a carbon-detonation supernova, which occurs when a white dwarf in a binary system collapse and then explodes as its carbon ignites. We can see evidence for a past supernova in the form of a supernova remnant, a shell of exploded debris surrounding the site of the explosion and expanding into space at a speed of thousands of kilometers per second. All elements heavier than helium formed in evolved stars or in supernova explosions. With each new generation of stars the fraction of heavy elements in the universe increases. Comparisons between theoretical predictions of element production and observations of element ebundance in the Galaxy provide strong support for the theory of stellar evolution. The theory of stellar evolution can be tested by observing star clusters. At any instant, stars with masses above the cluster's main-sequence turnoff have evolved off the main sequence. By comparing a cluster's main-sequence turnoff mass with theoretical predictions, astronomers can determine the cluster's age. Chapter 13: Neutron Stars and Black Holes A core-collapse supernova may leave behind an ultracompressed ball of material called a neutron star. This is the remnant of the inner core that rebounded and blew the rest of the star apart. Neutron stars are extremely dense and, at formation, are predicted to be extremely hot, strongly magnetized, and rapidly rotating. They cool down, lose much of their magnetism, and slow down as they age. According to the lighthouse model, neutron stars, because they are magnetized and rotating, send regular bursts of electromagnetic energy into space. The beams are produced by charged particles confined by the strong magnetic fields. 11 When we can see the beams from Earth, we call the source neutron star a pulsar. The pulse period is the rotation period of the neutron star. A neutron star that is a member of a binary system can draw matter from its companion, forming an accretion disk, which is usually a strong source of X-rays. As gas build up on the star's surface, it eventually become hot enough to fuse hydrogen. When hydrogen burning starts on a neutron star, it does so explosively, and an X-ray burster results. The rapid rotation of the inner part of the accretion disk causes the neutron star to spin faster as new gas arrives on its surface. The eventu al result is a rapidly rotating neutron star--a millisecond pulsar. Many millisecond pulsars are found in the hearts of old globular clusters. They cannot have formed recently, and must have been spun up by interactions with other stars. Careful analysis of the radiation received has shown that some millisecond pulsars are orbited by planet-sized objects. Gamma-ray bursts are very energetic flashes of gamma rays that occur about once a day and are distributed uniformly over the entire sky. In some cases, their distances have been measured, placing them at very large distances and implying that they are extremely luminous. The leading theoretical models for these explosions involve the violent merger or neutron stars in a distant binary system, or the recollapse and subsequent violent explosion following a "failed" supernova in a very massive star. Einstein's special theory of relativity deals with the behavior of particles moving at speeds comparable to the speed of light. It agrees with Newton's theory at low velocities, but it makes many very different predictions for high-speed motion. All of its predictins have been repeatedly verified by experiment. The modern replacement for Newtonian gravity is Einstein's general theory of relativity, which describes gravity in terms of warping, or bending, of space by the presence of mass. The more mass, the greater the warping. All particles--including photons--respond to that warping by moving along curved paths. The upper limit on mass of a neutron star is about three solar masses. Beyond that mass, the star can no longer support itself against its own gravity, and it collapses to form a black hole, a region of space from which nothing can escape. The most massive star, after exploding in a super nova, form black holes rather than neutron stars. Conditions in and near black holes can only be described by general relativity. The radius at which the escape speed from a collapsing star equals the speed of light is called the Scharzchild radius. The surface of an imaginary sphere centered on the collapsing star and having a radius equal to the star's Schwarzchild radius is called the event horizon. To a distant observer, light leaving a spaceship that is falling into a black hole would be subject to gravitational redshift as the light climbed out of the hole's intense gravitational field. At the same time, a clock on the spaceship would show time dilation--the clock would appear to slow down as the ship approached the event horizon. The observer would never see the ship reach the surface of the hole. Once within the event horizon, no known force can prevent a collapsing star from contracting all the way to a point-like singularity, at which point both the density and the gravitational field of the star become infinite. This prediction of relativity theory has yet to be proved. Singularities are places where known laws of physics break down. Once matter falls into a black hole, it can no longer communicate with the outside. However, on its way in, it can form an accretion disk and emit X-rays. The best place to look for a black hole is on a binary system in which one component is a compact X-ray source. Cygnus X-1, a well-studies X-ray source in the constellation Cygnus, is a long-standing black hole 12 candidate. Studies of orbital motions imply that some binaries contain compact objects too massive to be neutron stars, leaving black holes as the only alternative. There is also substantial evidence for more massive black holes residing in or near the centers of many galaxies, including our own. Chapter 14: The Milky Way Galaxy A galaxy is a huge collection of stellar and interstellar matter isolated in space and bound together by its own gravity. Because we live within it, the Galactic disk of our own Milky Way Galaxy appears as a broad band of light across the Milky sky--the Way. Near the center, the Galactic disk thickens into the Galactic bulge. The disk is surrounded by a roughly spherical Galactic halo of old stars and star clusters. Our Galaxy, like many others visible in the sky, is a spiral galaxy. The halo can be studied using variable stars whose luminosity changes with time. Two types of pulsating variable stars of particular importance to astronomers are RR Lyrae variables and Cepheid variables. All RR Lyrae stars have roughly the same luminosity. For Cepheids, the luminosity can be determined using the period-luminosity relationship. Knowing the luminosity, astronomers can apply the inverse-square law to determine the distance. The brightest Cepheids can be seen at distances of millions of parsecs, extending the cosmic distance ladder well beyond our own Galaxy. In the early twentieth century, Harlow Shapley used RR Lyrae stars to determine the distances to many of the Galaxy's globular clusters and found that they have a roughly spherical distribution in space, but the center of the sphere lies far from the Sun. The center of their distribution is close to the Galactic center, about 8 kpc away. Disk and halo stars differ in their spatial distributions, ages, colors, and orbital motion. The luminous portion of our Galaxy has a diameter of about 30 kpc. In the vicinity of the Sun, the Galactic disk is about 300 pc thick. The halo lacks gas and dust, so no stars are forming there. All halo stars are old. The gas-rich disk is the site of current star formation and contains many young stars. Stars and gas within the Galactic disk move on roughly circular orbits around the Galactic center. Stars in the halo and bulge move on largely random three-dimensional orbits that pass repeatedly through the disk plane but have no preferred orientation. Halo stars appeared early on, before the Galactic disk took shape, when there was still no preferred orientation. As the gas and dust formed a rotating disk, stars that formed in the disk inherited its overall spin and so moved on circular orbits in the Galactic plane, as they do today. Radio observations clearly reveal the extent of our Galaxy's spiral arms, regions of the densest interstellar gas where star formation is taking place. The spirals cannot be "tied" to the disk material, as the disk's differential rotation would have wound them up long ago. Instead, they may be spiral density waves that move through the disk, triggering star formation as they pass by. Alternately, the spirals may arise from self-propagating star formation, when shock waves produced by the formation and evolution of one generation of stars trigger the formation of the next. The Galactic rotation curve plots the orbital speed of matter in the disk against distance from the Galactic center. By applying Newton's laws of motion, astronomers can determine the mass of the Galaxy. They find that the Galactic mass continues to increase beyond the radius defined by the globular clusters and the spiral structure than we observe, indicating that our Galaxy has an invisible dark halo. The dark matter making up this dark halo is of unknown composition. Leading candidates include low-mass stars and exotic subatomic particles. Recent attempts to detect stellar dark matter 13 have used the fact that a faint foreground object can occasionally pass in front of a more distant star, deflecting the star' s light and causing its apparent brightness to increase temporarily. This deflection is called gravitational lensing. Astronomers working at infrared and radio wavelengths have uncovered evidence for energetic activity within a few parsecs of the Galactic center. The leading explanation is that a black hole roughly 4 million times more massive than the Sun resides there. The hole lies at the center of a dense star cluster containing millions of stars, which is in turn surrounded by a star-forming disk of molecular gas. The observed activity is thought to be powered by accretion onto the black hole, as well as by supernova explosions in the cluster surrounding it. Chapter 15: Normal and Active Galaxies The Hubble classification scheme divides galaxies into several classes, depending on their appearance. Spiral galaxies have flattened disks, central bulges, and spiral arms. Their halos consist of old stars, whereas the gas-rich disks are the sites of ongoing star formation. Barred-spiral galaxies contain an extended "bar" of material projecting beyond the central bulge. Elliptical galaxies have no disk and contain little or no cool gas or dust, although very hot interstellar dust is observed. In most cases, they consist entirely of old stars. They range in size from dwarf ellipticals, which are much l ess massive than the Milky Way Galaxy, to giant ellipticals, which may contain trillions of stars. S0 and SB0 galaxies are intermediate in their properties between ellipticals and spirals. Irregular galaxies are galaxies that do not fit into either of the other categories. Some may be the result of galaxy collisions or close encounters. Many irregulars are rich in gas and dust and are the sites of vigorous star formation. Astronomers often use standard candles as distance-measuring tools. These are objects that are easily identifiable and whose luminosities lie in some reasonably well-defined range. Comparing luminosity and apparent brightness, astronomers determine the distance using the inverse-square law. An alternative approach is the Tully-Fischer relation, an empirical correlation between rotational velocity and luminosity in spiral galaxies. The Milky Way, Andromeda, and several other smaller galaxies form the Local Group, a small galaxy cluster. Galaxy clusters consist of a collection of galaxies orbiting one another, bound together by their own gravity. The nearest large galaxy cluster to the Local Group of the Virgo Cluster. Distant galaxies are observed to be receding from the Milky Way at speeds proportional to their distances from us. This relationship is called Hubble's law. The constant of proportionality in the law is Hubble's constant. Its value is approximately 70 km/s/Mpc. Astronomers use Hubble's law to determine distances to the most remote objects in the universe simply by measuring their redshifts and converting the corresponding speed directly to a distance. The redshift associated with the Hubble expansion is called the cosmological redshift. Active galaxies are much more luminous than normal galaxies and have nonstellar spectra, emitting most of their energy outside the visible part of the electromagnetic spectrum. Often the nonstellar activity suggests that rapid internal motion and is associated with a bright central active galactic nucleus. Many active galaxies have high-speed, narrow jets of matter shooting out from their central nuclei. Often the jets transport energy from the nucleus, where it is generated, to enormous radio lobes lying far beyond the visible portion of the galaxy the lobes, where it is radiated into space. The jets often appear to be made up of distinct "blobs" of gas, suggesting that the process generating the energy is intermittent. 14 A Seyfert galaxy looks like a normal spiral, except that the Seyfert has an extremely bright central galactic nucleus. Spectral lines from Seyfert nuclei are very broad, indicating rapid internal motion, and rapid variability implies that the source of the radiation is much less than one light year across. Radio galaxies emit large amounts of energy in the radio part of the spectrum. The corresponding visible galaxy is usually elliptical. Quasars, or quasi-stellar objects are the most luminous objects known. In visible light they appear starlike, and their spectra are usually substantially redshifted. All quasars are very distant, indicating that we see them as they were in the distant past. The generally accepted explanation for the observed properties of all active galaxies is that they energy is generated by accretion of galactic gas into a supermassive (million to billion-solar-mass) black hole lying in the galactic center. The small size of the accretion disk explains the compact extent of the emitting region, and the high-speed orbit of gas in the black hole's intense gravity accounts for the rapid motion observed. Much of the high-energy radiation emitted from the disk is absorbed by a far, donut-shaped region outside the disk, and reemitted in the form of infrared radiation. Typical active galaxy luminosities require the consumption of about one solar mass of material every few years. Some of the infalling matter is blasted out into space, producing magnetized jets that create and feed the radio lobes. Charged particles spiraling around the magnetic field lines produce synchronotron radiation whose spectrum is consistent with the nonstellar radiation observed in radio galaxies and jets. Chapter 16: Galaxies and Dark Matter The masses of nearby spiral galaxies can be determined by studying their rotation curves. Astronomers also use studies of binary galaxies and galaxy clusters to obtain statistical mass estimates of the galaxies involved. Measurements of galaxy and cluster masses reveal the presence of large amounts of dark matter. The fraction of dark matter grows as the scale under consideration increases. More than 90 percent of the mass in the universe is dark. Substantial quantities of hot X-ray emitting gas have been detected among the galaxies in many clusters, but not enough to account for the dark matter inferred from dynamical studies. Researchers know of no simple evolutionary sequence that links spiral, elliptical, and irregular galaxies. Most astronomers think that large galaxies formed by the merger of smaller ones, and collisions and mergers of galaxies play very important roles in galactic evolution. A starburst galaxy may result when a galaxy has a close encounter or a collision with a neighbor. The strong tidal distortions caused by the encounter compress galactic gas, resulting in a widespread burst of star formation. Mergers between spirals most likely result in elliptical galaxies. Quasars, active galaxies, and normal galaxies may represent an evolutionary sequence. When galaxies form and merge, conditions may have been suitable for the formation of large black holes at their centers, and a highly luminous quasar could have been a result. The brightest quasars consume so much fuel that their energy-emitting lifetimes must be quite short. As the fuel supply diminished, the quasar dimmed, and the galaxy in which it was embedded became intermittently visible as an active galaxy. At even later times, the nucleus became virtually inactive, and a normal galaxy was all that remained. Many normal galaxies have been found to contain central massive black holes, suggesting that most galaxies in clusters have the capacity for activity if they should interact with a neighbor. 15 Galaxy clusters themselves tend to clump together into superclusters. The Virgo Cluster, the Local Group, and several other nearby clusters form the Local Super cluster. On even larger scales, galaxies and galaxy clusters are arranged on the surfaces of enormous "bubbles" of matter surrounding vast low-density regions called voids. The origin of this structure is thought to be closely related to conditions in the very earliest epochs of the universe. Quasars can be used as probes of the universe along the line of sight. Some quasars have been observed to have double or multiple images. These result from gravitational lensing, in which the gravitational field of a foreground galaxy or galaxy cluster bends and focuses the light from the more distant quasar. Analysis of the images of distant galaxies, distorted by the gravitational effect of a foreground cluster, provides a means of determining the masses of galaxy clusters--including the dark matter--far beyond the information that optical images of the galaxies themselves afford. Chapter 17: Cosmology On scales larger than a few hundred mega-parsecs, the universe appears roughly homogenous and isotropic. In cosmology--the study of the universe as a whole--researchers usually assume that the universe is homogenous and isotropic on large scales. This assumption is known as the cosmological principle. If the universe were homogenous, isotropic, infinite, and unchanging, then the night sky would be bright b ecause any line of sight would eventually intercept a star. The fact that the night sky is instead dark is called Olber's paradox. Its resolution is that we only see a finite part of the universe--the region within which light has had time to reach us since the universe formed. Tracing the observed motion of galaxies back in this time implies that, about 14 billion years ago, the universe consisted of a single point that expanded rapidly in the Big Bang. Space itself was compressed to a point at that instant-- the Big Bang happened everywhere at once. The cosmological redshift occurs as a photon's wavelength is stretched by cosmic expansion. The extent of the observed redshift is a direct measure of the expansion of the universe since the photon was emitted. There are only two possible outcomes to the current expansion: Either the universe will expand forever, or it will recollapse to a point. The critical density is the density of matter needed for gravity to overcome the present expansion and cause the universe to collapse. The total density of the universe (including matter, radiation, and dark energy) determines the geometry of the universe on the largest scales, as described by general relativity. In a high (greater-than-critical) density universe, the curvature of space is sufficiently large that the universe "bends back" on itself and is finite in extent, somewhat like the surface of a sphere. Such a universe is said to be a closed universe. A low-density open universe is infinite in extent and has a "saddle-shaped" geometry. equal to the critical value and is spatially flat. The critical universe has a density precisely Luminous matter by itself contributes only about 1 percent of the critical density. galaxies and clusters is taken into account, the figure rises to 20 or 30 percent. When dark matter in Observations of distant supernovae suggest that the expansion of the universe may be accelerating, possibly driven by the effects of a force commonly called dark energy. One candidate for this dark energy is the 16 cosmological constant, a repulsive force that may exist throughout all space. unknown. Its physical nature is Other, independent observations are consistent with the idea that the universe is flat--that is, of exactly critical density--with (mostly dark) matter making up 27 percent of the total and dark energy making up the rest. Such a universe will expand forever. Its The cosmic microwave background is isotropic blackbody radiation that fills the entire universe. present temperature is about 3K. state. Its existence is evidence that the universe expanded from a hot, dense As the universe has expanded, the initially high-energy radiation has been redshifted to lower At the present time, the density of matter in the universe greatly exceeds the We live in a matter-dominated universe. The matter density was and lower temperatures. equivalent mass density of radiation. much greater in the past, when the universe was smaller. However, because radiation is redshifted as The early universe was radiation the universe expands, the density of radiation was greater still. dominated. All of the hydrogen in the universe is primordial, formed from radiation as the hot early universe expanded ad cooled. Most of the helium observed in the universe today is also primordial, created by This is known as primordial Detailed fusion between protons and neutrons a few minutes after the Big Bang. nucleosynthesis. Other, heavier elements were formed much later, in the cores of stars. studies of this process indicate that "normal" matter can account for at most 3 percent of the critical density. By the time the universe was about 1500 times smaller than it is today, the temperature had At that time, the (then visible) radiation background become low enough for the first atoms to form. decoupled from the matter. The photons that now make up the microwave background have been traveling freely through space ever since. According to modern Grand Unified Theories, the three nongravitational forces of nature first began to display their separate characters about 10-34 seconds after the Big Bang. A brief period of rapid cosmic expansion called the epoch of inflation ensued, during which the size of the universe increased by a factor of about 1050. The horizon problem is the fact that, according to the standard (that is, non-inflationary) Big Bang model, there is no good reason for widely separated parts of the universe to be as similar as they are. Inflation solve s the horizon problem by taking a small homogenous patch of the early universe and expanding it enormously. Inflation also solves the flatness problem, which is the fact that there is no obvious reason why the density of the universe is so close to critical. Inflation implies that the cosmic density is almost exactly critical. The large-scale structure observed in the universe today formed when density inhomogenities in the dark matter clumped and grew to create the "skeleton" of the structure now observed. Normal matter then flowed into the densest regions of space, eventually forming the galaxies we now see. "Ripples" in the microwave background are the imprint of these early inhomogenities on the radiation field and lend strong support to the inflationary prediction that we live in a flat, criticaldensity universe. Chapter 18: Life in the Universe 17 Cosmic evolution is the continuous process that has led to the appearance of galaxies, stars, planets, and life on Earth. Living organisms may be characterized by their ability to react to their environment, to grow by taking in nutrition from their surroundings, to reproduce, passing along some of their own characteristics to their offspring, and to evolve in response to a changing environment. Organisms that can best take advantage of their new surroundings succeed at the expense of those organisms that cannot make the necessary adjustments. selection. Intelligence is strongly favored by natural Powered by natural energy sourcesm reactions between simpe molecules in the oceans of the primitive Earth may have led to the formation of amino acids and nucleotide bases, the basic molecules of life. Alternatively, some complex molecules may have been formed in interstellar space and then The best hope for life beyond Earth in the solar system is the Jupiter's Europa and Saturn's delivered to Earth my meteors of comets. planet Mars, although no evidence for living organism has been found. Titan may also be possibilities, but conditions on both those bodies are harsh by terrestrial standards. The Drake equation provides a means of estimating the probability of other intelligent life in the Galaxy. The astronomical terms in the equation are the Galactic star-formation rate, the likelihood of Chemical and biological terms are the probability of life Culture and political terms planets, and the number of habitable planets. appearing and the probability that it subsequently develops intelligence. are the probability that intelligence leads to technology and the lifetime of a technological civilization. Taking an optimistic view of the development of life and intelligence leads to the conclusion that the total number of technologically competent civilizations in the Galaxy is approximately equal to the lifetime of a typical civilization, expressed in years. A technological civilization would probably "announce" itself to the universe by the radio and television signals it emits into space. with a 24-hour period, as different Observed from afar, our planet would appear as a radio source The water hole is a region in regions of the planet rise and set. the radio range of the electromagnetic spectrum, near the 21-cm line of hydrogen and the 18-cm line of hydroxyl, where natural emissions from the galaxy happen to be minimal. the best part of the spectrum for communications purposes. Many researchers regard this as 18 E-Introduction 1. Compare the size of the Earth with that of the Sun, the Milky Way galaxy, and the entire Universe. 2. What is a constellation? Why are constellations useful for mapping the sky? 3. Why does the Sun rise in the east and set in the west each day? Does the Moon also rise in the east and set in the west? Why? Do stars do the same? Why? 4. How and why does a day measured by the Sun differ from a day measure by the stars? 5. How many times in your life have you orbited the Sun? 6. Why do we see different stars at different times of the year? 7. Why are there seasons on Earth? 8. What is precession, and what is it's cause? 9. If one complete hemisphere of the Moon is always lit by the sun, why do we see different phases of the Moon? 10. What causes a lunar eclipse? A solar eclipse? Why aren't lunar and solar eclipses every month? 11. Do you think an observer on another planet might see eclipses? Why or why not? 12. What is a parallax? Five an everyday example. 19 13. space? Why is it necessary to have a long baseline when using triangulation to measure the distances to objects in 14. What two pieces of information are needed to determine the diameter of a faraway object? 15. What is the scientific method? In what ways does the science differ from religion? Chapter 1- The Copernican Revolution 1. Briefly describe the geocentric model of the universe. 2. basic flaw? The benefit of our current knowledge lets us see flaws in the Ptolemaic model of the universe. What is its 3. What was the great contribution of Copernicus to our knowledge of the solar system? 4. What was the Copernican Revolution? 5. What discoveries of Galileo helped confirm the views of Copernicus? 6. What is Galileo often thought of as the first experimental scientist? 7. State Kepler's three laws of orbital motion. 20 8. How did Tycho Brahe contributed to Kepler's laws? 9. What is meant by the statement that Keptler's laws are empirical in nature? 10. the Sun? If radio waves cannot be reflected from the Sun, how can radar be used to find the distance from Earth to 11. List the two modifications made by Newton to Kepler's laws. 12. Why do we say that a baseball falls towards Earth, and not Earth toward the baseball? 13. Why would a baseball thrown upward from the surface of the Moon go higher than one thrown with the same velocity from the surface of the Earth? 14. According to Newton, why does the Earth orbit the Sun? 15. What would happen to Earth if the Sun's gravity were suddenly "turned off"? Chapter 2: Light and Matter 1. Define the following properties: period, wavelength, amplitude, frequency. 2. Compare the gravitational force with the electric force. 3. Describe the way in which light radiation leaves a star, travels through the vacuum of space, and finally is seen by someone on Earth. 21 4. What do radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays have in common? How do they differ? 5. In what regions of the electromagnetic spectrum is the electromagnetic spectrum is the atmosphere transparent enough to allow observations from the ground? 6. What is a blackbody? What are the characteristics of the radiation it emits? 7. If Earth were completely blanketed with clouds and we couldn't see the sky, could be learn about the realm beyond the clouds? What forms of radiation might penetrate the clouds and reach the ground? 8. Describe how its blackbody curve changes as a red-hot glowing coal cools off. 9. What is spectroscopy? Why is it so important to astronomers 10. What is a continuous spectrum? An absorption spectrum? 11. What is the normal condition for atoms? What is an excited atom? What are orbitals? 12. Why do excited atoms absorb and reemit radiation at characteristic frequencies? 13. Suppose a luminous cloud of gas is discovered emitting an emission spectrum. What can be learned about the cloud from this observation? 14. What is the Doppler effect, and how does it alter the way in which we perceive radiation? 22 15. How do astronomers use the Doppler effect to determine the velocities of astronomical objects? Chapter 3: Telescopes 1. List three advantages of reflecting telescopes over refracting telescopes. 2. What are the largest optical telescopes in use today? 3. How does Earth's atmosphere affect what is seen through an optical telescope? 4. disadvantages. What advantages does the Hubble Space TEelscope have over ground-based telescopes? List some 5. What are the advantages of a CCD over a photographic plate? 6. Is the resolution of a 2-m telescope on Earth's surface limited more by atmospheric turbulence or by the effects of diffraction? 7. Why do radio telescopes have to be very large? 8. What kind of astronomical objects can we best study with radio techniques? 9. What is interferometry, and what problem in radio astronomy does it address? 23 10. Compare the highest resolution attainable with optical telescopes to the highest resolution attainable with radio telescopes (including interferometers). 11. What special conditions are required to conduct observations in the infrared? 12. In what ways do the mirrors in X-ray telescopes differ from those found in optical instruments? 13. Why was CGRO placed in space, rather than on the ground? 14. What are the main advantages of studying objects at many different wavelengths of radiation? 15. Our eyes can see light with an angular resolution of about 1' equivalent to about a third of a millimeter at arm's length. Suppose our eyes detected only infrared radiation, with 1 angular resolution. Would we be able to make our way around on Earth's surface? To read? To sculpt? To create technology? Chapter 4: The Solar System 1. Describe three differences between terrestrial and jovian planets. 2. Why are asteroids, comets, and meteoroids important to planetary scientists? 3. Do all asteroid orbits lie between Mars and Jupiter? 4. What might be the consequences of a 10-km asteroid striking Earth today? 24 5. system? What are comets like when they are far from the Sun? What happens when they enter the inner solar 6. Why do comets approach the Sun from any direction but asteroids generally close to the ecliptic plane? 7. Explain the difference between a meteor, a meteoroid, and a meteorite. 8. What causes a meteor shower? 9. Describe the basic features of the nebular theory of solar system formation, and give three examples of how this theory explains some observed features of the present-day solar system. 10. What key ingredient in the modern condensation theory of solar system formation was missing from the original nebular theory? 11. Why are the jovian planets so much larger than the terrestrial planets? 12. How did the temperature at various locations in the solar nebula determine planetary composition? 13. Where did Earth's water come from? 14. How do astronomers detect extrasolar planets? 15. In what ways do observed extrasolar planetary systems differ from our own solar system? 25 Chapter 5: Earth and its Moon 1. 1.Explain how the Moon produces tides in Earth's oceans. 2. What is a synchronous orbit? How did the Moon's orbit become synchronous? 3. a. What is convection? What effect does it have on Earth's atmosphere? b. Earth's interior? 4. In contrast to Earth, the Moon undergoes extremes in temperature. Why? 5. Use the concept of escape speed to explain why the Moon has no atmosphere. 6. What is the greenhouse effect? Is the greenhouse effect operating in Earth's atmosphere helpful or harmful? What are the consequences of an enhanced greenhouse effect? 7. The density of water in Earth's hydrosphere and the density of rocks in the crust are both lower than the average density of the planet as a whole. What does this fact tell us about Earth's interior? 8. Give two reasons geologists believe that part of Earth's core is liquid. 9. What clue does Earth's differentiation provide to our planet history? 10. What process is responsible for the surface mountains, oceanic trenches, and other large-scale features on Earth's surface? 26 11. In what sense were the lunar maria once "seas"? 12. on Earth? What is the primary source of erosion on the Moon? Why is the average rate of lunar erosion so much less than 13. Name two pieces of evidence indicating that the lunar highlands are older than the maria. 14. Give a brief description of Earth's magnetosphere. Why does the Moon have no magnetosphere? 15. Describe the theory of the Moon's origin currently favored by many astronomers. Chapter 9: The Sun 1. Name and briefly describe the main regions of the Sun. 2. How massive is the Sun, compared with Earth? 3. How hot is the solar surface? The solar core? 4. How do scientists construct models of the Sun? 5. Describe how electromagnetic energy generated at the center of the Sun reaches Earth. 6. Why does the Sun appear to have a sharp edge? 27 7. What evidence do we have for solar convection? 8. What is solar wind? 9. What is the cause of sunspots, flares, and prominences? 10. What is the Maunder minimum? 11. What fuels the Sun's enormous energy output? 12. What is the law of conservation of mass and energy? How is it relevant to nuclear fusion in the Sun? 13. process? What are the ingredients and the end result of the proton-proton chain? Why is energy released in the 14. Why have scientists been trying so hard to detect solar neutrinos? 15. What would be observe on Earth if the Sun's internal energy source suddenly shut off? Would the Sun darken instantaneously? If not, how long do you think it might take--minutes, days, years, millions of years--for the Sun's light to begin to fade? Repeat the question for solar neutrinos. Chapter 10: Measuring the Stars 1. How is stellar parallax used to measure the distances to stars? 28 2. What is a parsec? Compare it to the astronomical unit. 3. Explain how a star's real motion through space translates into motion observable from Earth. 4. Describe some characteristics of red giants and white giants. 5. What is the difference between absolute and apparent magnitude? 6. How do astronomers measure star temperatures? 7. Briefly describe how stars are classified according to their spectral characteristics. 8. What information is needed to plot a star on the Hertzsprung-Russell diagram? 9. What is the main sequence? What basic property of a star determines where it lies on the main sequence? 10. How are distances determined using spectroscopic parallax? 11. Which stars are most common in the Milky Way Galaxy? Why don't we see many of them in H-R diagrams? 12. How can stellar masses be determined by observing binary star systems? 13. longer? If a high-mass star starts off with much more fuel than a low-mass star, why doesn't the high-mass star live 29 14. R diagram? In general, is it possible to determine the life span of an individual star simply by noting its position on an H- 15. Visual binaries and eclipsing binaries are relatively rare compared to spectroscopic binaries. Why is this? Chapter 11: The Interstellar Medium 1. What is the composition of interstellar gas? Of interstellar dust? 2. How is interstellar matter distributed through space? 3. What are some methods that astronomers use to study interstellar dust? 4. What is an emission nebulae? 5. What is 21-centimeter radiation, and why is it useful to astronomers? 6. If our Sun were surrounded by a cloud of gas, would this cloud be an emission nebula? Why or why not? 7. Briefly describe the basic chain of events leading to the formation of a star like our Sun. 8. What is an evolutionary track? 9. Why do stars tend to form in groups? 30 10. What critical event must occur in order for a protostar to become a star? 11. What are brown dwarfs? 12. Because stars live much longer than we do, how do astronomers test the accuracy of theories of star formation? 13. At what evolutionary stages must astronomers use radio and infrared radiation to study prestellar objects? Why can't they use visible light? 14. Explain the usefulness of the Hertzsprung-Russell diagram in studying the evolution of stars. Why can't evolutionary stages 1-3 be plotted on the diagram? 15. Compare and contrast the properties of open and globular star clusters. Chapter 12: Stellar Evolution 1. For how long can a star like the Sun keep burning hydrogen in it's core? 2. Why is the depletion of hydrogen in the core of a star such an important event? 3. What makes an ordinary star become a red giant? 4. How big (in AU) will the Sun become when it enters the red giant phase? 5. How long does it take for a star like the Sun to evolve from the main sequence to the top of the red giant branch? 31 6. What is the helium flash? 7. How do stars of low mass die? How do stars of high mass die? 8. What is a planetary nebula? With what stage of stellar evolution is it associated? 9. What are white dwarfs? What is their ultimate fate? 10. Under what circumstances will a binary star produce a nova? 11. What occurs in a massive star to cause it to explode? 12. What are the observational differences between Type I and Type II supernova? 13. How do the mechanisms that cause Type I and Type II supernovae explain their observed differences? 14. What evidence do we have that many supernova have occurred in our Galaxy? 15. Why do the cores of massive stars evolve into iron and not heavier elements? Chapter 13: Neutron Star and Black Holes 1. How does the way in which a neutron star forms determine some of its most basic properties? 32 2. What would happen to a person standing on the surface of a neutron star? 3. Why aren't all neutron stars seen as pulsars? 4. What are X-ray bursters? 5. Why do astronomers think that gamma-ray bursts are very energetic? 6. What is the favored explanation for the rapid spin rates of millisecond pulsars? 7. Why do you think astronomers were surprised to fins a pulsar with a planetary system? 8. What does it mean to say that the measured speed of a light beam is independent of the motion of the observer? 9. Use your knowledge of escape speed to explain why black holes are said the be "black". 10. What is an event horizon? 11. Why is it so difficult to test the predictions of a theory of general relativity? Describe two test of the theory. 12. What would happen to someone falling into a black hole? 13. What makes Cygnus X-1 a good black hole candidate? 33 14. Imagine that you had the ability to travel at will through the Milky Way Galaxy. Explain why you would discover many more neutron stars than those known to observers on Earth. Where would you most likely find these objects? 15. Do you think that planet-sized objects discovered in orbit around a pulsar should be called planets? Why or why not? Chapter 14: The Milky Way Galaxy 1. What do globular clusters tell us about our Galaxy and our place within it? 2. How are Cepheid variables used in determining distances? 3. Roughly how far out into space can we use Cepheids to measure distance? 4. What important discoveries were made early in this century using RR Lyrae variables? 5. Why can't we study the central regions of the Galaxy using optical telescopes? 6. Of what use is radio astronomy in the study of Galactic structures? Why are radio studies often more useful than observations made in visible light? 7. Contrast the motions of disk and halo stars. 8. Explain why Galactic spiral arms are thought to be regions of recent and ongoing star formation. 9. Describe the behavior of interstellar gas as it passes through a spiral density wave. 34 10. What are self-propagating star formations? 11. What do the red stars in the Galactic halo tell is about its total mass? 12. What does the rotation curve of our Galaxy tell us about its total mass? 13. What evidence is there for dark matter in the Galaxy? 14. What are some possible explanations for dark matter? 15. Why do astronomers think that a supermassive black hole lies at the center of the Milky Way Galaxy? Chapter 15: Normal and Active Galaxies 1. What are the differences between the various types of spiral galaxy? 2. Compare and contrast an elliptical galaxy with the halo of the Milky Way. 3. Describe the four rungs in the distance-measurement ladder involved in determining the distance to a galaxy lying 5 Mpc away. 4. How does the Tully-Fisher relation allow astronomers to measure the distances to galaxies? 5. What is the Virgo Cluster? 35 6. What is Hubble's law, and how is it used by astronomers to measure distance to galaxies? 7. What is the most likely range of values for Hubble's constant? Why is the exact value uncertain? 8. Why do you think astronomers prefer to speak in terms of redshifts rather than distances to faraway objects. 9. Name two basic differences between normal and active galaxies. 10. What is the evidence that the radio lobes of some active galaxies consist of material ejected from the galaxy's nucleus? 11. How do astronomers know that the energy-producing region of an active galaxy must be very small? 12. Briefly describe the leading model for the central engine of an active galaxy. 13. How do astronomers account for the differences in the spectra observed from active galaxies? 14. What is synchronotron radiation, and what does it tell us about energy emission from active galaxies? 15. How do we know that quasars are extremely luminous? Chapter 16: Galaxies and Dark Matter 1. Describe two techniques for measuring the mass of a galaxy. 36 2. Why do astronomers think that galaxy clusters contain more mass than we can see? 3. Why do some clusters of galaxies emit X-rays? 4. What evidence to we have that galaxies collide with one another? 5. Describe the role of collisions in the formation and evolution of galaxies. 6. Do you think that the collisions between galaxies constitute "evolution" in the same sense as the evolution of stars? 7. What are starburst galaxies, and what do they have to do with galaxy evolution? 8. Why do astronomers think that quasars represent an early stage of galactic evolution? 9. Why does the theory of galaxy evolution suggest that there should be supermassive black holes at the centers of many normal galaxies? 10. What evidence do we have for supermassive black holes in galaxies? 11. How might a normal galaxy become active? 12. What is a redshift survey? 13. What are voids? What is the distribution of galactic matter on very large (more than 100 Mpc) scales? 37 14. How can observations of distant quasars be used to probe the space between us and them? 15. How do astronomers "see" dark matter? Chapter 17: Cosmology 1. What evidence do have that there is no structure in the universe on a very large scales? How large is "very large?"? 2. What is the cosmological principle? 3. What is Olber's paradox? How is it resolved? 4. Explain how an accurate measure of Hubble's constant can lead to an estimate of the age of the universe. 5. What isn't it correct to say that the expansion of the universe involves galaxies flying outward into empty space? 6. Why are recent observations of distant supernovae so important to cosmology? 7. Where and when did the Big Bang occur? 8. How does the cosmological redshift relate to the expansion of the universe? 9. What is the cosmic microwave background, and why is it so significant? 38 10. Why do all stars, regardless of their abundance of heavy elements, contain at least 25 percent helium by mass? 11. What is 0? How do measurements of the cosmic deuterium abundance provide a lower limit on 0? 12. When did they universe become transparent to radiation? 13. What is cosmic inflation? How does inflation solve the horizon problem? The flatness problem? 14. What is the connection between dark matter and the formation of large-scale structure in the universe? 15. What prediction of inflation theory has been since verified by observations of the microwave background? Chapter 18: Life in The Universe 1. Why is life difficult to define? 2. What is chemical evolution? 3. What is the Urey-Miller experiment? 4. What are the basic ingredients from which biological molecules formed on Earth. 5. How do we know anything at all about the early episodes of life on Earth 6. What is the role of language in cultural evolution? 39 7. Where else, besides Earth, have organic molecules been found? 8. Where--besides the planet Mars--might we find signs of life in our solar system? 9. Do we know whether Mars ever had life at any time during its past? What argues in favor of the position that it may once have harbored life? 10. What is generally meant by "life as we know it"? What other forms of life might be possible? 11. How many terms in the Drake equation are known with any degree of certainty? Which factor is least well known? 12. What is the relationship between the average lifetime of galactic civilizations and the possibility of our someday communicating with them? 13. How would Earth appear, at radio wavelengths, to extraterrestrial astronomers? 14. What are the advantages in using radio waves for communication over interstellar distances? 15. What is the water hole? What advantage does it have over other parts of the spectrum? 40

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