Chapter 10 : The Sun

THE SUNS ATMOSPHERE * the sun is so hot that it neither has a liquid or solid matter anywhere inside of it * moving down into the sun there is denser and hotter masses

Photosphere (“sphere of light”) * The photosphere is the innermost of layer of the three layers that comprise the suns atmosphere * A gas layer of the sun that has the most visible light * It is about 400 km thick * Density of the photosphere is low by the earth standards about 0.01% as the air we breathe * Photosphere has a blackbody spectrum that corresponds to an average temp of 5800K * The photosphere appears darkest toward the edge or limb of the solar disk , a phenomenon called limb darkening, * This occurs b/c we see regions of different temp at different depths of the photosphere

Granules * lightly colored convection features about 100 km in diameter seen constantly in the solar photosphere * time lapse photography shows that granules form, disappear then reform in cylces that last several minutes

Chromosphere (“sphere of color”) * is a dim layer of less dense stellar gas that is above the photosphere * It is the layer we normally see * Astronomers can also study the chromosphere through filters that pass light with specific wavelengths strongly emitted by it – but not by the photosphere – or through telescope sensitive to nonvisble wavelengths that the chromosphere emits intensely

Spicules
- Are narrow jets of rising gas in the solar chromosphere
- high resolutions images of the chromosphere reveal numerous spikes which are jets of gas called spicules.
- a typical spicule rises for several minutes at the rate of 72000 km/h to a height nearly 10,000 km.
- spicules are located on the boundaries of enormous regions of rising and falling chromospheric gas called supergranules
- Supergranules – a large convective cell in the Sun’s chromosphere containing many granules

Corona * outermost region of the suns atmosphere * corona extends between several million kilometers from the top of the chromosphere. * b/w the chromosphere and corona is a transition zone in which the temperature skyrockets to about 1 million K * Earths gravity prevents most of our atmosphere from escaping into space , so does the suns gravity keep of it outer layers from leaving. * A portion of gas directly comes from the corona, with the help of the suns magnetic fields , while some is it is funneled out from below the corona, also by magnetic field. * The outflow of particles(mostely e- and p+) from the Sun is called solar wind. * Hellosphere – is a bubble in space created by the solar wind that contains the suns and planets

THE ACTIVE SUN * granules , supergranules spicules and the solar wind occur continuously * the suns atmosphere is constantly disturbed by magnetic fields creating the phenomena known as the active Sun.

Sunspots * A temporary cool region in the solar photosphere created by protruding magnetic fields * Sunspots are regions of the photosphere that appear dark b/c they are cooler than the rest of the Sun’s lower atmosphere * Sunspots can occur in isolation but often they arise in clusters call sunpot groups * Sunspot cycle was discovered in 1843 by astronomer Samuel Schwabe * The average sunspot cycle last 11 years. Within an 11 yr cycle, most sunspots appear at a sunspot maximum * Sunspot maximum – a time during the solar cycle where the number of sunspot is the greatest. * Sunspot minimum – the suns is often devoid of sunspots * Each sunspot has 2 parts: * Dark, central region, called the umbra * Brighter ring that surrounds the umbra , called the penumbra * Sun spot activity also reveals that like the giant , planets , different latitudes of the Sun rotate at different rates , a phenomenon called differential rotation * The equatorial regions rotate more rapidly than the polar regions * Ex. A sunspot near the solar takes 25 days to go once around the sun whereas a sunspot at 30 o north or south takes about 27 days * George Ellery Hale * - discovered in 1908 that sunpots are directly linked to intense magnetic fields * when Hale focused a spectroscope on sunlight, he found that each spectral line in the normal solar spectrum is flanked by additional, closed spaced spectral lines not usually observed * The splitting of a single spectral line into 2 or more lines is called the Zeeman effect * Peter Zeeman who first observed it in the lab , showed that intense magnetic field splits the spectral lines of a light source inside the field. The more intense the magnetic field, the more the split lines are separated
How do these fields create the spots? * The answer lies in the interactions b/w the magnetic fields and the photosphere gases. Because of the photospheres high temp, many atoms in it are ionized: One or more of their electrons have beens tripped off by high-energy photons there. As a result the photosphere is a mixture of electrically charged ions and electrons called Plasma * Plasma * Are good conductors of electricity * They repel from regions of high magnetic field * The magnetic field protruding through the photosphere prevents hot, ionized gases inside the sun from convecting to the surface as they normally do. Thus such regions are left relatively devoid of hot gas and therefore cooler and darker than the surrounding solar surface leaving what is called sunspots

Helioseismology * the study of vibrations of the solar surface * solar cycle – is the time is takes solar magnetic fields to return to their original orientation

Explaination of the suns magnetic field * the field is created as a result of the suns rotation and the resulting motion of the ionized particles found throughout it * This theory was proposed by Horace Babcock as the magnetic dynamo * magnetic dynamo: A theory that explains phenomena of the solar cycle as a result of periodic winding and unwinding of the Sun’s magnetic field in the solar atmosphere.
Plages
* Plages-a bright spots on the sun believed to be associated with an emerging magnetic field * By studying the light emitted by calcium or hydrogen atoms in the plages, we know that they are hotter and therefore brighter than the surrounding chromosphere * Plages which often appear to be just before nearby sunspots form, are thought to be created by the magnetic field under the photosphere crowding upward just before they emerge through the photosphere * The fields compress the gases of the upper Sun, causing this gas to become hotter and therefore to glow more brightly

Filaments * Are dark streaks features in the corona * These features are huge volumes of gas lofted upward from the photosphere by the Sun’s magnetic field * Filaments form loop or arches called prominences * The temp of the prominences can reach 50,000 K. the gases in the most energetic prominences escape the magnetic fields that confine them and surge out into space

Coronal holes - a dark region of the Suns inner corona as seen at x-ray wavelengths

Solar flares * Are violent eruptive events on the sun * they release vast quantities of high energy particles as well as X-ray and ultraviolet radiation from the sun * solar flares sometimes occur when sun spots collide * typical flares emit as much energy as is contained in all the fossil fuel ever stored inside the earth * flares are very powerful that they leave the region of the surface of the sun in their vicinity quaking for an hour or more * at maximum of the sunspot cycles , about 1100 flares occur per year

Coronal mass ejections
- are large volumes of high-energy gas being ejected from the Suns corona
- expel 2 trillion tons of matter as 1.4x10 ^6km/h and each one last for up t a few hours
- coronal mass ejections have enough to break through the suns magnetic field that
- flares have been observed to create shock waves that start some of the coronal mass ejections

THE SUNS INTERIOR * Einstein provided an important clue to the source of the sun energy with his special theory of relativity * E=mc2 * M-mass ( can be converted to the amount of energy ) * C- speed of light

Core
- is the center of the Sun
- in 1920 eddington proposed that the temp at the center of the Suns core is much greater then ever
- the suns core temp is about 15.5x 10^6 K

Thermonuclear Fusion * is the process of fusing nuclei at extreme temps

Hydrogen fusion aka hydrogen burning * is the conversion of hydrogen into helium * hydrogen fusion is also called hydrogen burning even though nothing is burned in a sense

Solar model * explains how the energy from nuclear fusion in the Suns core gets to it photosphere * the model begins with the inward force due to the suns gravity , the force increases the pressure and temp in the suns core , causing Hydrogen fusion to occur there

Hydrostatic equilibrium * A balance between the weight of a layer in a star and the pressure that supports it.

* The outward movement of energy by photons hitting particles, which then bounce off other particles and thereby reemit photons, is called radiative transport, because individual photons are responsible for carrying energy from collision to collision Radiative zone * A region inside a star where energy is transported outward by the movement of photons through a gas from a hot location to a cooler one.

Convective Zone * A layer in a star where energy is transported outward by means of convection; also known as the convective envelope or convection zone. * Hot gas travels up to the top of this zone by convection and from there there radiate photons into space. These photons are what we see as sunlight and you can see that the photosphere

Cerenkov Radiation * Radiation produced by particles traveling through a substance faster than light can.

SUMMARY
The Sun’s Atmosphere * The thin shell of the Sun’s gases we see are from its photosphere, the lowest level of its atmosphere. The gases in this layer shine nearly as a blackbody. The photosphere’s base is at the top of the convective zone. * Convection of gas from below the photosphere produces features called granules. * Above the photosphere is a layer of hotter, but less dense, gas called the chromosphere. Jets of gas, called spicules, rise up into the chromosphere along the boundaries of supergranules. * The outermost layer of gases in the solar atmosphere, called the corona, extends outward to become the solar wind at great distances from the Sun. The gases of the corona are very hot, but they have extremely low densities.
The Active Sun * Some surface features on the Sun vary periodically in an 11-year cycle. The magnetic fields that cause these changes actually vary over a 22-year cycle. * Sunspots are relatively cool regions produced by local concentrations of the Sun’s magnetic field protruding through the photosphere. The average number of sunspots and their average latitude vary in an 11-year cycle. * A prominence is gas lifted into the Sun’s corona by magnetic fields. A solar flare is a brief, but violent, eruption of hot, ionized gases from a sunspot group. Coronal mass ejections send out large quantities of gas from the Sun. Coronal mass ejections and flares that head in Earth’s direction affect satellites, communication, and electric power, and cause aurorae. * The magnetic dynamo model suggests that many transient features of the solar cycle are caused by the effects of differential rotation and convection on the Sun’s magnetic field.
Why did the earlier neutrino detectors not detect the predicted number of neutrinos from the Sun?

The Sun’s Interior * The Sun’s energy is produced by the thermonuclear process called hydrogen fusion, in which four hydrogen nuclei release energy when they fuse to produce a single helium nucleus. * The energy released in a thermonuclear reaction comes from the conversion of matter into energy, according to Einstein’s equation E = mc2. * The solar model is a theoretical description of the Sun’s interior derived from calculations based on the laws of physics. The solar model reveals that hydrogen fusion occurs in a core that extends from the center to about a quarter of the Sun’s visible radius. * Throughout most of the Sun’s interior, energy moves outward from the core by radiative diffusion. In the Sun’s outer layers, energy is transported to the Sun’s surface by convection. * Neutrinos were originally believed to be massless. The electron neutrinos generated and emitted by the Sun were originally detected at a lower rate than is predicted by our model of thermonuclear fusion. The discrepancy occurred because electron neutrinos have mass, which causes many of them to change into other forms of neutrinos before they reach Earth. These alternative forms are now being detected.

CHAPTER 11 SUMMARY | |
Stars differ in size, luminosity, temperature, color, mass, and chemical composition—facts that help astronomers understand stellar structure and evolution.
Absolute Magnitude * M, is the brightness each star would have at a distance of 10pc
Inverse square law * The inverse-square law provides the rule for just how quickly the brightness of an object changes with distance.
The Distance-Magnitude Relationship * The closer a star, the brighter it appears. The inverse-square law leads to a simple equation for absolute magnitude, M. Suppose a star’s apparent magnitude is m and its distance from Earth is d (measured in parsecs). Then * M = m − 5 log (d/10) * where log stands for the base-10 logarithm. This distance-magnitude relation can be rewritten as * m − M = 5 log d − 5 * Example: Consider Proxima Centauri, the nearest star to Earth (other than the Sun). By measuring its parallax angle, we know this star is at a distance from Earth of d = 1.3 pc. Its apparent magnitude is m = +11.1. Therefore, its absolute magnitude is * M = 11.1 − 5 log (1.3/10) = 11.1 − (−4.4) = +15.5 *
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Magnitude Scales * Determining stellar distances from Earth is the first step to understanding the nature of the stars. Distances to the nearer stars can be determined by stellar parallax, which is the apparent shift of a star’s location against the background stars while Earth moves along its orbit around the Sun. The distances to more remote stars are determined using spectroscopic parallax. * The apparent magnitude of a star, denoted m, is a measure of how bright the star appears to Earth-based observers. The absolute magnitude of a star, denoted M, is a measure of the star’s true brightness and is directly related to the star’s energy output, or luminosity. * The luminosity of a star is the amount of energy emitted by it each second. * The absolute magnitude of a star is the apparent magnitude it would have if viewed from a distance of 10 pc. Absolute magnitudes can be calculated from the star’s apparent magnitude and distance from Earth.
The Temperatures of Stars * Stellar temperatures can be determined from stars’ colors or stellar spectra. * Stars are classified into spectral types (O, B, A, F, G, K, and M) based on their spectra or, equivalently, their surface temperatures. * photometry: The measurement of light intensities * stellar spectroscopy: The study of the properties of stars encoded in their spectra. * spectral type: A classification of stars according to the appearance of their spectra. * OBAFGKM sequence: The sequence of stellar spectral classifications from hottest to coolest stars. Mnemonic : Oh Be A Fine Guy Kiss Me – Class O stars are the hottest and Class M stars are the coolest
Types of Stars * main sequence: A grouping of stars on the Hertzsprung-Russell diagram extending diagonally across the graph from the hottest, brightest stars to the dimmest, coolest stars. * giant star: A star whose diameter is roughly 10 to 100 times that of the Sun * .red giant: A large, cool star of high luminosity. * supergiant: A star of very high luminosity. * white dwarf: A low-mass stellar remnant that has exhausted all its thermonuclear fuel and contracted to a size roughly equal to the size of Earth. * luminosity class: The classification of a star of a given spectral type according to its luminosity and density; the classes are supergiant, bright giant, giant, subgiant, and main sequence. * Luminosity classes Ia and Ib encompass all of the supergiants; giants of various luminosity are assigned classes II, III, and IV; and main-sequence stars are luminosity class V * spectroscopic parallax: A method of determining a star’s distance from Earth by measuring its surface temperature, luminosity, and apparent magnitude. * The Hertzsprung-Russell (H-R) diagram is a graph on which luminosities of stars are plotted against their spectral types (or, equivalently, their absolute magnitudes are plotted against surface temperatures). * The H-R diagram reveals the existence of four major groupings of stars: main-sequence stars, giants, supergiants, and white dwarfs. * The mass-luminosity relation expresses a direct correlation between a main-sequence star’s mass and the total energy it emits. * Distances to stars can be determined using their spectral types and luminosity classes.
Stellar Masses * optical double: A pair of stars that appear to be near each other but are unbound and at very different distances from Earth. * binary star: Two stars revolving about each other; a double star. * visual binary: A double star in which the two components can be resolved through a telescope. * mass-luminosity relation: The direct relationship between the masses and luminosities of main-sequence stars. * spectroscopic binary: A double star whose binary nature can be deduced from the periodic Doppler shifting of lines in its spectrum. * Binary stars are fairly common. Those that can be resolved into two distinct star images (even if it takes a telescope to do this) are called visual binaries. * The masses of the two stars in a binary system can be computed from measurements of the orbital period and orbital dimensions of the system. * Some binaries can be detected and analyzed, even though the system may be so distant (or the two stars so close together) that the two star images cannot be resolved with a telescope. * A spectroscopic binary is a system detected from the periodic shift of its spectral lines. This shift is caused by the Doppler effect as the orbits of the stars carry them alternately toward and away from Earth. * An eclipsing binary is a system whose orbits are viewed nearly edge on from Earth, so that one star periodically eclipses the other. Detailed information about the stars in an eclipsing binary can be obtained by studying the binary’s light curve. | | |
CHAPTER 12: THE LIVES OF STARS FROM BIRTH THROUGH MIDDLE AGE
Protostars and Pre–Main-Sequence Stars * Enormous, cold clouds of gas and dust, called giant molecular clouds, are scattered about the disk of the Galaxy. * Star formation begins when gravitational attraction causes clumps of gas and dust, called protostars, to coalesce in Bok globules within a giant molecular cloud. As a protostar contracts, its matter begins to heat and glow. When the contraction slows down, the protostar becomes a pre–main-sequence star. When the pre–main-sequence star’s core temperature becomes high enough to begin hydrogen fusion and stop contracting, it becomes a main-sequence star. * The most massive pre–main-sequence stars take the shortest time to become main-sequence stars (O and B stars). * In the final stages of pre–main-sequence contraction, when hydrogen fusion is about to begin in the core, the pre–main-sequence star may undergo vigorous chromospheric activity that ejects large amounts of matter into space. G, K, and M stars at this stage are called T Tauri stars. * A collection of a few hundred or a few thousand newborn stars formed in the plane of the Galaxy is called an open cluster. Stars escape from open clusters, most of which eventually dissipate.
Main-Sequence and Giant Stars * The Sun has been a main-sequence star for 4.6 billion years and should remain so for about another 5 billion years. Less massive stars than the Sun evolve more slowly and have longer main-sequence lifetimes. More massive stars than the Sun evolve more rapidly and have shorter main-sequence lifetimes. * Main-sequence stars with mass between 0.08 and 0.4 M convert all of their mass into helium and then stop fusing. Their lifetimes last hundreds of billions of years, so none of these stars has yet left the main sequence. * Core hydrogen fusion ceases when hydrogen in the core of a main-sequence star with M > 0.4 M is gone, leaving a core of nearly pure helium surrounded by a shell where hydrogen fusion continues. Hydrogen shell fusion adds more helium to the star’s core, which contracts and becomes hotter. The outer atmosphere expands considerably, and the star becomes a giant. * When the central temperature of a giant reaches about 100 million K, the thermonuclear process of helium fusion begins. This process converts helium to carbon, then to oxygen. In a massive giant, helium fusion begins gradually. In a less massive giant, it begins suddenly in a process called the helium flash. * The age of a stellar cluster can be estimated by plotting its stars on an H-R diagram. The upper portion of the main sequence disappears first, because more massive main-sequence stars become giants before low-mass stars do. * Giants undergo extensive mass loss, sometimes producing shells of ejected material that surround the entire star. * Relatively young stars are metal-rich (Population I); ancient stars are metal-poor (Population II).
Clusters of stars * Groups of between a few hundred and a few thousand stars, formed together from a single interstellar cloud in the disk of our Galaxy, are called open clusters. * Stars in open clusters go their separate ways. * Groups of hundreds of thousands to millions of stars formed together from a common interstellar cloud are called globular clusters. * Stars in globular clusters remain bound together.
Variable Stars * When a star’s evolutionary track carries it through a region called the instability strip in the H-R diagram, the star becomes unstable and begins to pulsate. * RR Lyrae variables are low-mass, pulsating variables with short periods. Cepheid variables are high-mass, pulsating variables exhibiting a regular relationship between the period of pulsation and luminosity. * Mass can be transferred from one star to another in close binary systems. When this occurs, the evolution of the two stars changes. |
CHAPTER 13 THE DEATH OF STARS * Stars with higher masses fuse more elements than do stars with lower masses. * Stars lose mass via stellar winds throughout their lives.
Low-Mass Stars and Planetary Nebulae * A low-mass (below 8 M) main-sequence star becomes a giant when hydrogen shell fusion begins. It becomes a horizontal-branch star when core helium fusion begins. It enters the asymptotic giant branch and becomes a supergiant when helium shell fusion starts. * Stellar winds during the thermal pulse phase eject mass from the star’s outer layers. * The burned-out core of a low-mass star becomes a dense carbon-oxygen body, called a white dwarf, with about the same diameter as that of Earth. The maximum mass of a white dwarf (the Chandrasekhar limit) is 1.4 M. * Explosive hydrogen fusion may occur in the surface layer of a white dwarf in some close binary systems, producing sudden increases in luminosity that we call novae. * An accreting white dwarf in a close binary system can also become a Type Ia supernova when carbon fusion ignites explosively throughout such a degenerate star.
High-Mass Stars and Supernovae * After exhausting its central supply of hydrogen and helium, the core of a high-mass (above 8 M) star undergoes a sequence of other thermonuclear reactions. These reactions include carbon fusion, neon fusion, oxygen fusion, and silicon fusion. This last fusion eventually produces an iron core. * A high-mass star dies in a supernova explosion that ejects most of the star’s matter into space at very high speeds. This Type II supernova is triggered by the gravitational collapse and subsequent bounce of the doomed star’s core. * Neutrinos were detected from Supernova 1987A, which was visible to the naked eye. Its development supported theories of Type II supernovae.
Neutron Stars, Pulsars, and (perhaps) Quark Stars * The core of a high-mass main-sequence star containing between 8 and 25 M becomes a neutron star. The cores of slightly more massive stars may become quark stars. A neutron star is a very dense stellar corpse consisting of closely packed neutrons in a sphere roughly 20 km in diameter. The maximum mass of a neutron star, called the Oppenheimer-Volkov limit, is about 3 M. * A pulsar is a rapidly rotating neutron star with a powerful magnetic field that makes it a source of periodic radio and other electromagnetic pulses. Energy pours out of the polar regions of the neutron star in intense beams that sweep across the sky. * Some X-ray sources exhibit regular pulses. These objects are believed to be neutron stars in close binary systems with ordinary stars. * Explosive helium fusion may occur in the surface layer of a companion neutron star, producing a sudden increase in X-ray radiation, called an X-ray burster. | | |
CHAPTER 14 : BLACK HOLE MATTERS OF GRAVITY

SUMMARY OF KEY IDEAS | | eBook Tools | * If a stellar corpse is more massive than about 3 M, gravitational compression overcomes neutron degeneracy and forces it to collapse further and become a black hole. * A black hole is an object so dense that the escape velocity from it exceeds the speed of light.
The Relativity Theories * Special relativity reveals that space and time are intimately connected and change with an observer’s relative motion. * As seen by observers moving more slowly, the faster an object moves, the slower time passes for it (time dilation) and the shorter it becomes (length contraction). * According to general relativity, mass causes space to curve and time to slow down. These effects are significant only near large masses or compact objects.
Inside a Black Hole * The event horizon of a black hole is a spherical boundary where the escape velocity equals the speed of light. No matter or electromagnetic radiation can escape from inside the event horizon. The distance from the center of the black hole to the event horizon is called the Schwarzschild radius. * The matter inside a black hole collapses to a singularity. The singularity for nonrotating matter is a point at the center of the black hole. For rotating matter, the singularity is a ring inside the event horizon. * Matter inside a black hole has only three physical properties: mass, angular momentum, and electric charge. * Nonrotating black holes are called Schwarzschild black holes. Rotating black holes are called Kerr black holes. The event horizon of a Kerr black hole is surrounded by an ergoregion in which all matter must constantly move to avoid being pulled into the black hole. * Matter that approaches a black hole’s event horizon is stretched and torn by the extreme tidal forces generated by the black hole, light from the matter is redshifted, and time slows down. * Black holes can evaporate by the Hawking process, in which virtual particles near the black hole become real. These transformations of virtual particles into real ones decrease the mass of a black hole until, eventually, it disappears.
Evidence of Black Holes * Observations indicate that some binary star systems harbor black holes. In such systems, gases captured by the black hole from the companion star heat up and emit detectable X rays and jets of gas. * Supermassive black holes exist in the centers of many galaxies. Intermediate-mass black holes appear to exist in globular clusters of stars. Very low mass (primordial) black holes may have formed at the beginning of the universe.
Gamma-Ray Bursts * Gamma-ray bursts are events believed to be caused by some supernovae and by the collisions of dense objects, such as neutron stars or black holes. Some occur in the Milky Way and nearby galaxies, whereas many others occur billions of light-years away from Earth. * Typical gamma-ray bursts occur for a few tens of seconds and emit more energy than the Sun will radiate over its entire 10-billion-year lifetime.
CHAPTER 15: THE MILKY WAY GALAXY
Discovering the Milky Way * A century ago, astronomers were divided on whether or not the Milky Way Galaxy and the universe were the same thing. * The Shapley–Curtis debate was the first major public discussion between astronomers as to whether the Milky Way contains all the stars in the universe. * Cepheid variable stars are important in determining the distance to other galaxies. * Edwin Hubble proved that there are other galaxies far outside of the Milky Way.
The Structure of Our Galaxy * Our Galaxy has a disk about 100,000 ly in diameter and about 2000 ly thick, with a high concentration of interstellar dust and gas. It contains around 200 billion stars. * Interstellar dust obscures our view into the plane of the galactic disk at visual wavelengths. However, hydrogen clouds can be detected beyond this dust by the 21-cm radio waves emitted by changes in the relative spins of electrons and protons in the clouds, as well as by other nonvisible emissions. * The center, or galactic nucleus, has been studied at gamma-ray, X-ray, infrared, and radio wavelengths, which pass readily through intervening interstellar dust and H II regions that illuminate the spiral arms. These observations have revealed the dynamic nature of the galactic nucleus, but much about it remains unexplained. * A supermassive black hole of about 4.3 × 106 M exists in the galactic nucleus. * The galactic nucleus of the Milky Way is surrounded by a flattened sphere of stars, called the central bulge, through which a bar of stars and gas extends. * A disk with two bright arms of stars, gas, and dust spirals out from the ends of the bar in the galactic central bulge. * Young OB associations, H II regions, and molecular clouds in the galactic disk outline huge spiral arms where stars are forming. * The Sun is located about 26,000 ly from the galactic nucleus, between the spiral arms. The Sun moves in its orbit at a speed of about 878,000 km/h and takes about 230 million years to complete one orbit around the center of the Galaxy. * The entire Galaxy is surrounded by two halos of matter. The inner halo includes a spherical distribution of globular clusters and field stars, as well as large amounts of dark matter. It orbits in the same general direction as the disk. The outer halo is composed of dark matter and very old stars, which have retrograde orbits.
CHAPTER 16 : GALAXIES
Types of Galaxies * The Hubble classification system groups galaxies by their shapes into four major types: spiral, barred spiral, elliptical, and irregular. * The arms of spiral and barred spiral galaxies are sites of active star formation. * According to the theory of self-propagating star formation, spiral arms of flocculent galaxies are caused by the births and deaths of stars over extended regions of a galaxy. Differential rotation of a galaxy stretches the star-forming regions into elongated arches of stars and nebulae that we see as spiral arms. * According to the spiral density wave theory, spiral arms of grand-design galaxies are caused by density waves. The gravitational field of a spiral density wave compresses the interstellar clouds that pass through it, thereby triggering the formation of stars, including OB associations, which highlight the arms. * Elliptical galaxies contain much less interstellar gas and dust than do spiral galaxies; little star formation occurs in elliptical galaxies. * Irregular galaxies are rich in gas and dust, and star formation occurs in them. * Lenticular galaxies are disk galaxies without spiral arms.
Clusters and Superclusters * Galaxies group into clusters rather than being randomly scattered through the universe. * A rich cluster contains at least a thousand galaxies; a poor cluster may contain only a few dozen up to a thousand galaxies. A regular cluster has a nearly spherical shape with a central concentration of galaxies; in an irregular cluster, the distribution of galaxies is asymmetrical. * Our Galaxy is a member of a poor, irregular cluster, called the Local Group. * Rich, regular clusters contain mostly elliptical and lenticular galaxies; irregular clusters contain more spiral and irregular galaxies. Giant elliptical galaxies are often found near the centers of rich clusters. * Each galaxy is held together with the aid of dark matter. * No cluster of galaxies has an observable mass large enough to account for the observed motions of its galaxies; a large amount of unobserved mass must be present between the galaxies. * Hot intergalactic gases emit X rays in rich clusters. * When two galaxies collide, their stars initially pass each other, but their interstellar gas and dust collide violently, either causing gas and dust to be stripped from the galaxies or triggering prolific star formation. The gravitational effects of a galactic collision can cast stars out of their galaxies into intergalactic space. * Galactic mergers occur. A large galaxy in a rich cluster may also grow steadily through galactic cannibalism.
Superclusters in Motion * A simple linear relationship exists between the distance from Earth to galaxies in other superclusters and the redshifts of those galaxies (a measure of the speed at which they are receding from us). This relationship is the Hubble law: Recessional velocity = H0 × distance, where H0 is the Hubble constant. * Astronomers use standard candles—Cepheid variables, the brightest supergiants, globular clusters, H II regions, supernovae in a galaxy, and the Tully-Fisher relation—to calculate intergalactic distances. Because of difficulties in measuring the distances to remote galaxies, the value of the Hubble constant, H0, is not known with complete certainty.

CHAPTER 17: QUASARS AND OTHER ACTIVE GALAXIES | * The development of radio astronomy in the late 1940s led to the discovery of very powerful and extremely distant energy sources.
Quasars and Other Active Galaxies * An active galaxy is an extremely luminous galaxy that has one or more unusual features: an unusually bright, starlike nucleus; strong emission lines in its spectrum; rapid variations in luminosity; and jets or beams of radiation that emanate from its core. Active galaxies include quasars, Seyfert galaxies, radio galaxies, double-radio sources, and BL Lacertae objects. * A quasar, or quasi-stellar radio source, is an object that looks like a star but has a huge redshift. This redshift corresponds to a distance of billions of light-years from Earth, according to the Hubble law. * To be seen from Earth, a quasar must be very luminous, typically about 100 times brighter than an ordinary galaxy. Relatively rapid fluctuations in the brightness levels of some quasars indicate that they cannot be much larger than the diameter of our solar system. * An active spiral galaxy with a bright, starlike nucleus and strong emission lines in its spectrum is categorized as a Seyfert galaxy. * An active elliptical galaxy is called a radio galaxy. It has a bright nucleus and a pair of radio-bright jets that stream out in opposite directions. * BL Lacertae (BL Lac) objects (some of which are called blazars) have bright nuclei whose cores show relatively rapid variations in luminosity. * Double-radio sources contain active galactic nuclei located between two characteristic radio lobes. A head-tail radio source shows evidence of jets of high-speed particles that emerge from an active galaxy.
Supermassive Central Engines * Many galaxies contain huge concentrations of matter at their centers. * Some matter that spirals in toward a supermassive black hole is squeezed into two oppositely directed beams that carry particles and energy into intergalactic space. * The energy sources from quasars, Seyfert galaxies, BL Lac objects, radio galaxies, and double-radio sources are probably matter ejected from the accretion disks that surround supermassive black holes at the centers of galaxies.
CHAPTER 18: COSMOLOGY * Modern cosmology almost began in 1915, when Einstein published his theory of general relativity. To his surprise and dismay, the relativity equations predicted that the universe is not static: They indicated that it should be either expanding (which it is) or contracting * Newton believed that each star is fixed in place and held under the influence of a uniform gravitational pull from every part of the cosmos. If the stars were not uniformly distributed, he argued, one region would have more mass than another. The denser region’s gravity would then attract other stars, causing them to further clump together. Because he did not observe this clumping, Newton concluded that the stars in the universe must be distributed uniformly over an infinite space. * Einstein missed the opportunity to say that the universe is changing . so he added a repulsive (out-pushing term) called the cosmological constant which states that gravity’s normal attractive force would be counterbalanced and the universe would be static. * Although the value of the cosmological constant that Einstein inserted was wrong, the concept of such a constant may be correct. Observations since 1997 indicate that the universe is not just expanding but actually accelerating outward. This acceleration means that there must be an outward pressure that more than counteracts the effects of normal gravitation, which is trying to slow the universe’s expansion
Edwin Hubble * Discovered that we live in an expanding universe * expanding universe: The motion of the superclusters of galaxies away from each other. * The redshifts of clusters and superclusters of galaxies that Hubble found moving away from us appear to be produced by the Doppler effect, but they actually are not.. * cosmological redshift: An increase in wavelength from distant galaxies and quasars caused by the expansion of the universe.
Hubble’s law gives us a way to estimate the age of the universe. * time since the big bang= seperation distance/ recessional velocity * recessional velocity =H0 x separation distance * H 0 = recessional velocity/ separation distance

The Big Bang * Big Bang: An explosion that took place roughly 15 billion years ago, creating all space, time, matter, and energy in which the universe emerged. * cosmic microwave background: Photons from every part of the sky with a blackbody spectrum at 2.73 K; the cooled-off radiation from the primordial fireball that originally filled all space. * isotropy: The fact that the average number of galaxies at different distances from Earth is the same in all directions; also, the fact that the temperature of the cosmic microwave background is essentially the same in all directions. * homogeneity: The property of the universe being smooth or uniform as measured over suitably large distance intervals. * Astronomers believe that the universe began as an exceedingly dense cosmic singularity that expanded explosively in an event called the Big Bang. The Hubble law describes the ongoing expansion of the universe and the rate at which superclusters of galaxies move apart. * The observable universe extends about 13.7 billion light-years in every direction from Earth to what is called the cosmic light horizon. We cannot see any objects that may exist beyond the cosmic light horizon because light from these objects has not had enough time to reach us. * According to the theory of inflation, early in its existence, the universe expanded very rapidly for a short period, spreading matter that was originally far from our location (and hence at different temperatures and densities) throughout a volume of the universe so large that we cannot yet observe it. The observable universe today is thus a growing volume of space containing matter and radiation that was in close contact with our matter and radiation during the first instant after the Big Bang (and hence at the same temperature, pressure, and density). Inflation explains the isotropic and homogeneous appearance of the universe.
A Brief History of Spacetime, Matter, Energy, and Everything * strong nuclear force: The force that binds protons and neutrons together in nuclei. * weak nuclear force: A nuclear interaction involved in certain kinds of radioactive decay. * quark: A particle that is a building block of the heavy nuclear particles such as protons and neutrons.
Grand Unified Theory (GUT) * A theory that describes and explains the four physical forces. * It describes how the energies or, equivalently, temperatures at which this unification of forces occurs are greater than can ever be achieved in any laboratory on Earth.
Planck time * The earliest time at which our current equations can explain the behavior of the universe is about 10−43 s after the Big Bang. * Planck era: Time from the Big Bang until the Planck time (10−43 s). * At the Planck time, gravity is thought to have become a separate force, leaving electromagnetism and the weak and strong nuclear forces still united as the GUT force. Which of the present GUT models, if any, is correct, remains to be seen. * * horizon problem: The difficulty in explaining why seemingly disconnected regions of the universe have the same temperature. * inflationary epoch: A brief period shortly after the Big Bang during which the scale of the universe increased very rapidly. * cosmic light horizon: A sphere, centered on Earth, whose radius equals the Four basic forces—gravity, electromagnetism, the strong nuclear force, and the weak nuclear force—explain the interactions observed in the universe. * pair production: The creation of a particle and an antiparticle from energetic photons. * primordial nucleosynthesis: The transformation by fusion of protons and electrons into hydrogen isotopes, helium, and some lithium in the first few minutes of the existence of the universe. * radiation-dominated universe: The time at the beginning of the universe when the electromagnetic radiation prevented ions and electrons from combining to make neutral atoms. * matter-dominated universe: A universe in which the radiation field that fills all space is unable to prevent the existence of neutral atoms. * primordial fireball: The extremely hot gas that filled the universe immediately following the Big Bang. * decoupling: The epoch in the early universe when electrons and ions first combined to create stable atoms; the time when electromagnetic radiation ceased to dominate over matter. * era of recombination: The time, roughly 500,000 years after the Big Bang, when the universe became transparent.
Summary of key ideas * According to current theory, all four forces were identical just after the Big Bang. At the end of the Planck time (about 10−43 s after the Big Bang), gravity became a separate force. A short time later, the strong nuclear force became a distinct force. A final separation created the electromagnetic force and the weak nuclear force. * Before the Planck time, the universe was so dense that known laws of physics did not describe the behavior of spacetime, matter, and energy back then. * In its first 30,000 years, the universe was radiation-dominated, during which time photons prevented matter from forming clumps. Then it was matter-dominated, during which time superclusters and smaller clumps of matter formed. Today it is dark-energy–dominated. Dark energy of some sort supplies a repulsive gravitational force that causes superclusters to accelerate away from each other. * During the first 380,000 years of the universe, matter and energy formed an opaque plasma, called the primordial fireball. Cosmic microwave background radiation is the greatly redshifted remnant of the universe as it existed about 380,000 years after the Big Bang. * About 380,000 years after the Big Bang, spacetime expansion caused the temperature of the universe to fall below 3000 K, allowing protons and electrons to combine and thereby form neutral hydrogen atoms. This period is called the era of recombination. The universe became transparent during the era of recombination, with the photons that existed back then still traveling through space today. In other words, the microwave background radiation is composed of the oldest photons in the universe. * Clusters of galaxies and individual galaxies formed from pieces of enormous hydrogen and helium clouds, each of which became a separate supercluster of galaxies. * All of the superclusters and some of the clusters of galaxies within each supercluster are moving away from one another. * Supermassive black holes appear to have “seeded” the formation of most galaxies. * During the matter-dominated era, structure formed in the universe. As the universe goes farther into the dark-energy–dominated era, the large-scale structure of superclusters of galaxies will fade away.
The Fate of the Universe * The average density of matter and dark energy in the universe determines the curvature of space and the ultimate fate of the universe. * Observations show that the universe is flat and that the cosmic microwave background is almost perfectly isotropic, resulting from a brief period of very rapid expansion (the inflationary epoch) in the very early universe. * The universe is accelerating outward and it will expand forever. | | |