Astronomy Study Guide Exam 3
Astronomy Study Guide Exam 3 ASTRON 0089
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Astronomy Study Guide Exam 3 317 Chapter 12 Star Formation and Evolution A Nova in a Binary system Hydrogen Ignition on a WD Life after death of a WD Examples semidetached binary semidetached binary with mass transfer overcontact binary Hydrogen can accumulate on the surface of the white dwarf if enough builds up it can reexperience the hydrogen shell burning phase However there s no upper star to hold the fusion in an explosive burning phase engulfs the surface and there s a huge explosion as material is ejected The body of the white dwarf barely cares and settles back down to receiving more hydrogen so novae can reoccur many times Electron degeneracy pressure can only support a mass less than 14 solar masses this is called the Chandrasekhar Limit So things could get out of hand for a white dwarf that s almost 14 solar masses that s being fed more hydrogen Detonating a White Dwarf A Type Ia Supernova Carbon Detonation Supernova l 2 Spectra of these Supernovae don t show hydrogen or helium lines The more massive member of a pair of Sunlike stars exhausts its fuel and turns into a whitedwarf star The white dwarf sucks in gas from its companion eventually reaching a critical mass A ame runaway nuclear reaction ignites in the turbulent core of the dwarf The ame spreads outward converting carbon and oxygen to radioactive nickel Within a few seconds the dwarf has been completely destroyed over the following weeks the radioactive nickel decays powering the bright glow of the debris Type Ia Supernova are much brighter and completely different from nova explosions but both involve a white dwarf Evolution of Stars More Massive than the Sun Stars more massive than the Sun follow very different paths when leaving the Main Sequence Mass Loss from a Supergiant Star Supergiants lose mass at a rapid rate in a strong stellar wind even while on the main sequence As this wind collides with the surrounding interstellar gas and dust it creates a bubble Highmass stars like all stars leave the Main Sequence when there s no more hydrogen fuel in their cores The rst few events are similar to those in lowermass stars 0 First a hydrogen shell then a core burning helium to carbon surrounded by helium and hydrogenburning shells High mass stars burn hydrogen with the CNO cycle not the pp chain and there is no Helium ash The protonproton cycle isn t the only path stars take to fuse hydrogen to helium for stars with masses gt 4 solar masses core temps exceed 20000000 K at these temps the CNO cycle dominates Hydrogen is converted to Helium via a path including carbon the carbon12 nucleus acts as a catalyst C 4 H gives C He neutrinos energy The positrons encounter electrons and are annihilated this produces more energy For stars more massive than 4 solar masses Very high central temps and pressures mean that electron degeneracy pressure won t stop the collapse of the star and elements heavier than C and O are produced Electron degeneracy pressure can only support a mass lt 14 solar masses If the star s CO mass is greater than this then gravity takes over and C begins to burn when the temperature reaches 600 million K Carbon12 Helium4 9 Oxygenl6 this is more likely Or Carbon12 Carbon12 9 Magnesium24 Every successive element is formed by adding on a helium4 nucleus 16 Oxygen 4 Helium 9 20 Neon energy Carbon can fuse either with itself or with a helium nucleus to form more nuclei Evolutionary Stages of a 25 solar mass star Each successive element has smaller returns from fusion it has to burn faster and more vigorously to hold up the star In the stability of nuclei iron is the crossing point when the core has fused to iron no more fusion can take place On the left of iron nuclei gain energy through fusion On the right they gain it through fission Iron56 is the most stable nucleus so it neither fuses nor decays Iron is very stable energy isn t produced in nuclear reactions with iron but is used up For example iron Fe absorbs energy gamma rays and breaks up into helium nuclei and neutrons y 56Fe 9 l3 4He 4n The Beginning of the End Now there is no pressure left to hold up the star gravity takes over As the core collapses energy isn t produced The outer layers of the star lose their pressure support they begin to implode falling inward due to the immense gravity Catastrophic collapse follows in a fraction of a second Formation of a Neutron Core As the core gets denser protons and electrons will combine with one another to become neutrons p e 9 n neutrino Neutrons are more massive than protons so this takes some energy Emc2 cooling the core more reducing pressure and accelerating the collapse With the charged particles gone electrical repulsion is now a nonissue The neutrinos very low mass neutral particles mostly escape just like the ones produced in fusion The neutrons offer rapidly increasing resistance to further compression neutron degeneracy pressure kicks in and stops the collapse By the time the collapse can stop densities are enormous 10Al8 kgmA3 but the outer layers are still falling in The imploding outer layers rebound off the nowrigid core in an enormous explosion helped by the neutrinos pouring out of the core a Type 11 core collapse supernova ensues blowing up most of the star A CoreCollapse Supernova l 2 As a massive star nears its end it takes on an onionlayer structure at this point in its evolution the star is hundreds of millions of kilometers in radius Iron doesn t undergo nuclear fusion so the core becomes unable to generate heat the gas pressure drops and overlying material suddenly rushes in Within a second the core collapses to nuclear density inwardfalling material rebounds off the core setting up an outwardgoing pressure wave Neutrinos pouring out of the developing neutron star propel the shock wave outward unevenly The shock wave sweeps through the entire star blowing it apart Elements heaver than iron are created inside supernovae by neutron capture reactions The pathways by which heavier elements are created are quite complicated Luminous Supernovae Maximum luminosity as great as 10A9 solar L rivaling the light output of an entire galaxy for a brief period The Tarantula Nebula in the LMC is 51000 pc from Earth but was the SN bright enough to be seen without a telescope An Unusual Supernova SN 1987A appears to have a set of 3 glowing rings Relics of a hydrogenrich outer atmosphere ejected by gentle stellar winds from the star when it was a red supergiant The gas expanded in an hourglass shape bc it was blocked from expanding around the star s equator by a preexisting ring of gas These rings were ionized by the initial ash of ultraviolet radiation from the supernova Outer ring at edge sweptup gas from earlier mass loss Inner ring sweptup redsupergiant gas Supernova remnant dark invisible outer portion surrounding the brighter inner region lit by radioactive decay A Supernova in a Distant Galaxy The progenitor star that later exploded into SN 1993J was a K0 red supergiant SN 1993J resulted from the core collapse and subsequent explosion of a massive star Type I vs Type II Supernovae Type I Supernova binary star system 9 white dwarf amp planetary nebula 9 growing white dwarf from the red giant 9 detonation Type II Supernova He C and H normal star fusion 9 massive star imploding 9 core rebound 9 explosion Telling the Supernova Apart Type II Supernova massive stars collapsing due to gravitational energy 0 A onetime event releasing metals and emitting a hydrogen spectrum Type Ia Supernovae white dwarfs exploding 100 of their mass into space 0 A onetime event releasing oxygen carbon and silicon with almost no hydrogen spectrum Supernova remnants material blown out by a supernova an expanding cloud of material from the explosion Gum Nebula our supernova neighbor exploded around 9000 BC 0 At max brilliance the exploding star probably was as bright as the Moon at first quarter like the firstquarter moon it would ve been visible in the daytime Seeing Supernova Many supernova remnants are invisible at the visible wavelengths our human eyes can see however when the expanding gases collide with the interstellar medium they emit energy at a wide range of wavelengths from X rays through radio waves This is a radio image of the supernova remnant Cassiopeia A as a rule radio searches for supernova remnants are more fruitful than visiblelight searches Supernova of the Past Historically supernovae were recorded as guest stars giving us clues as to where old cores may be found Some of these cores are neutron stars or pulsars 324 Chapter 12 Part II Neutron Stars After a Type I carbon detonation supernova little or nothing remains of the original white dwarf After a Type 11 core collapse supernova part of the core may survive it is very dense as dense or denser than an atomic nucleus and is called a neutron star because it s made entirely of neutrons In the most massive stars black holes form The existence of neutron stars was theorized in 1933 but weren t discovered until 1967 Neutron stars are 13 times as massive as the Sun but are so dense that they re extremely small Incredible Densities 1 cubic cm of neutron star material size of grape has a mass of about 500 billion kg or 500 million tons as much as 10000 battleships The force of gravity at the surface of a white dwarf is about 300000 times that on Earth The force of gravity at the surface of a neutron star is about 1 trillion times that on Earth A neutron star is like a single gigantic atomic nucleus a unique form a matter that s poorly understood Other Properties of Neutron Stars Rotation as the parent star collapses the neutron core spins more and more rapidly 0 Typical periods are fractions of a second 0 If you shrank the Sun down to a neutronstar size it would rotate once every 10 seconds Magnetic field as the star collapses the neutron star s magnetic field becomes more and more concentrated and hence enormously strong 0 Like on Earth a neutron star s magnetic pole may not match up exactly with its rotation axis 0 The powerful magnetic field can accelerate particles and produce radiation with observable consequences Pulsars The first pulsar was discovered by Jocelyn Bell in l967 it was an object emitting extraordinarily regular pulses of radio waves After some initial confusion it was realized that this was a neutron star spinning very rapidly nothing else would be small enough to emit bright pulses with such high frequency A 001 second pulse must come from something smaller than 001 sec X c 3000 km in diameter or it would be smeared out What causes the pulses The lighthouse effect strong jets of light are emitted at the north and south magnetic poles of a neutron star we see a pulse when the beam points at us Evolution of a pulsar Pulsars radiated their energy away quite rapidly the radiation weakens and stops in a few tens of millions of years making the neutron star virtually undetectable Pulsars won t be visible on Earth if their jets aren t pointing our way About 1500 pulsars are known but our galaxy should contain many more neutron stars some are just too old to be seen or their jets point in wrong direction for us to ever see them Pulsars are sometimes found in supernova remnants there is a pulsar at the center of the Crab Nebula Pulsars are also observed to pulse in the gammaray spectrum and at radio and visible wavelengths matter is accelerated to high energies by a pulsar s magnetic field The Crab Nebula is close enough we can observe it in detail we can see ripples extending outward from the Crab pulsar at half the speed of light NeutronStar Binaries Bursts of Xrays have been observed from many objects in our galaxy Xray Bursts Xray bursts are thought to originate on neutron stars that have binary partners the process is similar to a nova occasional fusion areups However the gravitational eld is much stronger around a neutron star so much more energy is released r radius is tiny in a neutron star compared to a white dwarf so the gravitational force is much bigger Gammaray Bursts Gammaray bursts have also been observed rst spotted by satellites looking for violations of treaties banning aboveground nuclear tests that emit gamma rays For many years their existence was kept secret from astronomers At rst gammaray bursts were thought to be higherenergy versions of Xray bursts However Xray bursts tend to lie in the plane of our galaxy gammaray bursts are spread uniformly over the sky This suggests they re far away from us in other galaxies Models for GammaRay Bursts Long GammaRay Bursts o A redgiant star collapses onto its core becoming so dense that it expels its outer layers in a supernova explosion o Hypernova star explodes at the end of its life 0 A black hole forms and exploded material forms an accretion disk gamma rays are released in a jet Short GammaRay Bursts less than 2 seconds 2 neutron stars merging 0 Stars in a compact binary system begin to spiral inward eventually colliding o The resulting torus has at its center a powerful black hole Black Holes The mass of a neutron star can t exceed about 3 solar masses if the core remnant is more massive than that nothing will stop its collapse and it will become smaller and smaller and denser and denser Eventually the gravitational force is so intense that even light can t escape the remnant has become a black hole The formation of a black hole 1 A supergiant star has relatively weak gravity so emitted photons travel in essentially straight lines 2 As the star collapses into a neutron star the surface gravity becomes stronger and photons follow curved paths 3 Continued collapse intensifies the surface gravity and so photons follow paths more sharply curved 4 When the star shrinks past the critical size it becomes a black hole photons follow paths that curve back into the black hole so no light escapes When the star becomes a black hole not even photons emitted directly upward from the surface can escape they undergo an infinite gravitational redshift and disappear Schwarzschild radius the radius at which the escape speed from the black hole equals the speed of light 0 The Earth s Schwarzschild radius is about a centimeter the Sun s is about 3 km 0 Once the black hole has collapsed the Schwarzschild radius is also called the event horizon nothing within the event horizon can escape the black hole 0 The Schwarzschild radius or size of black hole is R 2GMcA2 o G gravitational constant c speed of light M mass of black hole Matter encountering a black hole will experience enormous tidal forces that will both heat it enough to radiate and tear it apart The human body can t withstand stress greater than 1020 g pull of gravity on Earth The breaking point would occur around 3000 km from a 10 solar mass black hole We can orbit a black hole a safe distance away but can t get too close Space Travel Near Black Holes The gravitational effects of a black hole are unnoticeable outside of a few Schwarzschild radii at this distance black holes don t suck in material any more than an extended mass would The orbit of an object near a black hole is basically the same as its orbit near a star of the same mass More on the Formation of a Black Hole 1 2 3 A black hole sharply curves the spacetime around it Far from the black hole spacetime is nearly flat close to the black hole the curvature forms a well that is infinitely deep Objects that venture too close to the black hold can t escape from the well Neither light nor anything else is able to escape the gravitational attraction of the crushed core In a sense a hole is punched in the fabric of the universe and the dying star seems to disappear into this cavity None of the star s mass is lost when it collapses to form a black hole and a black hole s gravitational in uence can still be felt by other objects How do you observe a black hole The Effects of Black Holes In binary systems the mass from one star may be captured by the black hole This material spirals into the black hole with so much speed that it emits X ray light Gases from the supergiant are captured into an accretion disk around the black hole As gases spiral toward the black hole they are heated by friction just outside the black hole they re hot enough to emit Xrays The Environment of an Accreting Black Hole If a black hole is rotating it can generate strong electric and magnetic elds in its immediate vicinity These elds draw material from the accretion disk around the black hole and accelerate it into oppositely directed jets along the black hole s rotation axis Xrays from the hot disk excite iron atoms in the torus making them glow Fastmoving jets of subatomic particles are formed and ejected by electric and magnetic elds General Relativity and Gravity Matter tends to warp spacetime and in doing so rede nes straight lines the path a light beam would take A black hole occurs when the indentation caused by the mass of the hole becomes in nitely deep 326 Tests of General Relativity De ection of starlight by the sun s gravity was measured during the solar eclipse of 1919 the results agreed with the predictions of general relativity Another prediction the orbit of Mercury should precess due to general relativistic effects near the Sun again the measurement agreed with the prediction Gravity Waves Just as we detect electromagnetic waves when charges accelerate in a radio antenna or on a star general relativity predicts that accelerating masses should radiate gravity waves Orbiting objects should lose energy by emitting gravitational radiation the amount of energy is tiny and these waves haven t yet been observed directly However a neutronstar binary system has been observed the orbits of the star are slowing at just the rate predicted if gravity waves are carrying off the lost energy Laser Interferometric Gravitywave Observatory designed to detect gravitational waves no waves have been detected yet acts as a gravity wave telescope LISA laser interferometer space antenna Three antennae orbiting the Sun forming an equilateral triangle 5 million km apart it will be sensitive to waves from superrnassive black holes near centers of galaxies Chapter 13 Our Parent Galaxy The Milky Way From Earth we see few stars when looking out of our galaxy and many stars when looking in the Milky Way is what our galaxy looks like in the night sky The Milky Way is 30 kpc or 100000 ly across and contains 100 billion stars We determine our location in the Galaxy by observing globular clusters which form a spherical halo centered on the center of the Galaxy Measuring Distances in the Milky Way Cataclysmic variables variable stars novae supernovae Xray busts and related phenomena Intrinsic variables other stars whose luminosity varies in a regular way but much more subtly 0 RR Lyrae stars and Cepheids 2 types of intrinsic variables that are useful for determining distances to clusters and galaxies in which they reside Cepheid Variables Henrietta Leavitt discovered the periodluminosity relation for Cepheid variables The longer a Cepheid s period the greater its luminosity The periodluminosity law can be used to determine distances Knowing the star s periodicity you can nd out how far away the star must be to give the observed brightness inverse square law of radiation Found throughout the Galaxy Pulsation periods of l 50 days Average luminosity related to pulsation period The more luminous the Cepheid the longer its pulsation period RR Lyrae Variables Horizontalbranch stars that all have roughly the same average luminosity of about 100 L Found in globular clusters these clusters aren t all in the plane of the galaxy so they aren t obscured by dust and can be measured Pulsation periods less than a day All have about the same luminosity By using the periodluminosity relationship for these stars we can determine the distances to globular clusters The variability of Cepheid and RR Lyrae stars comes from a dynamic balance between gravity and pressure they have large oscillations around stability They get bigger and smaller as they try to adjust themselves to gravity and radiation pressure Galactic Structure Disk contains gas and dust along with metalrich Population 1 stars Halo composed almost exclusively of old metalpoor Population 11 stars Central bulge mixture of Population I and Population 11 stars Stellar orbits in the disk move in a plane and in the same direction orbits in the halo and bulge are much more random they all orbit the center of the Galaxy If you were looking to nd the most recently formed stars in the Galaxy where would you look disk The Formation of the Milky Way The formation of the galaxy is believed to be similar to the formation of the solar system but on a much larger scale Globular clusters formed rst during the initial collapse of the Milky Way and remained in random orbits around the center The Mass of the Milky Way Galaxy The orbital speed of an object depends only on the amount of mass between it and the galactic center Total mass solar masses orbit size AUquot3 orbit period yearsquot2 The sun is 8 kpc from the center and takes 225 million years to orbit the Galaxy this gives the MWG a mass of at least 90 billion solar masses As in a binary star system if the size and period of the orbit are known the mass can be calculated As you move away from the visible galaxy the velocity speed of outermost gas should diminish with distance 0 But it doesn t there must be much more mass outside the visible part to reproduce the observed curve 0 We know this from 21 cm measurements of Hydrogen This means that there s much more mass than we can see this was observed in other galaxies as well and is evidence for dark matter in the Universe The measured speed implies that the Milky Way s mass is 600 billion The Galaxy and its Dark Matter Halo The dark matter in our Galaxy forms a spherical halo whose center is at the center of the visible Galaxy The extent of the dark matter halo is unknown but its diameter is at least 100 kpc The total mass of the dark matter halo is at least 10 times the combined mass of all the stars dust gas and planets in the Milky Way Rotation curves of other galaxies the variation in the orbital circular velocity of stars or gas clouds at different distances from the center What could this dark matter be It s dark at all wavelengths not just the visible Stellarmass black holes Not enough of them could ve been created Brown faint whitered dwarfs Currently the best starlike option but not enough could ve been formed Weird subatomic particles Could be although no direct evidence so far Dark Matter Speculations MACHOS Massive Compact Halo Objects 0 Brown dwarfs white dwarfs black holes 0 Can only account for about half of the dark matter halo 0 Searched for via gravitational lensing WIMPS Weakly Interacting Massive Particles 0 Theoretical and notyetdetected More evidence for dark matter Galaxies in clusters are moving faster than can be explained by the observable mass At the speed they re moving they should ve escaped the gravitational pull of the cluster And much more mass than we can see is responsible for bending the light from background objects A galaxy must be surrounded by an invisible halo of dark matter 85 of matter in the Universe is dark The nature of dark matter remains one of the biggest unsolved problems in modern astrophysics Size of the black hole in the center of the Milky Way Galaxy is 37 million solar masses Infrared and Radio Observations of the Galactic Nucleus In an infrared image the reddish band is dust in the plane of the Galaxy and the fainter bluish blobs are interstellar clouds heated by young 0 and B stars Adaptive optics reveal stars densely packed around the galactic center The galactic center appears to have A stellar density a million times higher than near the Sun A ring of molecular gas 400 pc across Strong magnetic fields A rotating ring or disk of matter a few parsecs across A strong Xray source at the center Apparently there is an enormous black hole at the center of the galaxy which is the source of these phenomena it s called Sagittarius A An accretion disk surrounding the black hole emits enormous amounts of radiation 42 Galaxies Island Universes Prior to the construction of large telescopes it wasn t clear that galaxies really were other galaxies Edwin Hubble was the rst to demonstrate that M31 is actually a galaxy that lies far beyond the Milky Way Measuring Galaxy Distances with Cepheid Variables Hubble examined photographic plates and discovered what he at rst thought to be a nova Referring to previous plates of that region he soon realized that the object was actually a Cepheid variable star Hubble realized that for these luminous stars to appear as dim as they were on his photographs of the Andromeda Nebula they must be extremely far away Cepheid variables allow measurement of galaxy distances to about 25 Mpc away The Hubble Classi cation of Galaxies The Tuning Fork Diagram Spiral Galaxies Edwin Hubble classi ed spiral galaxies according to the texture of their spiral arms and the relative size of their central bulges 0 Sa galaxies have smooth broad spiral arms and the largest central bulges 0 Se galaxies have narrow wellde ned arms and the smallest central bulges Barred Spiral Galaxies As with spiral galaxies Hubble classi ed barred spirals according to the texture of their spiral arms Barred spirals are named this bc the bulge has a bar going through it and the spiral arms originate from the ends of the bar SBa galaxies have the smoothest spiral arms and the largest central bulges SBc galaxies have narrow wellde ned arms and the smallest central bulges Hubble s Galaxy Classification Type Sa largest central bulge most tightly bound spiral arms Type Sb smaller bulge progressively less tight spiral arms correlation isn t perfect Type Sc smallest bulge progressively less tight spiral arms correlation isn t perfect The components of spiral galaxies are the same as in our own galaxy disk halo bulge and spiral arms Elliptical Galaxies Hubble classi ed elliptical galaxies according to how round or flattened they look A galaxy that appears round is labeled E0 and the attest appearing elliptical galaxies are labeled E7 Elliptical galaxies have no spiral arms and no disk they come in many sizes from giant ellipticals of trillions of stars to dwarf ellipticals of less than a million stars Giants and Dwarfs Most galaxies are about the same size except for the giant and dwarf ellipticals Giant ellipticals are rare Dwarf ellipticals are more common and transparent Irregular Galaxies Irregular galaxies small form new stars rapidly have wide variety of disorganized shapes Small and Large Magellanic Clouds are largest close neighbors to our own Milky Way Active Galaxies The radiation from these galaxies is called nonstellar radiation bc most of their light doesn t come from stars Active galactic nuclei are unusual bc their luminosity is dominated by activity in and around the galactic center Active galaxies are classi ed into 3 types Seyfert galaxies radio galaxies and quasars Seyfert galaxies resemble normal spiral galaxies but their cores are thousands of times more luminous Radio galaxies emit very strongly in the radio portion of the sptectrum they have enormous lobes invisible to optical telescopes perpendicular to the plane of the galaxy o Centaurus A Active Galactic Nuclei Quasars quasistellar objects are starlike in appearance but have very unusual spectral lines Eventually it was realized that quasar spectra were normal but enormously redshifted The Central Engine of an Active Galaxy Active galactic nuclei have some or all of the following properties 0 High luminosity o Nonstellar energy emission 0 Variable energy output indicating a small nucleus 0 Jets and other signs of explosive activity 0 Broad emission lines indicating rapid rotation Leading theory for energy source in an active galactic nucleus A black hole surrounded by an accretion disk the strong magnetic eld around the black hole channels particles into jets In an active galaxy the central black hole may be billions of solar masses but it s only about 20 AU in size the Schwarzschild radius The accretion disk is whole clouds of interstellar gas and dust they may radiate away as much as 1020 of their mass before disappearing a black hole in the center of a quasar might consume 10 stars per year The central portion of M87 shows rapid motion and jets characteristic of material surrounding a black hole red and blue shift of spectral lines gives the velocity of the spinning disk 0 Size few parsecs mass 3 billion solar masses Cosmic Distances Some galaxies have no cepheids and most are farther away than 25 Mpc The TullyFisher relation correlates a galaxy s rotation speed measure w Doppler effect to its luminosity higher rotation speed larger mass more stars 9higher luminosity Distances can be measured to about 200 Mpc Mega parsec million parsecs with the TF relation Type Ia Supernovae as Standard Candles Mass of white dwarf 14 solar masses Chandrasekhar Limitz accretes material mass increases core collapses star explodes 9 supernova Supernova are Extremely bright outshining the entire galaxy of stars in which they reside Easily detected but very rare occurring only about once every 50 years in our galaxy Very uniform in brightness at max light They can be used as a standard candle The Cosmic Distance Ladder With these additions the cosmic distance ladder can be extended to about 1 Gpc gigaparsec a billion parsecs Local Group of Galaxies 54 galaxies within about 1 Mpc of the Milky Way 3 spirals in this group Milky Way Andromeda M31 and M33 These and their satellites form the Local Group This type of group of galaxies is also called a galaxy cluster Dust in the plane of the Galactic disk obscures any additional dwarf galaxies that may be behind it The nearest rich galaxy cluster is the Virgo cluster much larger than Local Group contains 2000 galaxies spans over 10 degrees on the sky diameter of 9 million ly center is 16 Mpc away 0 M86 Virgo s massive central galaxy 50 kpc in diameter SO type or lenticular galaxy Members of a galaxy cluster are in constant motion around each other The Local Supercluster Clusters of the galaxies are themselves grouped together in huge associations called superclusters A typical supercluster contains dozens of individual clusters spread over a region of space up to 150 million ly across Virgo Supercluster is another name for the Local Supercluster Structure in the Nearby Universe Even superclusters aren t randomly distributed but lie along filaments The decreasing density of galaxies at the farthest distance is due to the dif culty of observing them Hubble Ultra Deep Field an 11 day HST exposure of a narrow pencil beam of sky consisting of 10000 galaxies Galaxy Formation Large systems are built up from smaller onces through collisions and mergers this is called the hierarchical theory of galaxy formation The protogalactic fragments galaxy shardsfragments will merge to form a galaxy 0 We see them as they were 10 billion years ago The Sloan Digital Sky Survey has mapped out a quarter of the sky Cosmology the Big Bang and Fate of the Universe Redshift and Hubble s Law Universal recession all galaxies seem to be moving away from us the redshift of their motion is correlated with their distance Recessional velocity Ho x distance Currently accepted value for Hubble s constant Ho 6780 077 kmsMpc The farther away a galaxy is the faster it s moving away from us Hubble s Law has important implications for how our universe behaves it s the rate of expansion of the Universe the subscript 0 indicates the current rate of expansion Measuring the redshift of a galaxy gives its recession velocity which in turn through Hubble s Law gives its distance Redshift is proxy for distance high redshift means very far away Although the high redshifts of quasars imply that they must be enormously far away and therefore enormously luminous in fact they re the most luminous objects in the Universe Hubble s Law puts the nal step on the Distance Ladder
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