Introduction to Stars, Galaxies, and the Universe
Introduction to Stars, Galaxies, and the Universe ASTR 1220
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Chapter 21 Notes Galaxy Evolution 0 Looking Back Through Time I How do we observe the life histories of galaxies 0 Pictures are taken of single galaxies at a single stage in its life and measuring the redshift of each galaxy allows us to place it on a timeline of the universe I How did galaxies form 0 Most Successful models for galaxy formation 0 Hydrogen and helium gas filled all of space more or less uniformly when the universe was very young say in the first million years after its birth 0 The distribution of matter in the universe was not perfectly uniform certain regions of the universe started out ever so slightly denser than others 0 We model galaxies off of this assumption with well established laws ofphysics to trace how the denser regions in the early universe grew into galaxies 0 Many galaxies have formed from the merger of multiple protogalactic clouds o The Lives of Galaxies I Why do galaxies differ 0 So if galaxies form the same way why are there different ones 0 Galaxies may have ended up looking different because they began with slightly different conditions in their protogalactic clouds o Galaxies may have begun their lives similarly but later changed due to interactions with other galaxies 0 Conditions in the Protogalactic Cloud 0 Two plausible explanations for the differences between spiral and elliptical galaxies trace a galaxy s type back to the protogalactic cloud from which it formed I Protogalactic spin A galaxy s type might be determined by the spin of the protogalactic cloud from which it formed If the original cloud had a significant amount of angular momentum it would have rotated quickly as it collapsed The galaxy it produced would therefore have tended to form a disk and the resulting galaxy would be a spiral galaxy If the protogalactic cloud had little or no angular momentum its gas might not have formed a disk at all and the resulting galaxy would be elliptical Protogalactic density A galaxy s type might be determined by the density of the protogalactic cloud from which it formed A protogalactic cloud with relatively high gas density would have radiated energy more effectively and cooled more quickly thereby allowing more rapid star formation If the star formation proceeded fast enough all the gas could have been turned into stars before any of it had time to settle into a disk The resulting galaxy would therefore lack a disk making it an elliptical galaxy In contrast a lowerdensity cloud would have formed stars more slowly leaving plenty of gas to form the disk of a spiral galaxy o Galactic Collisions o The latter two situations ignore one key fact Galaxies rarely evolve in perfect isolation o Galaxies are always surrounded or close to other nearby galaxies o Galaxies in Clusters o Galaxies in clusters support the idea that at least some elliptical result from collisions and subsequent mergers o Stronger evidence comes from structural details of elliptical galaxies which often attest to a violent past 0 Most decisive evidence that collisions affect evolution of elliptical galaxies comes from observations of the central dominant galaxies I Found at the center of many clusters I Giant elliptical galaxies that apparently grew to a huge size by consuming other galaxies through collisions o Galactic cannibalism I What are starbursts o Starbursts galaxies small percentage of the galaxies in the presentday universe are currently forming stars at a prodigious rate 0 Observations of Starbursts Galaxies 0 Dust grains in the molecular cloud absorb most of the visible and ultraviolet radiation streaming from the many hot young stars ofa starburst galaxy 0 Because we cannot see the active star formation in starburst galaxies with visible light astronomers didn t recognize their nature until they began to study them in longwavelength infrared light light that can be observed only with telescopes in space 0 Galactic Winds o The hot bubbles created by supernovae burst and when the hot gas erupts into intergalactic space it s created into galactic wind 0 Quasars and Other Active Galactic Nuclei I Some galaxies display an incredible phenomena of extreme amounts of radiation and sometimes powerful jets of material emanating from deep in their centers 0 Unusually bright galactic centers are called active galactic nuclei 0 Brightest active nuclei are know as quasars I What are quasars o The energy output ofa quasar comes from a gigantic accretion disk surrounding a supermassive black hole a black hole with a mass millions to billions of times that of our sun The Discovery of Quasars o Maarten Schmidt was busy identifying cosmic sources of radiowave emission o Realized wavelengths he couldn t identify were emission lines of hydrogen that were hugely redshifted from their normal wavelengths o The light we see from the farthest known quasar began its journey when the universe was less than one billion years old 0 Evidence from Nearby Active Galactic Nuclei 0 About 1 ofpresentday galaxies that is galaxies we see nearby have active galactic nuclei that look very much like quasars except that they are less powerful 0 Radio Galaxies and lets 0 Radio astronomers noticed that certain galaxies now called radio galaxies emit unusually strong radio waves I Learned that much of the radio emission comes not from the galaxies themselves but rather from pairs of huge radio lobes one on either side of the galaxy Radio galaxy I The active galactic nucleus is the power source and it drives two jets ofparticles that stream outward in opposite directions at nearly the speed of light I These jets shoot out far beyond the bounds of the stars in the radio galaxy but they eventually ram into surrounding intergalactic gas What is the power source for quasars and other active galactic nuclei 0 One explanation seems to fit how radio galaxies quasars and other active galactic nuclei release so much energy within such small central volumes 0 The energy comes from matter falling into a supermassive black hole Gravitational potential energy of matter falling toward the black hole is converted into kinetic energy and collisions between infalling particles convert the kinetic energy into thermal energy Do supermassive black holes really exist 0 Hunting for Supermassive Black Holes 0 Detailed observations of matter orbiting at the centers of nearby galaxies suggest that supermassive black holes are quite common 0 Black Holes and Galaxy Formation 0 Evidence for supermassive black holes is also found in galaxies whose centers are not currently active and the masses of those black holes follow a very interesting pattern I Mass of the black hole at the center ofa galaxy appears to be closely related to the properties of the galaxy s spheroidal component Astronomers have long suspected that galaxy evolution goes hand in hand with the formation of supermassive black holes because quasars O O were so much more common early in time when galaxies were growing rapidly Through all the work done so far by astronomers the origins of supermassive black holes and their connection to galaxy evolution remain mysterious I How do quasars let us study gas between the galaxies 0 Most mysterious part of galaxy evolution is the part we ve not yet observed 0 The formation and development ofprotogalactic clouds o Quasar spectra contain valuable information about the properties of hydrogen clouds in the early universe 0 We are only beginning to learn how to read the clues that hydrogen absorption lines have etched into the spectra of quasars but the evidence gathered so far supports our general picture of spiral galaxy evolution 0 Chapter 19 Our Galaxy 191 The Milky Way Revealed 0 Band oflight we say in the sky on dark nights is the Milky Way 0 Galaxy echoes Greek word quotgalactosquot for milk What Does Our Galaxy Look Like 0 A vast spiral galaxy has spiraling arms Fairly at disc and bright center is called the bulge 100000 lightyears in diameter Our galaxy is relatively large in size I Andromeda is the closest in size How Do Stars Orbit In Our Galaxy 0 Orbits of Disk Stars I Gravity makes stars bob up and down like horses on a merry goround I Orbital velocities of stars near the edge of the galaxy and those near the center are about the same 0 Orbits Of Halo And Bulge Stars I Orbits of stars in the halo and bulge are much less organized I Neighboring halo stars can circle the galactic center in opposite directions They swoop from high above the disk to far below it and back again plunging through the disk at velocities so great that the disks gravity hardly alters their trajectories 0 Makes bulge much puffier than the rest of the galaxy Galactic Recycling 0 Howls Gas Recycled In Our Galaxy I Recycling process proceeds in several stages making up the stargasstar cycle I Stars are born when gravity causes the collapse ofmolecular clouds I After dying they ultimately return much of their material back to the interstellar medium I Gas is ejected by dying stars and finishes back at star birth 0 Gas From Dying Stars I All stars return much of their original mass to interstellar space in two basic ways through stellar winds that blow throughout their lives and through quotdeath events of planetary nebulae for lowmass stars or supernovae for highmass stars I Lowmass stars generally have weak stellar winds while on the main sequence I Winds grow stronger and carry more material into space when they become red giants I Highmass stars lose mass much more dynamically and explosively OOO Astronomy Chapter 23 Notes The Beginning of Time The Big Bang 0 What were the conditions really like in the early universe I Universe is cooling and becoming less dense as it expands I Must have been hot and denser in the past I Universe was so hot during the first few seconds that photons could transform themselves into matter and vice versa 0 In accordance with Einstein s formula E mcquot2 I Electron s a particle of matter I Antielectron is a particle of antimatter o The reaction that creates an electronantielectron pair also runs in reverse 0 When they meet they annihilate each other totally transforming all their massenergy back into photon energy I Early universe was hot dense and had photons matter and antimatter furiously converting back and forth 0 What is the history of the universe according to the Big Bang theory I Big Bang Theory scientific theory of the universe s earliest moments is based on applying known and tested laws of physics to the idea that all we see today from Earth to the cosmic horizon began as incredibly tiny hot and dense collection of matter and radiation I We divide each series of the universe s life into sets of eras o Planck Era is the limit to as far back was when we can predict the physical conditions of the universe 0 GUT Era was when two forces operated in the universe gravity and the GUT force 0 GUT force is a unified force representing the merger of the strong weak and electromagnetic forces 0 Electroweak Era was when the GUT force split at the end of the GUT era and the universe entered an era during which three distinct forces operated gravity the strong force and the electroweak force 0 Indicated that the electromagnetic and weak forces were still unified in the electroweak force 0 Particle Era was the time between the end of the electroweak era and the moment when spontaneous particle production ceased to emphasize the importance of subatomic particles during this period Era of Nucleosynthesis was when the heat of the universe remained so high that most nuclei broke apart as fast as they formed Era of Nuclei was when the fully ionized nuclei moved independently of electrons during this period rather than being bound with electrons in neutral atoms 0 Throughout this era photons bounced rapidly from one electron to the next just as they do deep inside the Sun today Era of Atoms and Era of Galaxies 0 End of the era of nuclei marked the beginning of the era of atoms when the universe consisted of a mixture of neutral atoms and plasma along with a large number ofphotons o Era of galaxies was when the first full edged galaxies had formed when the galaxy was 1 billion years old Evidence for the Big Bang 0 Big Bang model predicts that the radiation that began to stream across the universe at the end of the era of nuclei should still be present today Sure enough we find that the universe is filled with what we call the cosmic microwave background Its characteristics precisely match what we expect according to the Big Bang Model 0 The Big Bang model predicts that some of the original hydrogen in the universe should have fused into helium during the era of nucleosynthesis Observations of the actual helium content of the universe closely match the amount of helium predicted by the Big Bang model 0 How do the abundances of elements support the Big Bang theory I Why didn t the Big Bang produce heavier elements By the time stable helium nuclei formed when the universe was about a minute old the temperature and density of the rapidly expanding universe had already dropped too far for a process like carbon production I Density of Ordinary Matter Calculations within the Big Bang model allow scientists to estimate the density of ordinary baryonic matter in the universe from the observed amount of deuterium in the universe today Observations show that about one out of every 40000 hydrogen atoms contains a deuterium nucleus that is a nucleus with a neutron in addition to its proton The Big Bang and In ation o What Aspects of the universe were originally unexplained by the Big Bang model I Where does the structure com e from Recall that our models of the formation of galaxies and larger structures all assume that gravity collected matter around regions of slightly enhanced density in the early universe Explaining the origin of structure requires that the Big Bang must have somehow produced these slight density enhancements The subtle temperature differences seen in the cosmic microwave background tell us that regions of enhanced density did indeed exist at the end of the era of nuclei when the universe was 380000 years old But we still need to explain where the density enhancements came from I Why is the largescale universe so uniform 7 Although the slight temperature variations in the cosmic microwave background show that the universe is not perfectly uniform on large scales the overall smoothness is nonetheless remarkable I Why is the density of the universe close to the critical density The total density of dark matter plus dark energy in the universe appears to be remarkably close to the critical density so close that it is difficult to consider it a coincidence After all there is no obvious reason why the density could not have been say 1000 times the critical density of 0000001 times the critical density But without in ation the Big Bang model is unable to explain the nearcritical density of the universe as anything other than luck 0 How can we test the idea of in ation I Strongest test ofin ation to date come from detailed studies of the cosmic microwave background and in particular of the map made by the WMAP satellite I Careful observations of the temperature variations in the microwave background can therefore tell us about the structure of the universe during its first instant existence Observing the Big Bang for Yourself 0 The Big Bang theory s very success has made it a target for respected scientists skeptical nonscientists and crackpots alike 0 Why is the darkness of the night sky evidence for the Big Bang I If the universe were infinite unchanging and everywhere the same then the entire night sky would blaze as brightly as the Sun 0 Olber s paradox I Big Bang solves paradox o Tells us that we can see only a finite number of stars because the universe began at a particular moment Chapter 22 Dark Matter Dark Energy and the Fate of the Universe Unseen In uences in the Cosmos o By carefully observing gravitational effects on matter than we can see such as stars or glowing clouds of gas we ve learned that there must be far more matter than meets the eye Because this matter appears to give off little or no light we call it dark matter 0 Dark matter is simply a name we give to whatever unseen in uence if causing the observed gravitational effects 0 Dark energy is the most common name given to whatever it is that may be causing the expansion to accelerate but it is not the only name I Quintessence or cosmological constant I Dark energy term is popular because it echoes dark matter Evidence for Dark Matter 0 A diagram called a rotation curve which plots the rotational velocity of stars or gas clouds against their distance form the center of the galaxy summarizes the results of these orbital velocity measurements I Example MerryGoRound 0 Every object on MGR goes around the center with the same rotational period but objects farther from the center move in larger circles In contrast the rotation curve for our solar system drops off with distance from the Sun I Closer planets orbit faster than outer planets The atness of the Milky Way s rotation curve once plotted on a graph implies that most of our galaxy s mass lies well beyond our Sun tens of thousands of lightyears form the galactic center For Elliptical Galaxies we generally weigh the inner parts of elliptical galaxies by observing the motions of the stars themselves We compare spectral lines form different regions of an elliptical galaxy and we find that the speeds of the stars remain fairly constant as we look farther from the galaxy s center Evidence for dark matter in cluster galaxies comes from three different ways ofmeasuring cluster masses I Measuring the speeds of galaxies orbiting the center of the clusters I Studying the Xray emission from hot gas between the cluster galaxies I Observing how the clusters bend light as gravitational lenses 0 Gravitational lenses bend light beams passing nearby All evidence for dark matter rest on our understanding of gravity I For individual galaxies the case for dark matter rests primarily on applying Newton s laws of motion and gravity to observations of the orbital speeds of stars and gas clouds O O O O O O O ASTR124 CHAPTER 22 DARK MATTER DARK ENERGY AND THE FATE OF THE UNIVERSE 221 UNSEEN INFLUENCES IN THE COSMOS we don t know what the universe is made of I Why when we know what stars and gas clouds are made up of He with small amounts of heavier elements 0 All familiar objects are made of atoms but this may not be true of other things in the universe 0 They probably consist of lldark matter and lldark energy its nature is unknown A What do we mean by dark matter and dark energy Just names given to quotunseen influences in the cosmos By observing gravitational effects on matter that we can see starsglowing clouds of gas we see that there must be far more matter than meets the eye quotdark matter a Discussed briefly in Chapters 1 and 19 Milky Way s rotation suggests most of the mass is in the halo but most of the stars and gas clouds are in the galactic disk quotdark energy is the name given to whatever is causing the expansion of the universe to accelerate also called a Quintessence b Cosmologicalconstant Dark matter and dark energy exist for completely different reasons Astronomers find the existence of dark matter is more acceptable than the existence of dark energy a Dark matter is llindispensible to explaining the current structure of the universe I 222 EVIDENCE FOR DARK MATTER A What is the evidence for dark matter in galaxies a Distribution of Mass in the Milky Way i We determine the amount of mass within the Sun s orbit by observing its motion around the galaxy 1 The orbital motion of other stars can be used in the same way to measure the mass within that star s orbit 2 The complete distribution of mass in the Milky Way could be determined by doing this with orbits of stars at every different distance from the galactic center in principle but interstellar dust obscures the view of disk stars more than a few thousand LY away a Makes it difficult to detect velocity b Radio waves can penetrate the dust i Clouds of atomic H gas emit a spectral line at the radio A of 21 cm the orbital velocity of the cloud can be determined by measuring the Doppler shift I Rotation curve plots the rotational velocity of stars or gas clouds against their distance from the center of the galaxy and summarizes the results of these orbital velocity measurements p683 1 For our solar system the rotational curve drops off as you move further from the sun a The inner planets move faster than the outer planets this happens because most of the mass in the solar system is in the Sun b Gravitational force of holding a planet decreases as distance increases smaller force lower orbital speed c The rotation curve of any astronomy system whose mass is concentrated toward the center must drop similarly 2 For the Milky Way the rotation curve remains constant beyond a few thousand LY making the curve flat a which contrasts with the evidence found within our solar system b therefore most of the mass must NOT be concentrated at the center of the galaxy the orbits of more distant gas clouds must encircle more and more mass b Dark Matter in Other Spiral Galleries i Other galaxies contain large amounts of dark matter this can be determined by comparing the galaxies mass to its luminosity 1 Luminosity is used to determine the mass the galaxy contains in the form of stars 2 The total mass is determined by observing the orbital velocities of stars and gas clouds 3 If the total mass is gt the mss of the stars there must be dark matter a Measuring the total mass involves measuring the orbital speeds of starsgas clouds as far from the galaxy s center as possible 4 Measuringluminosity a Measure brightness b Calculate luminosity by distance and the inverse square law of light 5 Rotational curve is surprisingly flat a lmply that there is a great deal of matter in the halos b 10x more mass in dark matter than in stars c Dark Matter in Elliptical Galaxies i Different technique must be used because they do not produce the detectable 21cm radiation ii Motions of the starts themselves must be observed 1 disorganized so the velocities cannot be organized into a sensible rotational curve 2 velocity is measured using the mass within the star s orbit as usual a some move towards us and some move away so the Doppler Shift can be observed 3 combined effects of these Doppler shifts can be seen in a spectral line which indicates wavelength iii Evidence 1Speed of stars remain constant as you look further from the center of the galaxy B What is the evidence for dark matter in clusters of galaxies a In single galaxies dark matter makes up 90 of the mass In cmsters ofga ames observauons suggest tnat even a nrgner MATHEMATICAL INSIGHT 3 S 3 s ofga ames orbmngthe center ofthe dusts erav ermssrons from not gas between tne duster ga ames Howtne duster bend hgm as gmwmnnna lenses d OrbwtsofGa amesm C usters r 15305 astronomer FrancesZwrckvargded for theemstence ofdark matter 1 on nenrst z Thoughtthatgabmesma cmster Shou d orbrt each other We starsm acm ter dsrngtnerr red snnt mm 1121 m Hth m wackv swmm a eraged tne speed ofthe MW e duster ortne recessmn speed d sdbtracted tne recessmn speed Fromthe speed of eacn mdwwdua gaan to deterrmne tne speed re atwe to tbe c uster39s centerrthwsm r tne average radwa component e Esumated tne c uster39s mass and compared rttotne drmnosrtv r rodnttnattne mass gt drmnosrtv sdggests DARK MATrER e Hot Gas m duster r obsennng xrravsfrom notgas mtrac uster medmm between ga amesz l 3 co How rt teHs us about d srsa astate ofgrathauona eddmbndm be used to deterrmnetne 013 mass motrons 6 a smg e ga awmgether how r Grathauona Lensmg r Occurs because massesdrstort spacerume Massive objects acting as gravitational lenses bend light beams passing by Verified 1919 during eclipse of the sun Einstein s Theory of Relativity Light bending angle depends on the mass of the object doing the bending so mass can be measured by observing how strongly they distort light paths ii Careful analysis allows measurement of cluster masses without using Newton s laws C Does dark matter really exist W J a All evidence rests on our understanding of gravity and Newton s laws of motion and gravity b One of the following must be true i Dark matter exists ii There is something wrong with our understanding of gravity unlikely D What might dark matter be made of a Some must be quotordinaryquot made of protons electrons neutrons baryonic b The rest must be extraordinary made of particles that have not been discovered nonbaryonic c Ordinary dark matter i Dark llnot as bright as a normal star and therefore not visible across vast distances of space ex 1 Me 2 Everything I own 3 Earth amp other planets 4 llfailed stars brown dwarfs 5 Faint red mainsequence stars of spectral type M 6 MACHOs Massive Compact Halo Objects ii MACHOs too faint to be seen 1 Gravitational lensing will sometimes focus a star s light more directly at the earth rare a Duration of the lensing the object s mass 2 Do exist but not in large enough numbers to account for all dark matter d Extraordinary Dark Matter i Maybe Neutrinos dark by their nature 1 no electrical charge no radiation 2 never bind together cannot be revealed by lightemitting particles 3 they are llweakly interacting particles interact with other forms of matter through only 2 of the 4 forces gravity and the weak force 4 CAN T be neutrinos they travel too fast amp escape the galaxy s gravitational pull ii Other particles WlMPs 1 Weakly Interacting Massive Particles Heavier than neutrinos llcold dark matter Likely Dark Rarely interactexchange energy with other protons The most common form of matter in the universe Newbww 223 STRUCTURE FORMATION A What is the role of dark matter in galaxy formation a Must have provided the gravity to form most of the protogalactic clouds i H amp He collapsed to form stars ii WlMPs left out b Clusters still forming Virgo Cluster i 60 million LY away ii Appears to be drawing in the Milky Way and other galaxies of the Local Group pulling us against the universe s expansion iii Superclusters clusters of clusters gravitationally bound to each other B What are the largest structures in the universe a Mapping LargeScale Structures i Enormous amounts of data 1 Red shift data 2 Distance ii Galaxies are arranged in chains 1 Clusters located at the intersection of these chains 2 Between chains are empty areas voids iii Sloan Great Wall gt 1 billion LY from end to end b Origin of Large Structures i Galaxies clusters superclusters and the Sloan Great Wall probably all started as high density of different sizes ii Why did the universe start with slightly dense spots next chapter 224 THE FATE OF THE UNIVERSE A Will the universe continue expanding forever a Gravity expected to slow expansion i Strength of pull is dependent on the density of the matter in the universe greater density greater strength of gravity eventually halting expansion ii Gravity can win over expansion if the current density of the universe exceeds 10 29 gcm2 or a few H atoms in an average closet critical density iii Mass falls far short of critical density 1 visible parts of galaxy 05 of the matter density needed to halt the expansion 2 dark matter needs to be at least 200x that of the mass of stars a single galaxies have 10x mass of dark matter as they do of stars b cluster galaxies have 50x mass of dark matter as they do of stars c NOT 200x almost of what is needed B Is the expansion of the universe expanding a Appears to be speeding up dark energy b Four Expansion Patterns i Two possibilities expanding forever or collapsing divided into 4 categories 1 Recollapsing no dark energy matter density gt critical density a Universe ends in a fiery llBig Crunch b Aka llclosed universe 2 Critical universe no dark energy and the matter density is at the critical density a Never collapses b Expands slowly c Destiny critical density d Aka llflat universe 3 Coasting universe a No dark energy b Matter density smaller than critical density c Keep expanding with no change d llopen universe 4 Accelerating universe a Dark energy exerts a repulsive force b Universe accelerates with time c universe become cold and dark d could be openclosedflat