Chapter 20, 21, 22, and 23 Bundle
Chapter 20, 21, 22, and 23 Bundle ASTR 1020
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Chapter 20: Galaxies & the Foundation of Modern Cosmology In Class Notes: Before Hubble, some scientists argued that “spiral nebulae” were entire galaxies like the Milky Way, while others maintained they were smaller collections of stars within the Milky Way. The debate remained unsettled until Edwin Hubble finally measured their distances. Great Debate of 1920: Shapely vs Curtis 1. The light curve of this pulsating variable star shows that its brightness alternately rises and falls over a 50-day period. 2. Hubble calculated a distance to M31 of 300000 PC; far greater than the ~50000 pc estimate of the Milky Way. 3. Andromeda Nebula = Andromeda Galaxy 4. Conclusion to debate: - Other galaxies - Milky Way actually smaller than estimate - Andromeda farther that estimate A galaxy’s age, its distance, and the age of the universe are all closely related. The study of galaxies is thus intimately connected with cosmology – the study of the structure and evolution of the universe. 3 major types of galaxies - Spiral galaxy - Elliptical galaxy - Irregular galaxy Spiral Galaxy - Stars of all ages and many gas clouds - Bulge and halo, old stars, few gas clouds - Blue-white color = ongoing star formation - Red-yellow color = older stars - Barred spiral galaxy has a bar of stars across the bulge - Often found in groups of galaxies (up to a few dozen galaxies) Elliptical galaxy - All spheroidal component, virtually no disk component - Red-yellow color indicates older star population - More common in huge clusters (hundreds to thousands of galaxies) Irregular galaxy - Blue-white color indicates ongoing star formation Brightness alone does not provide enough information to measure the distance to an object. Step 1: Determine the size of the solar system using radar Step 2: Determine the distances of stars out to a few hundred light-years using parallax White-dwarf supernovae can be used as standard candles. Slipher: the spectral features of virtually all galaxies are redshifted, which means that they’re all moving away from us. Modern Day Hubble’s Law Velocity = H 0 Distance or V=H D 0 H 0 22 km/s/Mly Redshift of a galaxy tells us its distance through Hubble’s law. (A particular galaxy originally emits this spectrum of light, but the spectrum we observe from it is shifted by 5% to longer wavelengths, indicating that the galaxy is moving away from us at 5% the speed of light) Velocity Distance = H 0 What does Hubble’s Law tell us about the age of the universe? 1. The expansion rate appears to be the same everywhere in space. The universe has no center and no edge (as far as we can tell). 2. One example of something that expands but has no center or edge is the surface of a balloon. 3. Hubble’s constant tells us the age of the universe because it related the velocities and distances of all galaxies. Age = Distance Velocity How does expansion affect distance measurements? 1. Expansion stretches photon wavelengths, causing a cosmological redshift directly related to look back time. 2. Distances between faraway galaxies change….. Why does the observable universe have a horizon? 1. Horizon = 14 billion light-years = age of universe 2. The cosmological horizon marks the limits of the observable universe. 3. It is a horizon in time rather than space. Since looking far away means looking back in time, there must be a limit – the beginning of the universe. Textbook Notes: The Hubble Deep Field is an image of a tiny patch of the sky that is taken by the Hubble Space Telescope. Spiral Galaxies - Example: Milky Way - Look like flat white disks with yellow bulge at the center - The disk population consists of stars in the disk of the galaxy - The spheroidal population consists of halo and bulge stars - All spiral galaxies have disk and spheroidal component - Large galaxies Elliptical Galaxies - Only have a spheroidal component - Sometimes called spheroidal galaxies - Giant elliptical galaxies are relatively rare and among biggest galaxies in universe - Dwarf elliptical galaxies are most common and have fewer than a billion stars; often found near larger spiral galaxies - Contain very little dust or cool gas - Little to no ongoing star formation - Red or yellow in color - Large range of sizes Irregular Galaxies - Small galaxies - Example: magellanic clouds & larger “peculiar” galaxies - Distant galaxies more likely to be irregular - More common when the universe was young Hubble’s Galaxy Classes proved to be of little scientific significance. A diagram, similar to an H-R diagram was created and plotted galaxies by color and luminosity. This created two major groups: blue cloud and red sequence. Radar Ranging is a technique astronomers use in which radio waves are transmitted from earth and bounced off a space object. Standard candles are light sources of a known, standard luminosity. Throughout the 20 century, one of the most important standard candle tchniques relied on the study of main-sequence stars. Apparent Brightness = Luminosity2 Distance = 4π∗(Distance) Luminosity √ 4π∗(Apparent Brightness) Example: The main sequence of the Hyades is 7.5 times as bright as that of the Pleiades, so the Pleiades must be√7.5≈ 2.75 times as far away. For measuring distances to other galaxies, the most useful standard candles are Cepheid variable stars. All Cepheids of a particular period have very nearly the same luminosity. Type I Cepheids: Sun-like heavy element content; found in galaxy disk Type II Cepheids: lower heavy-element conten; found in halo of galaxy The Great Debate Harlow Shapely: Spiral nebulae were gas clouds internal to the Milky Way Heber Curtis: In favor of Kant’s island of stars Edwin Hubble: Studied Andromeda spiral and used Henrietta Leavitt’s period-luminosity relation to estimate luminosities of Cepheids in Andromeda; underestimated distances of Cepheids but still proved that Andromeda was far beyond the Milky Way; ended debate The spectra of most spiral galaxies are redshifted; Hubble concluded: the more distant a galaxy, the greater its redshift and hence faster it moves away from us. λ −λ Redshift (z) = observedrest v = c * z λ rest Hubble’s Law: the idea that more distant galaxies move away from us faster is expressed with the formula… v = H 0 d Distance Chain Summary - Radar Ranging (bouncing radio waves off venus) - Parallax (observing how star positions change) - Main-sequence fitting (comparing apparent brightness of main- sequence stars in Hyades cluster with other clusters) - Cepheid variables (period-luminosity relation) - Distant standards (with previous step, we can learn luminosities of white dwarf supernovae) - Hubble’s Law (use H to0determine galaxy’s distance from its redshift) Cosmological Principle is the idea that the matter in the universe is evenly distributed, without a center or an edge. Example: As a balloon expands, the dots move apart in the same way that galaxies move apart in our expanding universe. The time it takes photons to reach us from a galaxy is called the lookback time. The expansion of the universe stretches out all the photons within it, shifting them to longer, redder wavelengths called a cosmological redshift. The cosmological horizon that marks the limits of the observable universe is a boundary in time, not space. Example: If the universe is 14 billion years old, then no object can have a lookback time greater than 14 billion years; limit of observable universe. Chapter 21 Section 3: Galaxy Evolution (Quasars) In Class Notes: - The highly redshifted spectra of quasars indicate large distances (Hubble’s Law). - From brightness and distance we find that luminosities of some quasars are greater than 10 12L . sun - Variability shows that all this energy comes from a region about the size of a solar system. Accretion of gas onto a supermassive black hole appears to be the only way to explain all the properties of quasars. Energy from a Black Hole 1. Gravitational potential energy of matter falling into black hole turns to kinetic energy. 2. Friction in an accretion disk turns kinetic energy into thermal energy (heat). 3. Heat produces thermal radiation (photons). 2 4. This process can convert 10 to 40% of E=mc into radiation. Jets are thought to come from twisting of magnetic field in the inner part of accretion disk. Reminder: Orbits of stars at the center of the Milky Way indicate a black hole with a mass of 4 x 10 M 6 sun rd Newton’s Version of Kepler’s 3 Law: The orbital speed and distance of gas orbiting the center of Galaxy M87 indicate a black hole with mass of 3 billion M sun (3 x 9 10 M sun Textbook Notes: Unusually bright galactic centers are called active galactic nuclei, and the brightest active galactic nuclei are quasars. Most powerful quasars produce more light than 1000 galaxies the size of the Milky Way. The energy output of a quasar comes form a gigantic accretion disk surrounding a supermassive black hole – a black hole with a mass millions to billions of times that of our Sun. Discovery of Quasars 1. Maarten Schmidt 2. Radio astronomers would tell him the coordinates of newly discovered radio sources, and he would try to match them with objects seen through visible-light telescopes. 3. A radio source, 3C 273, looked like a blue star through a telescope, but had abnormal emission lines. 4. Abnormal emission lines were not from an unfamiliar element; they were hydrogen emission lines that were hugely redshifted. 39 5. Luminosity of 10 watts The incredible luminosities of active galactic nuclei and quasars are being generated in a volume of space not much bigger than our solar system. 1% of present-day galaxies contain less active galactic nuclei and are often called Seyfert galaxies after Carl Seyfert, who grouped galaxies with active galactic nuclei into a special class. 90% of quasars are weak sources of radio waves, while 10% have stronger radio emission. Radio galaxies are certain galaxies that radio astronomers noticed that emit unusually strong radio waves. Possible theory to explain jets: 1. Magnetic fields thread the accretion disk surrounding a black hole. 2. As the accretion disk spins, it twists the magnetic field lines. 3. Charged particles at the accretion disk’s surface then fly outward along the twisted magnetic field lines. Mass accreted onto a black hole: The fact that 10-40% of the mass-energy of matter falling into a black hole is radiated away as energy allows us to determine how much mass is accreting onto the black hole in an active galactic nucleus. 10∗E Accreted mass = m = c2 Galaxies with large bulges have large black holes, while those with smaller bulges have smaller black holes. How interstellar hydrogen clouds affect the spectra of Quasars: 1. Absorbed by cloud 1 - Light from a quasar passes through many hydrogen clouds on its way to Earth. 2. Absorbed by cloud 2 - Each hydrogen cloud absorbs light of a particular wavelength. 3. Absorbed by cloud 3 - Clouds at different distances produce lines with different redshifts. 4. Final absorption spectrum - Each line in the quasar spectrum that reaches Earth tells us about a unique cloud. Weighing a supermassive black hole: 2 M = r∗v r G How are quasars powered? 1. Some galaxies have unusually bright centers known as active galactic nuclei; the most luminous of these are known as quasars. 2. Quasars are generally found at very great distances, telling us that they were much more common early in the history of the universe. 3. Quasars and other active galactic nuclei are thought to be powered by supermassive black holes. 4. As matter falls into a supermassive black hole through an accretion disk, its gravitational potential energy is efficiently transformed into thermal energy and then into light. Do supermassive black holes really exist? 1. Observations of orbiting stars and gas clouds in the nuclei of galaxies suggest that all galaxies may harbor supermassive black holes at their centers. 2. The masses we measure for central black holes are closely related to the properties of the galaxy around them, suggesting that the growth of these black holes is closely tied to the process of galaxy evolution. How do quasars let us study gas between the galaxies? 1. Each cloud of gas through which the quasar’s light passes on its long journey to Earth produces a hydrogen absorption line in the quasar spectrum. 2. Study of these absorption lines in quasar spectra allows us to study matter – including protogalactic clouds – that we cannot otherwise detect. Chapter 22: The Birth of the Universe In Class Notes: “Rewinding” the expansion of the universe tells us that the early universe must have been extremely hot and dense. Particle Creation: The early universe was full of particles and radiation because of its high temperature… 2 E = mc Particle Annihilation: Photons converted into particle-antiparticle pairs and vise verse Four known forces in universe: - Strong force - Electromagnetism - Weak force - Gravity Planck Era: before Planck time (~10 -4second); no theory of quantum gravity GUT Era: last from Planck time (~10 -43second) to end of GUT force (~10 -38second) -38 Electroweak Era: Lasts from end of GUT force (~10 second) to end of electroweak force (~10 -10second); pair production and annihilation Particle Era: pair production and annihilation ends as universe expands and cools; amounts of matter and antimatter nearly, but NOT equal (roughly 1 extra proton for every 10 proton – antiproton pairs!) Era of Nucleosynthesis: begins when matter annihilates remaining antimatter at ~0.001 second; protons and neutrons fuse into slightly heavier nuclei – mostly Helium, with some trace amounts of Deuterium and Lithium; fusion ceases t~5 minutes after Big Bang. E-a of Nuclei: universe still too hot for atoms to form; only H, He, and e . Era of Atoms: atoms form at age ~ 380,000 years; background radiation released. Era of Galaxies: galaxies form at age ~ 1 billion years. Primary evidence for Big Bang: 1. We have detected the leftover radiation from the Big Bang. 2. The Big Bang theory correctly predicts the abundance of Helium and other elements. The cosmic microwave background – the radiation left over from the Big Bang – was detected by Penzias and Wilson in 1965; awarded Nobel Prize in 1978. Background radiation from the Big Bang has been freely streaming across the universe since atoms formed at temperature ~ 3000 K: visible/IR. Background has perfect thermal radiation spectrum at temperature 2.73 K. (COBE) Expansion of the universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves. WMAP gives us detailed baby pictures of structure in the universe. How do abundances support the Big Bang theory? 1. Step 1: Proton and neutron fuse to form a deuterium nucleus. Step 2: Two deuterium nuclei fuse to make hydrogen-3. Step 3: Hydrogen-3 fuses with deuterium to create helium-4. 2. Protons and neutrons combined to make long-lasting helium nuclei when universe was ~ 5 minutes old. Big Bang theory prediction: 75% H, 25% He (by mass); this prediction matches observations of primordial gases. Before Helium synthesis: 14 protons, 2 neutrons After Helium synthesis: 12 Hydrogen, 1Helium Structure Problem: Where does the structure come from? Horizon Problem: Why is the overall distribution of matter so uniform? Flatness Problem: Why is the density of the universe so close to the critical density? Inflation can make all the structure by stretching tiny quantum ripples to enormous size; these ripples in density then become the seeds for all structures in the universe. Horizon problem: How can microwave temperature be nearly identical on opposites sides of the sky? Inflation solves Horizon Problem: regions now on opposite sides of the sky were close together before inflation pushed them far apart. Overall geometry of the universe is closely related to total density of matter and energy. Inflation solves flatness problem: inflation of the universe flattens its overall geometry like the inflation of a balloon, causing the overall density of matter plus energy to be very close to the critical density. Patterns observed by WMAP show us the “seeds” of structure in the universe. Overall geometry is flat - Total mass + energy has critical density Ordinary matter is ~ 4.4% of total Total matter is ~ 27% of total - Dark matter is ~ 23% of total - Dark energy is ~ 73% of total Age is 13.7 billion years Textbook Notes: The scientific theory that predicts what the universe was like early in time is called the Big Bang theory; it describes how expansion and cooling of this unimaginably intense mixture of particles and photons could have led to the present universe of stars and galaxies. The early universe was hotter and denser and cooled as it expanded. The electron is a particle of matter, and the antielectron is a particle of antimatter; when an electron and an antielectron meet, they annihilate each other which transforms all of their mass-energy back into photon energy. Everything that is happening in the universe today is governed by four distinct forces: 1. Gravity – the “glue” that holds planets, stars, and galaxies together. 2. Electromagnetism – the dominant force between particles in atoms and molecules; responsible for all chemical and biological reactions. 3. The strong force – binds atomic nuclei together. 4. The weak force – fission and fusion; only force besides gravity that affects weakly interacting particles such as neutrinos. The four forces are distinct at low temperatures but may merge at very high temperatures, such as those that prevailed during the first fraction of a second after the Big Bang. High temperatures can cause the merging of electromagnetic and weak forces to make the electroweak force. Grand unified theories, or GUTs, are theories that predict the merger of the electroweak and strong forces. Planck Era: first era after the Big Bang; represents time before the universe was 10 -43seconds old; temperatures greater than 10 32K; we lack a theory to describe conditions in the Planck era. GUT Era: two forces (gravity and the GUT force) are thought to have 29 operated during the GUT era; temperatures above 10 K; time when universe was 10 -38seconds old; inflation (the freezing out of the strong and electroweak forces release an enormous amount of energy, causing a sudden and dramatic expansion of the universe). Electroweak Era: elementary particles appeared spontaneously from energy, but also transformed rapidly back into energy; three distinct forces (gravity, the strong force, and the electroweak force) operate; 15 -10 temperatures of 10 K and age of 10 seconds old. Particle Era: elementary particles filled the universe, then quarks combined to make protons and antiprotons; all quarks had combined into protons and neutrons by the end of the particle era; era 12ded when universe was I millisecond and temperature was 10 K. Era of Nucleosynthesis: fusion produced helium from protons (H nuclei); protons and neutrons fuse into heavier nuclei; gamma rays destroyed the nuclei; ended when universe was 5 minutes old and temperature was 1 billion K. Era of Nuclei: a plasma of free electrons and H and He nuclei filled the universe; universe consisted of hydrogen nuclei, helium nuclei, and free electrons for 380,000 years; temperature 3000 K; hydrogen and helium nuclei formed stable atoms and free photons scattered across the universe. Era of Atoms: the era of atoms lasted until stars and galaxies began to form; universe consisted of neutral atoms and plasma; gravity caused protogalactic clouds; stars formed in these clouds and clouds merged to form galaxies. Era of Galaxies: the birth of stars and galaxies when the universe was a few hundred million years old; 1 billion years old. Evidence for the Big Bang: 1. Cosmic microwave background (radiation that began at the end of the era of nuclei) is still present today. 2. Observations of the actual helium content of the universe closely match the amount predicted by the Big Bang theory. Inflation is very rapid expansion as the universe cooled down. Arno Penzias and Robert Wilson discovered the cosmic microwave background with the Bell Labs microwave antenna. During the 5-minute period in the era of nucleosynthesis, almost all neutrons in the universe fused with protons to form helium-4. During inflation, ripples in spacetime would have stretched by a factor of 10 . Without inflation, light would have left the microwave-emitting regions that are on the opposite sides of the universe long before they could have communicated with each other and equalized their temperatures. With inflation, regions A and B could have been close enough to communicate and equalize before inflation pushed them far apart; we can see A and B but they cannot see each other. average density of matter + energy = critical density average density < critical density = saddle-shaped average density > critical density = spherical As a balloon expands, its surface seems increasingly flat to an ant crawling along it; inflation is thought to have made the universe seem flat in a similar way. Oblers’ paradox: if the universe were infinite, unchanging, and everywhere the same, then the entire night sky would blaze as brightly as the Sun; Big Bang theory solves paradox because universe is a finite age. Chemical composition of universe: 75% hydrogen, 25% helium Chapter 23: Dark Matter, Dark Energy, and the Fate of the Universe In Class Notes: Dark Matter: an undetected form of mass that emits little to no light, but whose existence we infer from its gravitational influence. Dark Energy: an unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate. “Ordinary” matter: ~ 4.4% - Ordinary matter inside stars: ~ 0.6% - Ordinary matter outside stars: ~ 3.8% Dark matter: ~ 23% Dark energy: ~ 73% We measure the mass of the solar system using the orbits of planets: - Orbital period Rotation curve 1. A plot of orbital velocity versus orbital radius. 2. The solar system’s rotation curve declines because the Sun has almost all the mass. 3. The rotation curve of the Milky Way stays flat with distance; mass must be more spread out than in the solar system. 11 Mass within the Sun’s orbit: 1.0 * 10 Msun The visible portion of a galaxy lies deep in the heart of a large halo of dark matter. We can measure the rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic hydrogen. Spiral galaxies all tend to have flat rotation curves, indicating large amounts of dark matter. We can measure the velocities of galaxies in a cluster from their Doppler shifts. The mass we find from galaxy motions in a cluster is about 50 times larger than the mass of the stars in the cluster. Clusters contain large amounts of X ray-emitting hot gas; temperature of hot gas (particle motions) tells us cluster mass (Math insight 23.3, page 674): 85% dark matter 13% hot gas 2% stars Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass. All three methods of measuring cluster mass indicate similar amounts of dark matter in galaxy clusters. Our options: 1. Dark matter really exists, and we are observing the effects of its gravitational attraction. 2. Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter. Because gravity is so well tested, most astronomers prefer option #1. Some observations of the universe are very difficult to explain without dark matter. Two basic options: 1. Ordinary dark matter a. Matter made of protons, neutrons, electrons, but too dark to detect with current instruments 2. Extraordinary dark matter a. Weakly interacting massive particles – mysterious neutrino- like particles Option #2 is best option. Measurements of light element abundances indicate that…. Why believe in WIMPs? 1. There’s not enough ordinary matter. 2. WIMPs could be left over from the Big Bang. 3. Models involving WIMPs explain how galaxy formation works. Textbook Notes: Dark matter is matter that gives off little to no light; also called transparent matter because light passes through it without interacting. Dark energy is the mysterious force that is counteracting the effects of gravity on very large scales, causing the expansion of the universe to accelerate. The amount of dark matter in a galaxy is determined by comparing the galaxy’s mass to its luminosity. The evidence for dark matter in clusters comes from three different ways of measuring cluster masses: - measuring the speeds of galaxies orbiting the center of the cluster - studying the x-ray emission from hot gas between the cluster’s galaxies - observing how the clusters bend light as gravitational lenses Fritz Zwicky discovered dark matter in clusters of galaxies. Finding cluster masses from galaxy orbits:2 r∗v M r G Gravitational equilibrium is when the outward gas pressure balances gravity’s inward pull. Gravitational lensing occurs because masses distort spacetime; massive objects act as gravitational lenses that bend light beams passing nearby; through a telescope we see multiple images of what is really a single galaxy. By measuring these distortions, astronomers can determine the total amount of mass in the cluster. What is dark matter made of? 1. It could be ordinary matter – protons, neutrons, and electrons 2. It could be one or more forms of exotic matter – particles of matter that are different from what we find in ordinary atoms and that do not interact with light at all 3. There is not enough ordinary matter based on the Big Bang model. 4. WIMPs, weakly interacting massive particles, are the leading explanation for dark matter. Four expansion models: 1. Recollapsing universe: in the case of extremely strong gravitational attraction and no repulsive force, the expansion would continually slow down with time and eventually would stop entirely and then reverse. 2. Critical universe: in the case of gravitational attraction that was not quite strong enough to reverse the expansion in the absence of a repulsive force, the expansion would decelerate forever, leading to a universe that would never collapse but would expand ever more slowly as time progressed. 3. Coasting universe: in the case of weak gravitational attraction and no repulsive force, galaxies would always move apart at approximately the speeds they have today. 4. Accelerating universe: In the case of a repulsive force strong enough to overpower gravity, the expansion would accelerate with time. Inventory of the universe: 1. Ordinary matter: makes up slightly more than 4% of the total mass-energy of the universe 2. Exotic dark matter makes up about 22% of the mass-energy of the universe 3. Dark energy makes up the remaining 74% of the mass-energy of the universe
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