Astronomy Section 5
Astronomy Section 5 ASTRO103
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This 45 page Bundle was uploaded by Morgan Oestmann on Wednesday May 4, 2016. The Bundle belongs to ASTRO103 at University of Nebraska Lincoln taught by Michael Sibbernsen in Spring 2016. Since its upload, it has received 15 views. For similar materials see Introductory Astronomy in Astronomy at University of Nebraska Lincoln.
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Date Created: 05/04/16
Milky Way I Lecture Summary Appearance of the Milky Way Shapley and Hubble Globular and Open Clusters Metallicity o Population I and II stars General properties of the Milky Way o Halo o Disk Andromeda Galaxy 2,000,000 Light years away Herschel’s “Grinding Stone” Model Major Questions circa 1915 What is the distribution of stars? What is the size scale of the Galaxy/Universe? What are the spiral nebula? Globular Cluster Distribution The Great Debate Shapley-Curtis debate in 1920 before the National Academy of Sciences. Both gave talks entitled “The Scale of the Universe” Shapley argued that “spiral nebulae” were just nearby gas clouds and that the Universe was composed of only one big galaxy. He had the scale of the Milky Way and out location in it largely correct. Curtis argued that the universe was composed of many galaxies like our own. He had the sun at the center of a small Milky Way. Milky Way ~100,000,000+ stars ~100,000 Ly in diameter (30 kpc) Sun 8.5 kpc from the center Sun orbits at 220 km/sec 240 million years for one orbit What is a metal? To astronomers anything that isn’t hydrogen and helium Population I—Stars with high metal content (2%-3%)—disk stars Population II—stars with low metal content (<1%)—halo star Creation of Metallicity Disk vs Halo Disk o Thin—around 1 kpc in thickness o Young—age around 5 billion years o Location of Spiral Arms—apparent because of the bright, young O and B stars they contain o The stars are in rapid circular rotation about the center of the galaxy. The sun has a rotational velocity of 220 km/s. o Metal content similar to that of the sun—2-3%(POP. 1) o Contains large groups of stars called open clusters, gas, and dust Halo o Roughly spherical—50-100 kpc o Old—10-15 billion years o Little or no present star formation o Stars in random, energetic elliptical orbits—little or no rotation o Metal content- 1/100 to 1/1000 that of the sun (POP II) o Contains globular clusters, old evolved stars, and low mass stars Galaxy Formation Refinements Cosmology I Summary Olber’s Paradox Homogeneity and Isotropy Countless Galaxies Big Bang Observation: Night Sky is Dark The night sky is dark, but why? A Big Forest Suppose a forest is… o Infinitely Old o Static Then… o We would see a tree in every direction at every point Incorrect Assumptions Static—new stars are born and old stars die all the time Eternal—Our universe began about 14 billion years ago Infinite—Observable universe is not infinite. These are stars whose light has not yet had time to reach us Uniform Distribution of Stars—The formation of galaxies and clusters of galaxies disprove this Correct Assumptions Homogeneity—There is no special vantage point in the universe —every place is pretty much the same Isotropy—There is no special direction in the universe—things pretty much look the same in every direction Cosmological Principle—Any observer in any galaxy sees the same general features in the universe Big Bang General Ideas Universe started out very hot, very dense Universe stars to expand (Big Bang) Expansion cause the universe to o Be less dense o Cool of Galaxies Lecture Summary Galaxy Classification o Hubble Tuning Diagram o Spirals o Ellipticals o Irregulars Distance Ladder o Hubble’s Law Why are there different types of Galaxies? o Formation Theories o Collisions Spirals Contain disks of young stars and lots of gas and dust Active star formation occurring Around 77% of the galaxies we see are spirals 10 to 10 11solar masses About a fifth of spirals contain bars through their centers Our Milky Way has a bar S0 Galaxies Spirals that have no spiral arms, very little gas and dust, very few hot, bright stars – but, an obvious disk component Ellipticals No visible gas, dust, or bright stars Recent star formation is not observed Consist primarily of old stars About 20% of the galaxies we see are ellipticals 10 to 10 13solar masses Largest galaxies Smallest galaxies o Dwarf ellipticals or dwarf spheroidal galaxies Irregular No predictable structure Both young and old stars are present Faint, difficult to detect Only about 3% of galaxies are irregular Very few are seen 8 10 10 to 10 solar masses Hubble Tuning Fork Diagram Normal Spirals Sa—Large nucleus, tightly wound spiral arms Sb—Medium Nucleus, average spiral arms (Milky Way and Andromeda) Sc—Small nucleus, Loosely wound spiral arms Barred Spirals SBa—Large nucleus, tightly wound spiral arms SBb—Medium Nucleus, average spiral arms SBc—Small nucleus, loosely wound spiral arms Irregulars Determining Galactic Properties Distance o CEPHIDS Only work for nearest galaxies (R<15 Mpc) o BRIGHTEST SUPERGIANTS M~ -8 to -9 (R< 40 Mpc) o BRIGHTEST GLOBULAR CLUSTERS M~ -10 (R< 60Mpc) o HII REGIONS Absolute Magnitude M~ -12 (R< 100 Mpc) Angular Diameter (R< 100 mpc) o TULLY-FISHER RELATION The broader the hydrogen 21-cm emission line of a spiral galaxy is the brighter the galaxy o SUPERNOVAE PEAK BRIGHTNESS M~ -20 (R< 1000 Mpc) o HUBBLE’S LAW Proportionality between velocity of a galaxy and its distance Vr~H D0 Hubble Constant H is b0tween 50 to 100 km-s - 1 -1 -Mpc Diameter Luminosity Mass Galactic Structure: Why are there Different types of galaxies? Why do some galaxies become spiral, some elliptical, and some irregular? There is something different about the protogalactic clouds from which they form They are somehow changed later Angular Movement Density Summation o Low Anglular momentum, high density Elliptical o High angular momentum, low density Spiral Rich Galaxy Clusters More than 1000 galaxies Containing mostly elliptical galaxies Spherically shaped, crowded concentration near center Often contain giant ellipticals at the center Poor Galaxy Clusters Less than 1000 galaxies Irregualry shaped: small groupings of galaxies throughout Large percentage of spirals than rich clusters Are collisions between Galaxies Common? How many suns put edge-to-edge would it take to reach the nearest star (Alpha Centari)? How many Milky Ways put edge-to-edge to would it take to reach the nearest large galaxy (Andromeda)? Starburst Galaxies Galaxy Collisions Spiral have been involved in few collisions Ellipticals have been involved in many collisions Summary Galaxy type may be determined at birth by its protogalactic cloud’s o Angular Momentum o Density Galaxy type may evolve due to interactions which “use up” gas and dust o Giant ellipticals form at the center of dense clusters They are “built up” over time o Central regions of dense cluster are filled with hot gas—it strips away the gas of spirals that pass through it It is unknown which is the dominant factor Quasars and Active Galaxies Lecture Summary Quasars High redshifts Gravitational Lensing Active Galaxies Spectrum of a Quasar Quasar Redshifts Hubble’s Law Quasar Brightness Vary in brightness over short time scales o Implies that they are small Density of Quasars at Various Distances Characteristics of Quasars Quasar spectra show very large redshifts Thus Quasars have large velocities of recession Thus by Hubble’s Law—Quasars must be at great distances Quasars are predominantly objects of the past For us to detect them at these great distances they must be incredibly energetic objects Quasars vary in brightness over short time periods o Thus they are very small Gravitational Lensing Active Galaxies Some galaxies produce far more energy than other normal galaxies Spirals producing large amounts of energy are known as Seyfert Galaxies Jets set to Radio Lobes Cosmology II Summary Big Bang Radiation and Matter Inflation Nucleosynthesis Dark Matter Dark Energy CMB Big Bang Universe began o 13.7 billion years ago o Very hot o High Density o And it has been cooling, expanding, and forming more complex structures every since Inflation Why Inflation? Inflation solves: o Horizon Problem (why the opposite sides of the universe to look the same—though they have never interacted) o Magnetic Monopole Problem (the fact that we don’t see them but GUT predicts them) Big Bang Nucleosynthesis Recombination Microwave Background Radiation Composition of the Universe Matter (~30%) o Dark ~75% o Protons and neutrons ~25% Radiation o Light and energy o Redshifted to be less significant Dark energy (~70%) o Unknown Matter Deceleration Enough matter and the universe stops expanding and will contract Not enough matter and the universe expands forever In between is a special “critical density” Dark Energy Acceleration Behaves very differently than matter The more space expands the more dark energy causes space to expand Is there Dark Energy? Cosmic Microwave Background Phtons from the era right at recombination Very slight density variations cause slightly different temperatures Measuring the CMB Geometry All the matter < Critical Density Gravitational Lensing Rotation Curves X-Ray gas Simulations Milky Way II Lecture Summary Rotation Curves Spiral Tracers Spiral Arms Self-sustaining star formation Center of the Milky Way Determining the Mass of the Galaxy Note that if you apply this to a star in the Milky Way for which you know R (the distance from the Center of the Galaxy) and v (the star’s orbital velocity) it only gives you the mass interior to the stars orbit. The gravitational effects of all material outside of a star’s orbit cancels out. Galactic Rotation Curve Galactic Rotation Curve indicates o A—Large Distribution of mass at center High Velocities Radius is small Mass is large o B—Lots of mass in outer parts of the galaxy implying “dark matter” Increasing velocities at large distances High Velocities Orbits encircle more mass Dark Matter Distribution of Matter 11 o Mass within 15 pc—2 x 10 11Solar masses o Mass within 40 pc—6 x 10 solar masses What could Dark Matter Be? o Black Holes—unlikely o Massive Compact Halo Objects (MACHOS) Brown Dwarfs White Dwarfs Red Dwarfs—Unlikely o Exotic SubAtomic Particles Weakly Interacting Massive Particles (WIMPS) MACHO Detection—Gravitational Lensing Bending of light due to the presence of a very large object What are Spiral Arms? Associations o Groups of hot O and B stars Spiral Tracers—Giant Molecular Clouds mapped by CO emission Density Wave Theory Spiral Arms are “compressions” Similar to the concept of sound waves Self-Sustaining Star Formation Spiral Arms in NGC 1566 What’s at the very center? A very massive, energetic object These orbits for stars orbiting very close to the Galactic Center suggest a mass of about 3 million solar masses
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