ASTR 1120 General Astronomy: Stars & Galaxies!NNOUNCEMENTS HOMEWORK #6 DUE TODAY, by 5pm HOMEWORK #7 DUE Nov. 10, by 5pm Dark matter halo for galaxies Dark matter extends beyond visible part of the galaxy -- mass is ~10x stars and gas! Probably not normal mass that we know of (protons, neutrons, electrons). Most likely subatomic particles, as yet unidentified (weakly interacting massive particles WIMPs?) REVIEW A Case for a Supermassive Black Hole at the Galactic Center Doppler shift measurements of spiraling stars and gas suggest 3 million M Sun black hole Still too far out to exclude other possibilities Enclosed mass (M o ) Distance from Sgr A* (pc) REVIEW Genzel 1996 We need to be able to see closer in to really prove there is a supermasive black hole! REVIEW Stars appear to be orbiting something massive but invisible a black hole! Orbits of stars indicate a mass of about 3-4 million M sun within 600 R Schwarzchild
REVIEW REVIEW State of Affairs at the Galactic Center Stellar orbits have made our Galaxy one of the best proofs of supermassive black holes at the center of most galaxies. Hubble Ultra Deep Field Elliptical EllipticalGalaxy Galaxy Millions to billions of times the mass of our Sun. Must be created by something entirely different than a massive star supernova. Flares are often observed in X-rays and IR Observations of occasional flares are interpreted as the result of occasional swallowing of a gas clump or a star by the giant black hole. Irregular Galaxies Spiral Galaxy Which type of galaxy contains a low percentage of cool gas and dust? A. B. C. D. E. Spiral Elliptical Irregular Barred spiral Everyone but B Which type of galaxy contains a low percentage of cool gas and dust? A. B. C. D. E. Spiral Elliptical Irregular Barred spiral Everyone but B
Hubble classification of galaxy types Ellipticals Spirals Where do spirals and ellipticals live? Spirals: mostly in groups (3-10 galaxies) HST: Hickson CG 44 Barred spiral Ellipticals - most often in dense clusters of galaxies (involve 100 s to 1000 s) The Big Picture: Universe is filled with network of galaxies in groups and clusters ~100 billion galaxies! HST: Abell 1689
Pattern of galaxies (3 million+),15 o portion of sky Brighter = more galaxies Which of the following is NOT a classification of a type of galaxy? A. Keplerian B. Spiral C. Lenticular D. Elliptical E. Irregular Our Local Group of galaxies Which of the following is NOT a classification of a type of galaxy? A. Keplerian B. Spiral C. Lenticular D. Elliptical E. Irregular 3 spirals: Andromeda (M31) Milky Way Triangulum (M33) 2 irregulars: LMC SMC 16+ dwarfs 3/2 M MW 1 M MW 1/5 M MW 1/8 M MW 1/30 M MW ~21 Galaxies
Biggest is Andromeda (Sb - M33) Andromeda is ~3 million light years away (or ~30 MW diameters), has ~1.5 mass of MW We see it as it was 3 million years ago, not as it is today! this is lookback time Triangulum (M33) 1/5 mass of MW, spiral classified as Sc Several bright (pink) star forming regions Oops! It may crash into MW in about 2 billion years Large & Small Magellanic Clouds LMC has 30 Doradus, home of SN 1987A SMC LMC
What are the Magellanic Clouds? A. Two nebulae in disk of Milky Way visible only in southern hemisphere B. Clouds of dust and gas in many places throughout the Milky Way galaxy C. Two small galaxies in the same group as the Milky Way D. Star-forming clouds in constellation Orion What are the Magellanic Clouds? A. Two nebulae in disk of Milky Way visible only in southern hemisphere B. Clouds of dust and gas in many places throughout the Milky Way galaxy C. Two small galaxies in the same group as the Milky Way D. Star-forming clouds in constellation Orion How do we get distances to things far outside our Galaxy? Mapping the Universe: We need Distances to Galaxies! The problem: or Methods we are familiar with: Radar and Stellar parallax Only useful inside the Solar System A few thousand ly
New Methods: Bootstrap our way Identify (and calibrate) objects that could serve as STANDARD CANDLES -- beyond direct measurement DISTANCE ESTIMATE 1 Start with cluster A (upper) whose distance known via parallax Main-Sequence Fitting A 1. Make some measure of an object which identifies its luminosity 2. Use this luminosity and measure apparent brightness to infer distance to it Compare with other cluster B (lower) Get distance to B from brightness difference B Distances up to ~1 million light years Which cluster is closer? Which cluster is closer? A. Hyades A. Hyades B. Pleiades C. Not enough information to tell B A B. Pleiades C. Not enough information to tell B A
Main Sequence Fitting pinned to nearby Hyades Cluster Only 151 ly away DISTANCE ESTIMATE 2 Instability strip -- region in H-R diagram with large, bright stars Outer regions of star are unstable and tend to pulsate Star expands and contracts, getting brighter and fainter Cepheid variable stars Reminder (Fig 15.14) DISTANCE ESTIMATE 2 Cepheid variable stars Period - Luminosity relation brighter Cepheids have longer periods Two Cepheid stars, Fred and Barney, have the same apparent brightness. Fred has a period of 5 days, and Barney of 10 days. Which is closer? A. Fred B. Barney
Why A. Fred? DISTANCE ESTIMATE 3 Tully-Fisher Relation Fred has a shorter period and so must be less luminous Less luminous but the same apparent brightness means that Fred is closer to us Period-Luminosity Relation Fast rotation speeds in spiral galaxies! more mass in galaxy! higher luminosity Measure rotation speeds to infer luminosity Need bright edge-on spirals, estimate tilt Distances up to ~1 billion ly DISTANCE ESTIMATE 4 Even brighter: White dwarf supernovae Bright enough to be seen halfway across observable universe Nearly the same amount of energy released every time. why? Standard explosion = fusion of 1.4 solar masses of material Useful for mapping the universe to the largest distances
Summary Distance Ladder to measure universe Different standard candles are useful for different distances