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Galaxies and the Distance Ladder

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Galaxies and the Distance Ladder Lecture 27 Galaxy Types and the Distance Ladder December 3, 2018 1 2 Early Observations • Some galaxies had been observed before 1900’s. – Distances were not known. – Some looked like faint spirals. • Originally thought to be nearby star forming nebulas. 3 Hubble Classification • Galaxies are classified by their appearance (morphology) • Main types – Spirals – Ellipticals – Lenticulars – Irregulars 4 Spirals • Flat disk, nuclear bulge, spiral arms • Contain young and old stars, gas, and dust. • Sub-classified by size of bulge and tightness of spiral arms. – Sa -- large bulge, tightly wound arms, less gas and dust. – Sb -- smaller bulge, more loosely wound arms – Sc -- small bulge, more loosely wound arms, more gas and dust. 5 Sa Sb Sc M81 M51 NGC 2997 NGC 4565 6 • Bars in Spirals – Galaxies have bar type shapes near the center – Labeled SBa, SBb, or SBc 7 Ellipticals • Balls of stars with round or oval shape • Classified E0 (round) to E7 (elliptical) • Usually have less dust and gas. • Contain older stars • Large range of sizes E0 M89 E2 M49 E4 NGC4889 8 Lenticulars (S0 / SB0) NGC 2859 type SB0 • Between elliptical and spiral galaxies • Some galaxies classified E4 to E7 may actually be lenticulars • Have disk and large bulge • Labeled S0 or SB0 (barred) • No spiral arms, usually little dust or gas NGC 1201 type S0 9 Irregulars • Do not fall into other categories • Often small NGC 4485 M82 LMC 10 Hubble Tuning Fork Diagram • Organizes classifications • Possible evolutionary track? Not so simple 11 Which type of galaxy tends to have the most gas and dust? A. Elliptical B. Irregular C. Lenticular D. Spiral 12 Distances to Galaxies • Galaxies were first thought to be star forming regions. • Proposed that the “spiral nebula” were “island universes”. • Main-sequence fitting is only good for distances up to 10,000 pc. Andromeda Galaxy 13 Using Cepheid Variable Stars to Measure Distances • Variable stars are stars whose brightness varies in a very smooth, predictable way. • Cepheid variables – periods vary from 1-100 days. Figure 23.5, Chaisson and McMillan, 6th ed. Astronomy Today, © 2008 Pearson Prentice Hall • RR Lyrae variables – periods are all less than 1 day. 14 • Cepheids are stars that have moved off of the main sequence – Star is expanding and contracting. – Luminosity rises and falls. Figure 23.6, Chaisson and McMillan, 6th ed. Astronomy Today, © 2008 Pearson Prentice Hall 15 Cepheid Variables • Average luminosity is related to pulsation period. • If luminosity and apparent brightness are known, distance can be determined Luminosity Apparent Brightness = 4d 2 Figure 23.7, Chaisson and McMillan, 6th ed. Astronomy Today, © 2008 Pearson Prentice Hall 16 Two Cepheid variable stars, A and B, have the same apparent magnitude, but star B has a shorter period. Which star is further away? A. Star A B. Star B C. They are at the same distance from us. 17 Two Cepheid variable stars, A and B, have the same apparent magnitude, but star B has a shorter period. Which star is further away? apparent brightness A. Star A bbAB= B. Star B LL AB= C. They are at the 22 44ddAB same distance dL from us. BB=1 dLAA 18 Supernovae as Standard Candles • Type Ia supernovae (exploding white dwarfs) all reach the same maximum luminosity, about 3 × 109 solar luminosities • If supernova is observed in another galaxy and the peak apparent brightness is measured, the distance can be calculated. Luminosity Apparent Brightness = 4d 2 19 Tully-Fisher Relation for Determining Distances • Used to determine distances to galaxies where individual stars cannot be seen. • Relates speed of rotation and luminosity of a galaxy. – The faster a galaxy rotates, the higher the luminosity. • If apparent brightness and luminosity are known, distance can be determined 20 • How is rotational speed measured? – Doppler Shift Figure 24.11, Chaisson and McMillan, 6th ed. Astronomy Today, • Tully-Fisher calibrated using © 2008 Pearson Prentice Hall nearby galaxies with variable stars 21 The Distance Ladder (or “Distance Chain,” see pp. 415-416) “main sequence fitting” Figure 26.12, Freedman and Kaufmann, 7th ed. Universe, © 2005 W. H. Freeman & Company 22 Which of the following distance calibration methods is most effective at short distances? A. Tully – Fisher B. Main-sequence fitting C. Cepheid variables D. Type Ia supernovae 23 Distribution of Galaxies • Most galaxies are clustered – Milky Way has 3 nearby companions (SMC, LMC, Sagittarius Dwarf) – Andromeda Galaxy (M31) is the largest in Local group – ~40 galaxies in Local Group (Size ~1 Mpc) Map of three-quarters of the members of the Local Group Figure 16-18, Comins and Kaufmann, Artist’s view of the Local Group 7th ed. Discovering the Universe, Figure 16.30, Arny and Schneider, 5th ed. Explorations, © 2005 W.H. Freeman and Company © 2008 The McGraw-Hill Companies, Inc. 24 Clusters of Galaxies • Galaxies often found in clusters – Rich cluster: many hundreds of galaxies – Poor cluster (or group): only a few dozen galaxies • Held together by gravity • Milky Way and Andromeda are the two large galaxies in a Local Group of about 40 to 70 galaxies (see p.4, Figure 1.1) • Milky Way is near the Virgo Cluster of ~1500 galaxies (Size ~4.4 Mpc = 14 Mly across), which is part of the Virgo Supercluster (33 Mpc = 110 Mly across) 25 Virgo Supercluster Figure 16.36, Arny and Schneider, 5th ed. Explorations, © 2008 The McGraw-Hill Companies 26 The Local Group of Galaxies Andromeda Milky Way Image from Wikipedia. See also Fig. 1.1, Bennett et al., The Essential Cosmic Perspective, 7th ed, p. 3. 27 Virgo Cluster – SDSS Image 28 Clusters and Superclusters • Large clusters – More ellipticals found near the center – More spirals found in outer regions • Superclusters – clusters of clusters of galaxies. – In between clusters -- no gas has been detected • Most must have been swept up during galaxy formation. 29 30 Abell 2218 Abell 2218 is a cluster of galaxies so dense that it warps spacetime and acts like a powerful lens, magnifying and distorting the galaxies that lie behind it. Credit: NASA/ESA 31 Role of Interactions • Small interactions may start formation of spiral structure. • Strong interactions (collisions, cannibalism) may alter structure completely – Spirals lose structure, become ellipticals. – Large galaxies “eat” many other galaxies, become very large 32 Galaxy Merger 33 Antennae Galaxy 34 Which of the following is NOT true concerning the Local Group? A. It contains about 50 member galaxies. B. It is roughly spherical in shape with the most massive galaxies near the center. C. It is a poor cluster. D. It is the galaxy cluster to which the Milky Way belongs..
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