The Milky Way. The Milky Way. First Studies of the Galaxy. Determining the Structure of the Milky Way. Galactic Plane.

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1 The Milky Way The Milky Way Almost everything we see in the night sky belongs to the Milky Way. 1 We see most of the Milky Way as a faint band of light across the sky. From outside, our Milky Way might very much look like our cosmic neighbor, the Andromeda Galaxy. 2 Determining the Structure of the Milky Way First Studies of the Galaxy Galactic Plane Galactic Center First attempt to unveil the structure of the galaxy by William Herschel (1785), based on optical observations. The shape of the Milky Way was believed to resemble a grindstone, with the sun close to the center 3 The structure of our Milky Way is hard to determine because: 1) We are inside. 2) Distance measurements are difficult. 3) Our view towards the center is 4 obscured by gas and dust. 1

2 Instability Strip Measuring Distances: The Cepheid Method The more luminous a Cepheid variable, the longer its pulsation period. Observing the period yields a measure of its luminosity and thus its distance! Cepheids allow us to measure the distances to star clusters throughout the Milky Way 5 Period-Luminosity Relation We now know there are 2 types of Cepheids Prototype is δ Cephei In-class project: determine period of δ Cephei using observations of AAVSO use period and periodluminosity relation to calculate M and distance next Mon, Wed 6 Extra Credit: Heanrietta Swan Leavitt Use the web to find out about Heanrietta Leavitt. Write a short summary (~2 paragraphs) about her life and work. Due Friday 3/31. Open clusters h and χ Persei Exploring the Galaxy Using Clusters of Stars Two types of clusters of stars: 1) Open clusters = young clusters of recently formed stars; within the disk of the Galaxy 7 Globular Cluster M 19 2) Globular clusters = old, centrally concentrated clusters of stars; mostly in a halo around the galaxy 8 2

3 Locating the Center of the Milky Way Globular Clusters Harlow Shapley used Cepheids to measure distances to globular clusters Distribution of globulars clusters was NOT centered on sun Globular Cluster M80 Center was in a heavily obscured location in the constellation of Sagittarius Sun lived on outskirts of giant wheel of stars Dense clusters of 50,000 a million stars Old (~ 11 billion years), lower-main-sequence stars Approx. 200 globular clusters in our Milky Way 9 10 The Structure of the Milky Way Model of the Milky Way 75,000 light years Before Shapley Sun is ~8.5 kpc from center Disk Nuclear Bulge Sun Open Clusters, O/B Associations Halo After Shapley 11 Globular Clusters 12 3

4 Stellar Populations Population I: Young stars: metal rich; located in spiral arms and disk Radio Maps of Milky Way 21-cm emission Hyperfine transition of neutral H Population II: Old stars: metal poor; located in the halo (globular clusters) and nuclear bulge Structure of Milky Way

5 The Structure of the Milky Way Revealed Distribution of stars and neutral hydrogen Orion star-forming region Distribution of dust Sun Sagittarius dwarf stream Bar Ring 17 Infrared View of the Milky Way Near-infrared image Galactic plane Interstellar dust (absorbing optical light) emits mostly infrared. Nuclear bulge Small Magellanic Cloud Large Magellanic Cloud 18 The Formation of the Milky Way Similar process as formation of solar system Infrared emission is not strongly absorbed and provides a clear view throughout the Milky Way 19 Far-infrared image 20 5

6 Modifications of the Traditional Theory Recently discovered ring of stars around the Milky Way may be the remnant of a merger. Orbital Motions in the Milky Way (II) Differential Rotation Sun orbits around galactic center at 220 km/s 1 orbit takes approx. 240 million years. Stars closer to the galactic center orbit faster. Stars farther out orbit more slowly Mass determination from orbital velocity: The Mass of the Milky Way If all mass was concentrated in the center, Rotation curve would follow a modified version of Kepler s 3rd law. The more mass there is inside the orbit, the faster the sun has to orbit around the galactic center. Combined mass: M = billion sun M sun sun 23 Rotation Curve = orbital velocity as function of radius. 24 6

7 One of Vera Rubin's important contributions to astrophysics is the collection of conclusive data pointing to the presence of dark matter in galaxies. These data are measurements of the orbital velocities of interstellar matter in galaxies. She studied the variation of these velocities with distance from the center of the galaxy. The Mass of the Milky Way (II) Total mass in the disk of the Milky Way: Approx. 200 billion solar masses It is assumed that matter orbits about the center of a galaxy owing to a centripetal force which is the gravitational attraction of other matter in the galaxy. Assuming all other matter in the galaxy is luminous, astrophysicists cannot account for the centripetal accelerations observed. These can be accounted for, however, if additional matter is present. Additional mass in an extended halo: Total: Approx. 1 trillion solar masses Most (>80%) of the mass is not emitting any radiation: dark matter! 25 Consequently, Rubin's measurements were of fundamental importance as empirical evidence for dark matter. The Galactic Center (I) The Nature of Spiral Arms Our view (in visible light) towards the Galactic center (GC) is heavily obscured by gas and dust: Spiral arms appear bright (newly formed, massive stars!) against the dark sky background, but dark (gas and dust in dense, star-forming clouds) against the bright background of the large galaxy Chance coincidence of small spiral galaxy in front of a large background galaxy Extinction by 30 magnitudes Only 1 out of 1012 optical photons makes its way from the GC towards Earth! galactic center Wide-angle optical view of the GC region 7

8 Radio View of the Galactic Center Measuring the Mass of the Black Hole in the Center of the Milky Way Many supernova remnants; shells and filaments By following the orbits of individual stars near the center of the Milky Way, the mass of the central black hole could be determined to be ~ 2.6 million solar masses. Arc Sgr A Sgr A Sgr A*: The center of our galaxy The galactic center contains a supermassive black hole of approx. 2.6 million solar masses. 29 Astrometry of Galactic Center Stars 30 Flare of Sag A* May 9,

9 X-Ray View of the Galactic Center Galactic center region contains many black-hole and neutron-star X-ray binaries. Supermassive black hole in the galactic center is unusually faint in X rays, compared to those in other galaxies. Chandra X ray image of Sgr A* 33 9