The Milky Way Galaxy. Some thoughts. How big is it? What does it look like? How did it end up this way? What is it made up of?

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1 Some thoughts The Milky Way Galaxy How big is it? What does it look like? How did it end up this way? What is it made up of? Does it change This is not a constant zoom

2 The Milky Way Almost everything we see in the night sky belongs to the Milky Way. We see most of the Milky Way as a faint band of light across the sky. 6 8 Our Milky Way Galaxy Spiral Galaxy (Seen in the Infrared) (Andromeda Galaxy 2-million LY away) 9 From outside, our Milky Way might very much look like our cosmic neighbor, the Andromeda Galaxy. 10

3 William Herschel The First Studies of the Galaxy Born Friedrich Wilhelm Herschel 15 November 1738 Hanover, Brunswick-Lüneburg, Holy Roman Empire Died 25 August 1822 (aged 83) Slough, England, UK Resting place St Laurence's Church, Slough Residence Observatory House Nationality Hanoverian; later British Fields Astronomy and music Known for Discovery of Uranus Discovery of infrared radiation Deep sky surveys First President of the Royal Astronomical Society 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 We need to develop a strategy to explore the structure of our Milky Way 13 14

4 Strategies to explore the structure of our Milky Way I. Select bright objects that you can see throughout the Milky Way and trace their directions and distances. II. Observe objects at wavelengths other than visible (to circumvent the problem of optical obscuration), and catalog their directions and distances. III. Trace the orbital velocities of objects in different directions relative to our position. 1 st Let s Work on Distances Direct Surveyors Astronomers HR Diagram Variable Stars Henrietta Swan Leavitt (July 4, 1868 December 12, 1921) was an American astronomer 17 18

5 Measuring Distances: The Cepheid Method Instability Strip The more luminous a Cepheid variable, the longer its pulsation period. Observing the period yields a measure of its luminosity and thus its distance! The Cepheid Method Allows us to measure the distances to star clusters throughout the Milky Way Harlow Shapley (November 2, 1885 October 20, 1972) was an American astronomer Dec Shapley took Henrietta s variable star calculations to determine where the center of the galaxy is located Henrietta Leavitt d Dec 12, 1921

6 23 24 Exploring the Galaxy Using Clusters of Stars Globular Clusters Two types of clusters of stars: 1) Open clusters = young clusters of recently formed stars; within the disk of the Galaxy Globular Cluster M80 Open clusters h and Persei Dense clusters of 50,000 a million stars Old (~ 11 billion years), lower-main-sequence stars Globular Cluster 2) Globular clusters = old, centrally concentrated M 19 clusters of stars; mostly in a halo around the galaxy 25 Approx. 200 globular clusters in our Milky Way 26

7 Locating the Center of the Milky Way If we look for the where the clusters are centered We could find the Center of the Milky Way Galaxy Distribution of globular clusters is not centered on the sun, but on a location which is heavily obscured from direct (visual) observation

8 Infrared The Galactic Center (I) Our view (in visible light) towards the Galactic center (GC) is heavily obscured by gas and dust: Extinction by 30 magnitudes Only 1 out of optical photons makes its way from the GC towards Earth! galactic center Wide-angle optical view of the GC region 33 34

9 Strategies to explore the structure of our Milky Way I. Select bright objects that you can see throughout the Milky Way and trace their directions and distances. II. Observe objects at wavelengths other than visible (to circumvent the problem of optical obscuration), and catalog their directions and distances. Observing Neutral Hydrogen: The 21-cm (radio) line (I) Electrons in the ground state of neutral hydrogen have slightly different energies, depending on their spin orientation. Equal magnetic Opposite magnetic Magnetic field due fields repel => fields attract => to proton spin Higher energy Lower energy 21-cm line III. Trace the orbital velocities of objects in different directions relative to our position. Magnetic field due to electron spin Infrared View of the Milky Way Near-infrared image Strategies to explore the structure of our Milky Way Nuclear bulge Galactic plane Infrared emission is not strongly absorbed and provides a clear view throughout the Milky Way Interstellar dust (absorbing optical light) emits mostly infrared. Far-infrared image I. Select bright objects that you can see throughout the Milky Way and trace their directions and distances. II. Observe objects at wavelengths other than visible (to circumvent the problem of optical obscuration), and catalog their directions and distances. III. Trace the orbital velocities of objects in different directions relative to our position

10 Orbital Motions in the Milky Way (I) Orbital Motions in the Milky Way (II) Disk stars: Nearly circular orbits in the disk of the galaxy Differential Rotation Sun orbits around galactic center at 220 km/s 1 orbit takes approx. 240 million years. Halo stars: Highly elliptical orbits; randomly oriented 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 Rotation Curve = orbital velocity as function of radius

11 The Mass of the Milky Way (II) Total mass in the disk of the Milky Way: Approx. 200 billion solar masses Additional mass in an extended halo: Total: Approx. 1 trillion solar masses Most of the mass is not emitting any radiation: dark matter! Metals in Stars Metal Abundances in the Universe Absorption lines almost exclusively from Hydrogen: All elements heavier than He are very rare. Many absorption lines also from heavier elements (metals): Population I At the time of formation, the gases forming the Milky Way consisted exclusively of hydrogen and helium. heavier elements ( metals ) were later only produced in stars. Linear Scale Logarithmic Scale 45 Population II => Young stars contain more metals than older stars. 46

12 Stellar Populations What the Milky Way is made of Population I Population II Population I: Young stars: metal rich; located in spiral arms and disk Population II: Old stars: metal poor; located in the halo (globular clusters) and nuclear bulge The History of the Milky Way The traditional theory: Quasi-spherical gas cloud fragments into smaller pieces, forming the first, metal-poor stars (pop. II); Rotating cloud collapses into a disk-like structure Later populations of stars (pop. I) are restricted to the disk of the galaxy Modifications of the Traditional Theory Ages of stellar population may pose a problem to the traditional theory of the history of the Milky Way. Possible solution: Later accumulation of gas, possibly due to mergers with smaller galaxies. Recently discovered ring of stars around the Milky Way may be the remnant of such a merger.

13 Exploring the structure of the Milky Way with O/B Associations Radio Observations O/B Associations 21-cm radio observations reveal the distribution of neutral hydrogen throughout the galaxy. Distances to hydrogen clouds determined through radial-velocity measurements (Doppler effect!) Sun Sun Galactic center O/B Associations trace out 3 spiral arms near the sun. Neutral hydrogen concentrated in spiral arms Distances to O/B Associations determined using Cepheid variables Determining the Structure of the Milky Way The Structure of the Milky Way 75,000 light years Galactic Plane Disk Nuclear Bulge Sun Galactic Center Halo Open Clusters, O/B Associations The structure of our Milky Way is hard to determine because: 1) We are inside. Globular Clusters 53 2) Distance measurements are difficult. 3) Our view towards the center is obscured by gas and dust. 54

14 The Structure of the Milky Way Revealed Distribution of stars and neutral hydrogen Distribution of dust Sun Star Formation in Spiral Arms (I) Shock waves from supernovae, ionization fronts initiated by O and B stars, and the shock fronts forming spiral arms trigger star formation. Spiral arms are stationary shock waves, initiating star formation. Bar Ring Star Formation in Spiral Arms (II) Spiral arms are basically stationary shock waves. Stars and gas clouds orbit around the galactic center and cross spiral arms. Shocks initiate star formation. Star formation selfsustaining through O/B ionization fronts and supernova shock waves. The Nature of Spiral Arms Chance coincidence of small spiral galaxy in front of a large background galaxy 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 57 58

15 Self-Sustained Star Formation in Spiral Arms Star forming regions get elongated due to differential rotation. Grand-Design Spiral Galaxies Grand-design spirals have two dominant spiral arms. Flocculent (woolly) galaxies also have spiral patterns, but no dominant pair of spiral arms. Star formation is self-sustaining due to ionization fronts and supernova shocks M 100 NGC 300 The Whirlpool Galaxy Grand-design galaxy M 51 (Whirlpool Galaxy): Radio View of the Galactic Center Many supernova remnants; shells and filaments Arc Self-sustaining star forming regions along spiral arm patterns are clearly visible. 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

16 Measuring the Mass of the Black Hole in the Center of the Milky Way X-Ray View of the Galactic Center Galactic center region contains many black-hole and neutron-star X-ray binaries. 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. 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*

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18 71 72 See you in one week! 73