The Milky Way. Overview: Number of Stars Mass Shape Size Age Sun s location. First ideas about MW structure. Wide-angle photo of the Milky Way

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1 Figure The Milky Way Wide-angle photo of the Milky Way Overview: Number of Stars Mass Shape Size Age Sun s location First ideas about MW structure Figure Shapely (~1900): The system of globular clusters is centered on a point some 26,000 light years away. If these clusters are evenly distributed, that should be where the center of the Milky Way is located. 1

2 Fig The shape of the Galaxy The Local Group of galaxies Disk: structure (thin and thick) Bulge + bar Halo Revolution of all matter around the Galactic center The Nearby Spiral M33 M31 Andromeda 2

3 The discovery of stellar populations Stellar groups with different distribution, composition, age Walter Baade, in 1944, based on blue and red photographs of Andromeda POPULATION I occurs in disk old and young stars open clusters (and isolated/binary stars) roughly solar metallicity (70% hydrogen, 28% helium, 2% metals) Associated with Population I: interstellar gas and dust star-forming gas clouds Type II supernovae POPULATION II occurs in the halo old stars only (no blue main sequence stars) globular clusters (and isolated/binary stars) low metallicity, 1/1000 to 1/100 solar [75% hydrogen, 24.99% helium, 0.01% metals] Associated with Population II: very little interstellar gas or dust no star formation only Type I supernovae 3

4 The formation of the MW The formation of the MW The formation of the MW 4

5 A Milky Way Companion? Interactions between galaxies This newfound star clump is in a region of the sky the size of the full Moon centered on Willman 1. The distribution of stars in the faraway cluster was filtered to enhance the signal relative to the much more numerous closer stars of the Milky Way Galaxy. Credit: B. Willman/NYU/SDSS The age of the MW Figure The age of the MW 5

6 The age of the MW Oldest globular clusters Turnoff point of MS ~ 13 billion yrs The evolution of the MW As stars form the amount of ISM decreases The rate of star-formation is not fixed Current rate of star-formation Chemical composition of ISM changes More metal rich stellar generations Fate of MW End of star-formation The last low-mass stars Merger events with nearby galaxies LMC, SMC Andromeda The Local Group - star formation and stellar evolution in differing environments Galaxies active in star formation: Cover a factor of 10 in metallicity Span a range of types from Elliptical Dwarfs to Irregular and Spiral. 200 million light years from Sun Arp 273 The larger galaxy: is strongly tidally distorted The companion: relatively undisturbed spiral disk, but a luminous, star burst nucleus (one in which there is a burst of new star formation). Colliding Galaxies 6

7 The MW structure Observing at different wavelengths Visible Far-infrared astronomy: observations from space Sagittarius Dwarf ~50,000 lry. from Gal. center In process of collision! 7

8 M83 Hydra Mapping the MW disk in visible light Spiral Structure of the Milky Way Spiral arms look strung out along the line of sight Different parts overlapped Need distances to bright spiral structure tracers Our understanding entirely depends on observational data. What do external galaxies tell us about the stellar content of the spiral arms? Hot, blue stars O, B spectral class Individual young stars Clusters of young stars Associations of young stars Interstellar matter Dist HII regions Atomic H GMC (H 2, CO, ) Star-forming regions Delineate recent starformation sites and therefore the spiral arms. Provide structural details of the galactic spiral patterns Smeni s M33 8

9 MW spiral structure In our Galaxy, about 30 large star-forming fields may be distinguished. Mapping of the spiral structure of the Milky Way Steps when mapping the Galactic structure using young stars: Identify the young, recently born stars Study the light that comes from them Measure the energy distribution in their spectra Obtain temperature and intrinsic brightness from the energy distribution Compare intrinsic brightness to apparent brightness to obtain the distance Use the coordinates and distances to obtain projection of the stars on the Galactic plane - should see spiral arms Studying stellar light Fig. 7.6 Emission depends on T Stellar photometry 1. Higher T shifts the maximum to a higher frequency, or bluer color Wien s law λ max ( m ) = T ( K) 2. Higher T, brighter object. Stefan Boltzmann law 4 E = σ T How can we get impression about the temperature and intrinsic brightness of the stars? By measuring the distribution of the energy along the rainbow Stellar photometry technique Telescope + set of colored filters + detector: shows how much energy the star emits at different parts of the rainbow: impression about the distribution of the star's radiation over the color scale: stellar surface temperature and intrinsic brightness 9

10 The technique of stellar photometry 1. Photometry allows us to find the spectral type and luminosity class 2. T, R 3. L = 4πR 2 σt 4 4. Absolute magnitude 5. Apparent magnitude 6. Distance 7. Mapping the MW Stellar Photometry KPNO 1950 present : tremendous amount of work to identify and obtain photometry of the brightest young stars in the MW from ground SAAO La Silla, ESO 10

11 Early-type stars as spiral tracers Mapping of the spiral structure of the Milky Way Early 1960s: Three distinct concentrations of young stars within 3 kpc of the Sun have been roughly identified: Perseus feature Local (also Orion or Cygnus) arm Sagittarius arm Local Sagittarius Perseus Galactic structure traced by young stars. Becker (1964) 1970s: a major addition to the Spiral structure Carina Arm an extended feature seen nearly edge-on and traced to about 8 kpc Young Stellar Clusters and Galactic Structure Moffat and Vogt (1974) Fig The spiral structure of the Milky Way Carina-Sagittarius Norma-Centaurus or Norma-Scutum 11

12 Young stellar population as tracer of Galactic structure Tracing MW spiral structure from space Need more! At present only about 4000 young stars are studied ESA, 2011, Multi-color photometry and parallaxes of 1% of the Galactic stellar population The spiral structure near Sun Scorpius Centaurus Association 12

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