Stellar Populations in the Galaxy

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1 Stellar Populations in the Galaxy Stars are fish in the sea of the galaxy, and like fish they often travel in schools. Star clusters are relatively small groupings, the true schools are stellar populations. Populations often correlate with specific components or structures of the galaxy.

2 Open Cluster NGC 6520 from CFHT Credit & Copyright: Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT Explanation: Did you ever have a day when it felt like a dark cloud was following you around? For the open cluster of stars NGC 6520, every day is like this. On the left of the above picture are many of NGC 6520's bright blue stars. They formed only millions of years ago - much more recently than our ancient Sun which formed billions of years ago. On the right is an absorption nebula, molecular cloud Barnard 86, from which the stars of NGC 6520 surely formed. This nebula contains much opaque dust that blocks light from the many stars that would have been visible in the background. Surrounding NGC 6520 is part of the tremendously dense starscape in the bulge of our Milky Way Galaxy, the extended halo of stars that surrounds the center of our Galaxy. NGC 6520 spans about 10 light years and lies about 5500 light years away toward the direction of Sagittarius.

3 The most basic populations Characteristic Population I II Age: Young-intermediate Old ( 3 x 10 9 yr.) ( 5 x 10 9 ) Color (Sp type): Blue (type O-M) Red (G-M) Heavy Elements: 2% 1% Preferred Habitat: Galactic disks, Galactic bulges and halos especially spiral arms Social Groupings: Young, open clusters, Globular clustersor alone. young stellar associations, travel with gas & dust clouds. Reproductive Rate: ~ 3 M sun /yr typically 0 (but can be much more prolific in short bursts!) (but not endangered.)

4 Refined Stellar Populations in the Milky Way Population Thin disk: Characteristics = Baade s Pop. I (young to intermediate). Near circular, high angular momentum orbits. High metallicity, well-defined age-metallicity relationship in solar neighborhood. Product of extended infall? Thick disk: Old, 12 Gyr. Intermediate metallicity ([Fe/H] -0.6, with significant spread. Distinct chemical enrichment history. Product of minor merger? Central bulge: Stellar halo: Most stars in the universe are probably in bulges. Old and metal-rich, with broad spread. Probably enriched rapidly by SNII, though need more data. High phase space density. Baade s Pop. II. Old and metal-poor. Element ratios like those produced in SNII, indicating short formation timescale. Small scatter in abundance ratios indicates efficient mixing. Low angular momentum like bulge. Formation probably connected to bulge pop., though only 1/10 the star mass.

5 Outer halo: Pop. III? Very little mass, much of it in streams from accreting dwarf galaxies. The first generation(s)? A few very low metallicity stars have been found.

6 Basic Milky Way Properties Property Disk Spheroid Spatial dimension 2 3 Size 20 kpc 6 x m 100 kpc (bulge smaller) Kinematics differential rotation random orbits 250 km/s 250 km/s Stellar Populations Pop. I, including: Pop. II, including: Young associations Field halo stars, & clusters, spiral arm nuclear bulge, globular pops., intermediate age clusters (130), dwarf pops. ( yrs) spheroidals (~ 10)

7 Milky Way Properties continued Property Disk Spheroid Metallicity Z Z sun Z ( ) Z sun ISM Gas & dust clouds, Few clouds, star-forming complexes, hot corona. warm edges, bubbles. % Dark matter 50% 90%

8 The Milky Way's Center Credit: DIRBE, COBE, NASA Explanation: Although the Earth is round, our Galaxy appears truly flat. This was shown in dramatic fashion by the COsmic Background Explorer (COBE) satellite which produced this premier view of the central region of our own Milky Way Galaxy in infrared light in1990. The Milky Way is a typical spiral galaxy with a central bulge and extended disk of stars. However, gas and dust within the disk obscure visible wavelengths of light effectively preventing clear observations of the center. Since infrared wavelengths are less affected by the obscuring material, the Diffuse InfraRed Background Experiment (DIRBE) on board COBE was able to detected infrared light from stars surrounding the Galactic center and produce this image. Of course, the edge on perspective represents the view from the vicinity of our Sun, a star located in the disk about 30,000 light years out from the center.