Lecture 30. The Galactic Center

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1 Lecture 30 History of the Galaxy Populations and Enrichment Galactic Evolution Spiral Arms Galactic Types Apr 5, 2006 Astro 100 Lecture 30 1 The Galactic Center The nature of the center of the Galaxy is poorly known. Most evidence is from infrared and radio (some strong γ-rays, too). (interstellar dust blocks visible): About 1 kpc from center, expanding gas 100 km/sec => some explosive event about 10 million years ago About 100 pc across: radio source Sagittarius A (strongest radio source in sky). Synchrotron radiation (light from accelerated subatomic particles). Radio images show effects of strong magnetic field. About 10 pc across: pinwheel-like arrangement of gas < 1 pc across: "Sagittarius A*" almost point-like radio/infrared/x-ray source. Orbits of nearby stars => 3 x 10 6 M sun. Nucleus: Supermassive Black Hole? Most believe that this much mass must mean a black hole: IR, X-rays from Sag A* varies with t(var) ~ 1 minute => size < 1 AU 10 6 M sun is far above the limit for a neutron star. Apr 5, 2006 Astro 100 Lecture 30 2

2 Stellar Populations The different regions of the Galaxy do not have the same kinds of stars Stellar Populations: Stars may be very roughly grouped into populations by the time of their formation using "enrichment dating". Careful spectral typing finds same spectral types sometimes show different strengths of lines from heavy elements. Enrichment dating: Elements in a star's atmosphere are representative of elements in the interstellar gas at the time of its birth. (New ones formed by fusion are hidden in core until after main sequence lifetime) Apr 5, 2006 Astro 100 Lecture 30 3 History of the Galaxy 1) "In the Beginning": "Population III" H,He interstellar gas -> Stars (first generation, no metal absorption lines) Stars fuse H,He -> C,N,O,"Metals" Supergiants, Planetary Nebulae return new fusion products CNO back to the interstellar gas Supernovae return all elements, including metals and heavies >Fe back to the interstellar gas 2) Next generation: "Population II" H,He + some C,N,O,metals -> Stars (weak absorption lines of metals) This happened about 13 Gyr (1 Gyr = 1"gigayear = 1 billion years) ago, from looking at the Main Sequence turnoff points of Pop II Globular Clusters Stars fuse H,He -> C,N,O,"Metals" Supergiants, Supernovae Planetary Nebulae -> Interstellar gas Many more generations: "Population I" H,He + more C,N,O,metals -> Stars (metal absorption lines comparable to Sun) Apr 5, 2006 Astro 100 Lecture 30 4

3 Distribution of Populations Find: the more recent the formation of the star, the more confined it is to the Galactic disk. Population III: Only a few candidates found. Age unknown. In halo. Population II: Old Globular Clusters, many M main sequence stars (10-13 billion years old). In central bulge and halo. "Old Disk Population I". (2-8 billion years old). Sun, planetary nebula nuclei, etc. Confined within about 500 pc of disk plane (Sun is about 8 pc N of plane). "Extreme Population I": (formed within last few million years). Up to twice metal line "enrichment" of Sun: interstellar gas, dust clouds, O,B stars, Supernova Remnants, Red Supergiants. All confined within about 100 pc of central plane of Disk. Apr 5, 2006 Astro 100 Lecture 30 5 Galactic history scenario Since once a star is formed, it stays (pretty much) in the same galactic orbit, this tells us the location of the gas clouds from which the star formed. Gas clouds orbits change with time as they run into each other When the galaxy first formed (13 Gyrs ago), Pop III and II stars formed out of gas in large chaotic spherical cloud (perhaps still contracting). Gas (enriched by SN of Pop II) then settled into ever flatter rotating disk; sun formed 5 Gyr ago in thick disk. New stars are now forming in thin disk. Apr 5, 2006 Astro 100 Lecture 30 6

4 The most recent action: Spiral Structure There is additional structure within the most recently formed material in the disk: Overall. Not observable in visible light due to dust extinction. Observe in radio: neutral hydrogen (21 cm line), molecules in interstellar clouds. Gas forms a rough spiral pattern (maybe 10 arms) centered on nucleus. Nearby (1 kiloparsec). Visible wavelength: map positions of star-forming regions, bright OB star clusters: Scorpio-Centaurus-Orion-Perseus the "Gould Belt". The Sun is near the edge of the "Orion arm". Apr 5, 2006 Astro 100 Lecture 30 7 Spiral Density Waves: The spiral pattern deduced for our Galaxy is also seen in other galaxies. What causes it? One explanation: 1) Some large event compresses stars and gas in one part of galaxy (central "bar", galactic collision) 2) Disturbance propagates like sound wave, where gravity is the "restoring force" instead of gas pressure. In a rotating medium, waves are spirals. Simulation: 3) Stars not much effected by waves, but speed of waves is faster than pressure sound waves in interstellar gas, causing shocks, and star formation. This gives "grand design" spiral The spiral arms are marked out by short lived massive stars, as observed. Apr 5, 2006 Astro 100 Lecture 30 8

5 Other Galaxies Classification Types classified by appearance: Elliptical Little dust and gas, squashed sphere distribution of stars, all stars old. Like central bulge of Milky Way. Masses from < 0.01 M Gal (dwarfs) to > 100 M Gal (giants) Spiral Like Milky Way, with varying proportions of bulge (old) to disk (new) in order Sa, Sb, Sc. Sc galaxies tend to have more open spiral arms. Smaller range of masses; MW is a giant spiral. Irregular. Most are "dwarfs" with lots of new stars and gas. Apr 5, 2006 Astro 100 Lecture 30 9 Groups and Clusters of Galaxies Many galaxies are in clusters ("group" = small cluster) Local Group. The small cluster containing the Milky Way about 20 galaxies in cubic Mpc. 3 spirals: MW, Andromeda ("M31"), M33 4 irr: including MW satellites Large, Small Magellanic Clouds (dist ~ 50 kpc) many dwarf ellipticals Virgo Cluster. Nearest rich cluster, ~ 2500 galaxies, distance 15 Mpc. Rich clusters have few spirals, thought to be stripped of gas by collisions. Apr 5, 2006 Astro 100 Lecture 30 10

6 Galactic Center X-rays Apr 5, 2006 Astro 100 Lecture Magnetic Arches Apr 5, 2006 Astro 100 Lecture 30 12

7 Galactic Center Stellar Orbits Black Hole? + Apr 5, 2006 Astro 100 Lecture Galactic Center Black Hole Apr 5, 2006 Astro 100 Lecture 30 14

8 Formation of the Milky Way Population II Age 9-13 Gyr Population I Age 0-9 Gyr Globular Clusters: Central bulge and halo Old Open Clusters, Sun: Thick Disk Forming Stars, OB stars: Thin Disk Apr 5, 2006 Astro 100 Lecture Milky Way Spiral Arms Sun Apr 5, 2006 Astro 100 Lecture 30 16

9 Spiral Density Waves Apr 5, 2006 Astro 100 Lecture Elliptical Galaxies M87 Giant Elliptical Leo dwarf Elliptical Apr 5, 2006 Astro 100 Lecture 30 18

10 Spiral Galaxies Apr 5, 2006 Astro 100 Lecture Irregular Galaxies Large Magellanic Cloud NGC Starburst Apr 5, 2006 Astro 100 Lecture 30 20

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