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The Milky Way Galaxy The infinitude of creation is great enough to make a world, or a Milky Way of worlds, look in comparison with it what a flower or an insect does in comparison with the Earth. Immanuel Kant (1724 – 1804) German philosopher WHAT DO YOU THINK? 1. How big is the Milky Way Galaxy? 2. Where is our solar system located? 3. Is there really a HUGE black hole at the center (and why weren’t we sucked into it in 2012 ??) WHAT DO YOU THINK? 1. How big is the Milky Way Galaxy? “Hella big” 2. Where is our solar system located? 25555 Hesperian Blvd, Hayward, CA, USA, Earth 3. Is there really a HUGE black hole at the center (and why weren’t we sucked into it in 2012 ??) Yes, and how do you know we weren’t?? Our View of the Milky Way… Our view Clearly a “disk” shaped, with us inside Optical view blocked by dust & cold gas Infer we probably look like other galaxies Other Galaxies we see – do we look like one of them?? A spiral galaxy “edge on” Is this how we look to the rest of the Universe? A spiral galaxy “edge on” Is this how we look to the rest of the Universe? Some other spirals… M51 (enhanced) M51 (enhanced) Infer we probably look like other galaxies See new, young stars, gas, dust See “pancake” shape across sky indicating a disk. Use other wavelengths to confirm spiral. => Milky Way is a Spiral Galaxy! Drawings – not photos – of our galaxy! Milky Way Structure •Disk •Bulge •Halo Structure of Milky Way Galaxy Disk open clusters younger generation of stars, gas, dust Population I stars Structure of Milky Way Galaxy Disk Bulge mixture of both young & old stars Population II Stars Structure of Milky Way Galaxy Disk Bulge Halo oldest generation of stars, no gas/dust globular clusters Regions of the Milky Way Galaxy diameter = 100,000 l.y radius = 50,000 l.y. thickness ~ 1,000 l.y. number of stars = ~200 Billion Sun is in disk, 28,000 l.y. out from center Regions of the Milky Way Galaxy diameter = 100,000 l.y radius = 50,000 l.y. thickness ~ 1,000 l.y. number of stars = ~200 Billion Sun is in disk, 28,000 l.y. out from center Mapping the Galaxy Using Optical light Using Radio Waves Using IR light Using X-rays & Gamma Views of the Milky Way Mapping the Galaxy- Optical Observation: Cepheid variables in “globular clusters” Cepheid Variable Stars in Globular Clusters Mapping the Galaxy- Optical Observation: Cepheid variables in “globular clusters” Hypothesis: Distribution around galactic center defines location of our solar system in Milky Way! Cepheids in Globular Clusters All we need to find is their distance! Henrietta Leavitt, along with Harlow Shapely, used Cepheid variable stars to determine direction & distance to center Shapely Curtis Debate - 1920! Another example of “good science”! Observation: Cepheid variables in “globular clusters” Hypothesis: Distribution around galactic center defines location of our solar system Critical Test: Distances average to ~ 30,000 light years, toward Sagittarius Globular Clusters Our Sun Not centered around Sun Towards the Center… Mapping the Galaxy Using Radio Waves Cool Hydrogen Gas emits 21-cm light Map out spiral arms Stellar Orbits in the Galaxy Stars in the disk all orbit the Galactic center: • in the same direction in the same plane (like planets orbit our sun) • they “bobble” up and down • gravitational pull from disk Stellar Orbits in the Galaxy Stars in the bulge and halo all orbit the Galactic center: • • • in different directions various angles to the disk higher velocities not slowed by disk as they plunge through it Spiral Arms M 51 Galactic disk does not appear solid • Spiral arms are not fixed strings of stars which revolve like the fins of a fan. Spiral Arms as “Waves” M 51 Caused by compression waves which move around the disk Increased density of matter at “crests” Density waves revolve at different speed than individual stars orbit Galactic center Does the Milky Way have a Bar? Mapping the Galaxy Using IR light Warm clouds of gas/dust locates star formation sites in spiral arms The Star–Gas–Star Cycle Where the new stars are…. Views of the Center! Infrared Visual The Center in Radio & X-Rays Although dark in visual light, there are bright radio, IR, and X-ray sources at the center of the Galaxy, known as Sgr A*. Radio Image of Center of Milky Way “SNR” = Supernova Remnant Sgr A = brightest radio source in Sagittarius 11 years of observation in IR 6 stars orbiting unseen central mass ~ 4 Million x Mass of Sun Mapping the Center - IR Mapping the Center in IR Use IR telescopes to measure orbits of fastmoving stars near the Galactic center. One star passed within 1 light-day of Sgr A* Using Kepler’s Laws, mass = 2.6 million M What can be so small, yet be so massive? Mapping the Galaxy in X-Rays Using X-rays Highest energy emissions from center of galaxy Confirms Supermassive Black Hole Chandra image of Sgr A* Missing Mass? Observation: Stars in edges orbit center “too fast” to stay attached using gravity … from mass we “see”. Conclusion: So there must be more mass we *don’t* see pulling as well? Hypothesis: Dark Matter (Missing Mass) Mapping the Galaxy in X-Rays Chandra image of Sgr A* Rapid flare rise/drop time (< 10 min) Tiny emission region only 20 times the size of event horizon of 2.6 million M black hole. Observations consistent w/ supermassive black hole at the center of our Galaxy. Energy from flare probably came from a comet-sized lump of matter…torn apart before falling beneath the event horizon!