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### 9 Cards in this Set

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 Sketch a side-on view of the galaxy with diameters for each section of it, including disk, bulge, and halo. Sketch a fried egg where yolk is bulge. Halo is a spherical distribution around the whole thing (150,000 LY), Bulge is the center (100,000 LY) and disk around it (white of the egg, 12,000 LY). Where are we in the galaxy? About 2/3 of the way out in the visible light portion of the disk. How do we know the Milky Way is a disk galaxy? Because when we look up in a dark place in the sky we can see the edge of our galaxy, the Milky Way. When we look above and below the plane of the galaxy, we can't see as many stars as when we look through it. How do we know the disk was not originally flat? Because the halo is made up of old globular clusters and they are in a spherical formation, so they must have started out in a more spherical shape. Over time, gas and dust collisions have flattened the inter stellar gas and dust out to make the disk and that's where all the new star formation takes place. How do we know that we aren't at the center of the galaxy? By looking at globular clusters (Harlow Shappley) with RR Lyrae variables and use it to give us the distance to the globular clusters. It turned out they formed a sphere and we weren't at the center of it! Contrast Pop. 1 and 2 stars. Population 1 stars are stars like our sun and they live in the disk. Pop. 2 stars are in the halo. Pop. 2 stars formed right after our galaxy's formation and are made pretty much only of hydrogen and helium (about 1/100 heavy element abundance as our sun.) Pop. 2 stars have more heavier elements in them. These are younger (2nd generation.) These are mostly blue because they are young. Population 1 stars are more red. Remember, though, you cannot determine the age of a star just by its color. How do we map out the spiral structure of our galaxy's disk when we're living in the disk? We study radio waves emitted by the clouds of gas that deliniate the spiral arms. Part of the way we do that is by studying the 21 cm radiation from hydrogen, where the electron will randomly flip over and give off 21 cm wave. Since this is a long radio wave, it can penetrate the gas and dust at the disk of the galaxy. We can seperate different clouds of gas within the arms because each has a different Doppler shift than gas within another arm. How can regions of high density create the bright star formation regions that deliniate the spiral arms? The pattern of the density wave gives us the pattern of the spiral arms. As a cloud of gas and dust rotates around in the galaxy and causes one of these density waves, it moves into a region of higher density and it can give the cloud of gas and dust the little push that it needs to start the star formation process. The spiral arms are so bright because they are comprised of bright, new stars. They don't live very long and their whole lives are spent in the traffic jam of the spiral density wave. Understand how rotation curves can be used to measure the distribution of mass through a galaxy. This is a Kepler's Law problem. If you take a star or a cloud of gas that is orbiting around the center of a galaxy to figure out the mass contained between the center of the galaxy and the object. If you look at two objects each at different distances from the center if the galaxy, figure out the mass between them, and subtract one from the other, you can see how mass is distributed throughout the galaxy. This ends up showing us that there is a lot of mass in the outer parts of galaxies even though there is not a lot of light (dark matter!!)